Evotree: Evolution Traversal Enhancements (#3936)

Like move validation, evolutions are the earliest thing we wish to traverse when determining what encounters may have originated the current Pokémon. To determine the permitted species-form-levels a Pokémon could originate with, we must devolve a Pokémon by traveling down-generation to origin. Once we have an encounter, we can then evolve it to the current species, traversing upwards from origin to the current format.

PKHeX.Core/Editing/Applicators/BallApplicator.cs

@@ -73,7 +73,7 @@ public static class BallApplicator
         return pk.Ball = (int)next;
     }
 
-    private static int ApplyFirstLegalBall(PKM pk, Span<Ball> balls)
+    private static int ApplyFirstLegalBall(PKM pk, ReadOnlySpan<Ball> balls)
     {
         foreach (var b in balls)
         {

PKHeX.Core/Editing/PKM/QR/QRMessageUtil.cs

@@ -112,7 +112,7 @@ public static class QRMessageUtil
         // Decode unicode string -- size might be pretty big (user input), so just rent instead of stackalloc
         var tmp = ArrayPool<byte>.Shared.Rent(url.Length);
         var result = Convert.TryFromBase64Chars(url, tmp, out int bytesWritten) ? tmp[..bytesWritten] : null;
-        ArrayPool<byte>.Shared.Return(tmp);
+        ArrayPool<byte>.Shared.Return(tmp, true);
         return result;
     }
 

PKHeX.Core/Game/Locations/LocationsHOME.cs

@@ -1,3 +1,5 @@
+using System;
+
 namespace PKHeX.Core;
 
 /// <summary>
@@ -106,4 +108,47 @@ public static class LocationsHOME
         SWSL when ver == (int)GameVersion.SW => true,
         _ => false,
     };
+
+    /// <summary>
+    /// Checks if the location is (potentially) remapped based on visitation options.
+    /// </summary>
+    /// <remarks>Relevant when a side data yields SW/SH side data with a higher priority than the original (by version) side data.</remarks>
+    /// <param name="original">Original context</param>
+    /// <param name="current">Current context</param>
+    public static LocationRemapState GetRemapState(EntityContext original, EntityContext current)
+    {
+        if (current == original)
+            return LocationRemapState.Original;
+        if (current == EntityContext.Gen8)
+            return LocationRemapState.Remapped;
+        return original.Generation() switch
+        {
+            < 8 => LocationRemapState.Original,
+            8 => LocationRemapState.Either,
+            _ => current is (EntityContext.Gen8a or EntityContext.Gen8b) // down
+                ? LocationRemapState.Either
+                : LocationRemapState.Original,
+        };
+    }
+
+    public static bool IsMatchLocation(EntityContext original, EntityContext current, int met, int expect, int version)
+    {
+        var state = GetRemapState(original, current);
+        return state switch
+        {
+            LocationRemapState.Original => met == expect,
+            LocationRemapState.Remapped => met == GetMetSWSH((ushort)expect, version),
+            LocationRemapState.Either => met == expect || met == GetMetSWSH((ushort)expect, version),
+            _ => false,
+        };
+    }
+}
+
+[Flags]
+public enum LocationRemapState
+{
+    None,
+    Original = 1 << 0,
+    Remapped = 1 << 1,
+    Either = Original | Remapped,
 }

PKHeX.Core/Items/ItemStorage9SV.cs

@@ -82,6 +82,11 @@ public sealed class ItemStorage9SV : IItemStorage
         2210, 2211, 2212, 2213, 2214, 2215, 2216, 2217, 2218, 2219,
         2220, 2221, 2222, 2223, 2224, 2225, 2226, 2227, 2228, 2229,
         2230, 2231,
+
+        // DLC additions
+        // 2232, 2233, 2234, 2235, 2236, 2237, 2238, 2239, 2240, 2241,
+        // 2242, 2243, 2244, 2245, 2246, 2247, 2248, 2249, 2250, 2251,
+        // 2252, 2253, 2254, 2255, 2256, 2257, 2258, 2259, 2260, 2261,
     };
 
     private static ReadOnlySpan<ushort> Pouch_Treasure_SV => new ushort[]

PKHeX.Core/Legality/Core.cs

@@ -139,20 +139,6 @@ public static class Legal
     internal static readonly ushort[] HeldItems_LA = Array.Empty<ushort>();
     internal static readonly ushort[] HeldItems_SV = ItemStorage9SV.GetAllHeld();
 
-    internal static int GetMaxSpeciesOrigin(int generation) => generation switch
-    {
-        1 => MaxSpeciesID_1,
-        2 => MaxSpeciesID_2,
-        3 => MaxSpeciesID_3,
-        4 => MaxSpeciesID_4,
-        5 => MaxSpeciesID_5,
-        6 => MaxSpeciesID_6,
-        7 => MaxSpeciesID_7b,
-        8 => MaxSpeciesID_8a,
-        9 => MaxSpeciesID_9,
-        _ => -1,
-    };
-
     internal static int GetMaxLanguageID(int generation) => generation switch
     {
         1 => (int) LanguageID.Spanish, // 1-7 except 6
@@ -231,11 +217,12 @@ public static class Legal
     /// </summary>
     public static bool GetIsFixedIVSequenceValidSkipRand(ReadOnlySpan<int> IVs, PKM pk, uint max = 31)
     {
-        for (int i = 0; i < 6; i++)
+        for (int i = 5; i >= 0; i--)
         {
-            if ((uint) IVs[i] > max) // random
+            var iv = IVs[i];
+            if ((uint)iv > max) // random
                 continue;
-            if (IVs[i] != pk.GetIV(i))
+            if (iv != pk.GetIV(i))
                 return false;
         }
         return true;

PKHeX.Core/Legality/Encounters/Data/EncounterUtil.cs

@@ -48,7 +48,7 @@ internal static class EncounterUtil
         return temp;
     }
 
-    internal static T? GetMinByLevel<T>(EvoCriteria[] chain, IEnumerable<T> possible) where T : class, IEncounterTemplate
+    internal static T? GetMinByLevel<T>(ReadOnlySpan<EvoCriteria> chain, IEnumerable<T> possible) where T : class, IEncounterTemplate
     {
         // MinBy grading: prefer species-form match, select lowest min level encounter.
         // Minimum allocation :)

PKHeX.Core/Legality/Encounters/EncounterMisc/EncounterInvalid.cs

@@ -14,7 +14,7 @@ public sealed record EncounterInvalid : IEncounterable
     public byte LevelMin { get; }
     public byte LevelMax { get; }
     public bool EggEncounter { get; }
-    public int Generation { get; init; }
+    public int Generation { get; }
     public EntityContext Context { get; }
     public GameVersion Version { get; }
     public bool IsShiny => false;

PKHeX.Core/Legality/Encounters/EncounterStatic/EncounterFixed9.cs

@@ -53,8 +53,25 @@ public sealed record EncounterFixed9 : EncounterStatic, IGemType
 
     protected override bool IsMatchLocation(PKM pk)
     {
-        if (pk is PK8)
-            return LocationsHOME.IsValidMetSV((ushort)pk.Met_Location, pk.Version);
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetSV(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
+    }
+
+    private bool IsMatchLocationExact(PKM pk)
+    {
         var loc = pk.Met_Location;
         return loc == Location0 || loc == Location1 || loc == Location2 || loc == Location3;
     }

PKHeX.Core/Legality/Encounters/EncounterStatic/EncounterMight9.cs

@@ -213,9 +213,23 @@ public sealed record EncounterMight9 : EncounterStatic, ITeraRaid9
 
     protected override bool IsMatchLocation(PKM pk)
     {
-        if (pk is PK8)
-            return LocationsHOME.IsValidMetSV((ushort)pk.Met_Location, pk.Version);
-        return base.IsMatchLocation(pk);
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationExact(PKM pk) => pk.Met_Location == Location;
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetSV(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
     }
 
     protected override EncounterMatchRating IsMatchDeferred(PKM pk)

PKHeX.Core/Legality/Encounters/EncounterStatic/EncounterStatic8a.cs

@@ -72,10 +72,38 @@ public sealed record EncounterStatic8a(GameVersion Version) : EncounterStatic(Ve
 
         if (pk is IScaledSize s)
         {
-            if (HasFixedHeight && s.HeightScalar != HeightScalar)
-                return false;
-            if (HasFixedWeight && s.WeightScalar != WeightScalar)
-                return false;
+            // 3 of the Alpha statics were mistakenly set as 127 scale. If they enter HOME on 3.0.1, they'll get bumped to 255.
+            if (IsAlpha && this is { HeightScalar: 127, WeightScalar: 127 }) // Average Size Alphas
+            {
+                // HOME >=3.0.1 ensures 255 scales for the 127's
+                // PLA and S/V could have safe-harbored them via <=3.0.0
+                if (pk.Context is EntityContext.Gen8a or EntityContext.Gen9)
+                {
+                    if (s is not { HeightScalar: 127, WeightScalar: 127 }) // Original? 
+                    {
+                        // Must match the HOME updated values AND must have the Alpha ribbon (visited HOME).
+                        if (s is not { HeightScalar: 255, WeightScalar: 255 })
+                            return false;
+                        if (pk is IRibbonSetMark9 { RibbonMarkAlpha: false })
+                            return false;
+                        if (pk.IsUntraded)
+                            return false;
+                    }
+                }
+                else
+                {
+                    // Must match the HOME updated values
+                    if (s is not { HeightScalar: 255, WeightScalar: 255 })
+                        return false;
+                }
+            }
+            else
+            {
+                if (HasFixedHeight && s.HeightScalar != HeightScalar)
+                    return false;
+                if (HasFixedWeight && s.WeightScalar != WeightScalar)
+                    return false;
+            }
         }
 
         if (pk is IAlpha a && a.IsAlpha != IsAlpha)
@@ -86,14 +114,16 @@ public sealed record EncounterStatic8a(GameVersion Version) : EncounterStatic(Ve
 
     protected override bool IsMatchLocation(PKM pk)
     {
-        if (pk is PK8)
-            return pk.Met_Location == LocationsHOME.SWLA;
-        if (pk is PB8 { Version: (int)GameVersion.PLA, Met_Location: LocationsHOME.SWLA })
-            return true;
-
-        return base.IsMatchLocation(pk);
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return base.IsMatchLocation(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMetRemappedSWSH(pk);
+        return base.IsMatchLocation(pk) || IsMetRemappedSWSH(pk);
     }
 
+    private static bool IsMetRemappedSWSH(PKM pk) => pk.Met_Location == LocationsHOME.SWLA;
+
     public override EncounterMatchRating GetMatchRating(PKM pk)
     {
         var result = GetMatchRatingInternal(pk);

PKHeX.Core/Legality/Encounters/EncounterStatic/EncounterStatic8b.cs

@@ -19,8 +19,25 @@ public sealed record EncounterStatic8b : EncounterStatic, IStaticCorrelation8b
 
     protected override bool IsMatchLocation(PKM pk)
     {
-        if (pk is PK8)
-            return LocationsHOME.IsValidMetBDSP((ushort)pk.Met_Location, pk.Version);
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetBDSP(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
+    }
+
+    private bool IsMatchLocationExact(PKM pk)
+    {
         if (!Roaming)
             return base.IsMatchLocation(pk);
         return IsRoamingLocation(pk);
@@ -48,18 +65,24 @@ public sealed record EncounterStatic8b : EncounterStatic, IStaticCorrelation8b
 
     protected override bool IsMatchEggLocation(PKM pk)
     {
-        if (pk is not PB8)
-        {
-            if (!EggEncounter)
-                return pk.Egg_Location == 0;
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchEggLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchEggLocationRemapped(pk);
+        // Either
+        return IsMatchEggLocationExact(pk) || IsMatchEggLocationRemapped(pk);
+    }
 
-            if (pk is PK8)
-                return LocationsHOME.IsLocationSWSHEgg(pk.Version, pk.Met_Location, pk.Egg_Location, (ushort)EggLocation);
-
-            // Hatched
-            return pk.Egg_Location == EggLocation || pk.Egg_Location == Locations.LinkTrade6NPC;
-        }
+    private bool IsMatchEggLocationRemapped(PKM pk)
+    {
+        if (!EggEncounter)
+            return pk.Egg_Location == 0;
+        return LocationsHOME.IsLocationSWSHEgg(pk.Version, pk.Met_Location, pk.Egg_Location, (ushort)EggLocation);
+    }
 
+    private bool IsMatchEggLocationExact(PKM pk)
+    {
         var eggloc = pk.Egg_Location;
         if (!EggEncounter)
             return eggloc == EggLocation;

PKHeX.Core/Legality/Encounters/EncounterStatic/EncounterStatic9.cs

@@ -21,9 +21,23 @@ public sealed record EncounterStatic9(GameVersion Version) : EncounterStatic(Ver
 
     protected override bool IsMatchLocation(PKM pk)
     {
-        if (pk is PK8)
-            return LocationsHOME.IsValidMetSV((ushort)pk.Met_Location, pk.Version);
-        return base.IsMatchLocation(pk);
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationExact(PKM pk) => pk.Met_Location == Location;
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetSV(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
     }
 
     protected override bool IsMatchPartial(PKM pk)

PKHeX.Core/Legality/Encounters/EncounterStatic/EncounterTera9.cs

@@ -97,9 +97,23 @@ public sealed record EncounterTera9 : EncounterStatic, ITeraRaid9
 
     protected override bool IsMatchLocation(PKM pk)
     {
-        if (pk is PK8)
-            return LocationsHOME.IsValidMetSV((ushort)pk.Met_Location, pk.Version);
-        return base.IsMatchLocation(pk);
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationExact(PKM pk) => pk.Met_Location == Location;
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetSV(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
     }
 
     protected override EncounterMatchRating IsMatchDeferred(PKM pk)

PKHeX.Core/Legality/Encounters/EncounterTrade/EncounterTrade8b.cs

@@ -36,17 +36,46 @@ public sealed record EncounterTrade8b : EncounterTrade, IContestStatsReadOnly, I
             return false;
         if (pk is IScaledSize w && w.WeightScalar != WeightScalar)
             return false;
+        if (!IsMatchLocation(pk))
+            return false;
         return base.IsMatchExact(pk, evo);
     }
 
+    private bool IsMatchLocation(PKM pk)
+    {
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationExact(PKM pk) => pk.Met_Location == Location;
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetBDSP(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
+    }
+
     protected override bool IsMatchEggLocation(PKM pk)
     {
-        var expect = EggLocation;
-        if (pk is not PB8 && expect == Locations.Default8bNone)
-            expect = 0;
-        return pk.Egg_Location == expect;
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchEggLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchEggLocationRemapped(pk);
+        // Either
+        return IsMatchEggLocationExact(pk) || IsMatchEggLocationRemapped(pk);
     }
 
+    private static bool IsMatchEggLocationRemapped(PKM pk) => pk.Egg_Location == 0;
+    private bool IsMatchEggLocationExact(PKM pk) => pk.Egg_Location == EggLocation;
+
     protected override void ApplyDetails(ITrainerInfo sav, EncounterCriteria criteria, PKM pk)
     {
         base.ApplyDetails(sav, criteria, pk);

PKHeX.Core/Legality/Encounters/EncounterTrade/EncounterTrade9.cs

@@ -41,6 +41,32 @@ public sealed record EncounterTrade9 : EncounterTrade, IGemType
     {
         if (TeraType != GemType.Random && pk is ITeraType t && !Tera9RNG.IsMatchTeraType(TeraType, Species, Form, (byte)t.TeraTypeOriginal))
             return false;
-        return base.IsMatchExact(pk, evo);
+        if (!base.IsMatchExact(pk, evo))
+            return false;
+        if (!IsMatchLocation(pk))
+            return false;
+        return true;
+    }
+
+
+    private bool IsMatchLocation(PKM pk)
+    {
+        var metState = LocationsHOME.GetRemapState(Context, pk.Context);
+        if (metState == LocationRemapState.Original)
+            return IsMatchLocationExact(pk);
+        if (metState == LocationRemapState.Remapped)
+            return IsMatchLocationRemapped(pk);
+        return IsMatchLocationExact(pk) || IsMatchLocationRemapped(pk);
+    }
+
+    private bool IsMatchLocationExact(PKM pk) => pk.Met_Location == Location;
+
+    private bool IsMatchLocationRemapped(PKM pk)
+    {
+        var met = (ushort)pk.Met_Location;
+        var version = pk.Version;
+        if (pk.Context == EntityContext.Gen8)
+            return LocationsHOME.IsValidMetSV(met, version);
+        return LocationsHOME.GetMetSWSH((ushort)Location, version) == met;
     }
 }

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator1.cs

@@ -13,6 +13,9 @@ public sealed class EncounterGenerator1 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = GetGifts();
@@ -109,6 +112,8 @@ public sealed class EncounterGenerator1 : IEncounterGenerator
         // Calculate all 3 at the same time and pick the best result (by species).
         // Favor special event move gifts as Static Encounters when applicable
         var chain = EncounterOrigin.GetOriginChain12(pk, game);
+        if (chain.Length == 0)
+            return Array.Empty<IEncounterable>();
         return GetEncounters(pk, chain, game);
     }
 

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator2.cs

@@ -13,6 +13,9 @@ public sealed class EncounterGenerator2 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         bool korean = pk.Korean;
         if (groups.HasFlag(Mystery))
         {
@@ -116,6 +119,8 @@ public sealed class EncounterGenerator2 : IEncounterGenerator
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, GameVersion game)
     {
         var chain = EncounterOrigin.GetOriginChain12(pk, game);
+        if (chain.Length == 0)
+            return Array.Empty<IEncounterable>();
         return GetEncounters(pk, chain, game);
     }
 

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator3.cs

@@ -14,6 +14,9 @@ public sealed class EncounterGenerator3 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM _, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncountersWC3.Encounter_WC3;
@@ -117,12 +120,15 @@ public sealed class EncounterGenerator3 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 3);
         return GetEncounters(pk, chain, info);
     }
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
+
         info.PIDIV = MethodFinder.Analyze(pk);
         IEncounterable? partial = null;
 

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator3GC.cs

@@ -13,6 +13,9 @@ public sealed class EncounterGenerator3GC : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncountersWC3.Encounter_WC3CXD;
@@ -67,12 +70,14 @@ public sealed class EncounterGenerator3GC : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 3);
         return GetEncounters(pk, chain, info);
     }
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         IEncounterable? partial = null;
         info.PIDIV = MethodFinder.Analyze(pk);
         foreach (var z in IterateInner(pk, chain))

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator4.cs

@@ -17,12 +17,17 @@ public sealed class EncounterGenerator4 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
-       return GetEncounters(pk, chain, info);
+        var chain = EncounterOrigin.GetOriginChain(pk, 4);
+        if (chain.Length == 0)
+            return Array.Empty<IEncounterable>();
+        return GetEncounters(pk, chain, info);
     }
 
     public IEnumerable<IEncounterable> GetPossible(PKM _, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             if (chain[^1].Species == (int)Species.Manaphy)

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator5.cs

@@ -13,12 +13,17 @@ public sealed class EncounterGenerator5 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 5);
+        if (chain.Length == 0)
+            return Array.Empty<IEncounterable>();
         return GetEncounters(pk, chain, info);
     }
 
     public IEnumerable<IEncounterable> GetPossible(PKM _, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G5;

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator6.cs

@@ -13,6 +13,9 @@ public sealed class EncounterGenerator6 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G6;
@@ -134,12 +137,14 @@ public sealed class EncounterGenerator6 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 6);
         return GetEncounters(pk, chain, info);
     }
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         var game = (GameVersion)pk.Version;
 
         IEncounterable? deferred = null;

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator7.cs

@@ -13,6 +13,9 @@ public sealed class EncounterGenerator7 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G7;
@@ -108,6 +111,8 @@ public sealed class EncounterGenerator7 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         var game = (GameVersion)pk.Version;
 
         bool yielded = false;

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator7GG.cs

@@ -15,6 +15,9 @@ public sealed class EncounterGenerator7GG : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM _, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G7GG;
@@ -102,6 +105,8 @@ public sealed class EncounterGenerator7GG : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         bool yielded = false;
         if (pk.FatefulEncounter)
         {

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator7GO.cs

@@ -12,6 +12,9 @@ public sealed class EncounterGenerator7GO : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Slot))
         {
             var areas = EncountersGO.SlotsGO_GG;
@@ -38,6 +41,8 @@ public sealed class EncounterGenerator7GO : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         if (!CanBeWildEncounter(pk))
             yield break;
 

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator8.cs

@@ -12,6 +12,9 @@ public sealed class EncounterGenerator8 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM _, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G8;
@@ -108,6 +111,8 @@ public sealed class EncounterGenerator8 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         if (pk.FatefulEncounter)
         {
             bool yielded = false;

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator8GO.cs

@@ -12,6 +12,9 @@ public sealed class EncounterGenerator8GO : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Slot))
         {
             var table = EncountersGO.SlotsGO;
@@ -38,6 +41,8 @@ public sealed class EncounterGenerator8GO : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         if (!CanBeWildEncounter(pk))
             yield break;
 

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator8a.cs

@@ -12,6 +12,9 @@ public sealed class EncounterGenerator8a : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G8A;
@@ -80,6 +83,8 @@ public sealed class EncounterGenerator8a : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         if (pk is PK8 { SWSH: false })
             yield break;
         if (pk.IsEgg)
@@ -139,8 +144,11 @@ public sealed class EncounterGenerator8a : IEncounterGenerator
         // Encounter Slots
         if (CanBeWildEncounter(pk))
         {
-            bool hasOriginalLocation = pk is not (PK8 or PB8 { Met_Location: LocationsHOME.SWLA });
             var location = pk.Met_Location;
+            var remap = LocationsHOME.GetRemapState(EntityContext.Gen8a, pk.Context);
+            bool hasOriginalLocation = true;
+            if (remap.HasFlag(LocationRemapState.Remapped))
+                hasOriginalLocation = location != LocationsHOME.SWLA;
             foreach (var area in Encounters8a.SlotsLA)
             {
                 if (hasOriginalLocation && !area.IsMatchLocation(location))

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator8b.cs

@@ -12,6 +12,9 @@ public sealed class EncounterGenerator8b : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM _, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G8B;
@@ -107,6 +110,8 @@ public sealed class EncounterGenerator8b : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, EvoCriteria[] chain, LegalInfo info)
     {
+        if (chain.Length == 0)
+            yield break;
         if (pk is PK8)
             yield break;
 
@@ -182,8 +187,11 @@ public sealed class EncounterGenerator8b : IEncounterGenerator
 
         if (CanBeWildEncounter(pk))
         {
-            bool hasOriginalLocation = pk is not PK8;
             var location = pk.Met_Location;
+            var remap = LocationsHOME.GetRemapState(EntityContext.Gen8b, pk.Context);
+            bool hasOriginalLocation = true;
+            if (remap.HasFlag(LocationRemapState.Remapped))
+                hasOriginalLocation = location != LocationsHOME.GetMetSWSH((ushort)location, (int)game);
             var encWild = game == GameVersion.BD ? Encounters8b.SlotsBD : Encounters8b.SlotsSP;
             foreach (var area in encWild)
             {

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/EncounterGenerator9.cs

@@ -13,7 +13,10 @@ public sealed class EncounterGenerator9 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 9);
+        if (chain.Length == 0)
+            return System.Array.Empty<IEncounterable>();
+
         return (GameVersion)pk.Version switch
         {
             SW when pk.Met_Location == LocationsHOME.SWSL => Instance.GetEncountersSWSH(pk, chain, SL),
@@ -24,6 +27,9 @@ public sealed class EncounterGenerator9 : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetPossible(PKM pk, EvoCriteria[] chain, GameVersion game, EncounterTypeGroup groups)
     {
+        if (chain.Length == 0)
+            yield break;
+
         if (groups.HasFlag(Mystery))
         {
             var table = EncounterEvent.MGDB_G9;

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/Lump/EncounterGenerator7X.cs

@@ -15,7 +15,7 @@ public sealed class EncounterGenerator7X : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 7);
         return GetEncounters(pk, chain, info);
     }
 

PKHeX.Core/Legality/Encounters/Generator/ByGeneration/Lump/EncounterGenerator8X.cs

@@ -17,7 +17,7 @@ public sealed class EncounterGenerator8X : IEncounterGenerator
 
     public IEnumerable<IEncounterable> GetEncounters(PKM pk, LegalInfo info)
     {
-        var chain = EncounterOrigin.GetOriginChain(pk);
+        var chain = EncounterOrigin.GetOriginChain(pk, 8);
         return GetEncounters(pk, chain, info);
     }
 

PKHeX.Core/Legality/Encounters/Generator/Moveset/EncounterMovesetGenerator.cs

@@ -31,10 +31,10 @@ public static class EncounterMovesetGenerator
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
     /// <param name="versions">Any specific version(s) to iterate for. If left blank, all will be checked.</param>
     /// <returns>A consumable <see cref="PKM"/> list of possible results.</returns>
-    /// <remarks>When updating, update the sister <see cref="GenerateEncounters(PKM,ITrainerInfo,ushort[],GameVersion[])"/> method.</remarks>
-    public static IEnumerable<PKM> GeneratePKMs(PKM pk, ITrainerInfo info, ushort[] moves, params GameVersion[] versions)
+    /// <remarks>When updating, update the sister <see cref="GenerateEncounters(PKM,ITrainerInfo,ReadOnlyMemory{ushort},GameVersion[])"/> method.</remarks>
+    public static IEnumerable<PKM> GeneratePKMs(PKM pk, ITrainerInfo info, ReadOnlyMemory<ushort> moves, params GameVersion[] versions)
     {
-        if (!IsSane(pk, moves))
+        if (!IsSane(pk, moves.Span))
             yield break;
 
         OptimizeCriteria(pk, info);
@@ -63,10 +63,10 @@ public static class EncounterMovesetGenerator
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
     /// <param name="versions">Any specific version(s) to iterate for. If left blank, all will be checked.</param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible results.</returns>
-    /// <remarks>When updating, update the sister <see cref="GeneratePKMs(PKM,ITrainerInfo,ushort[],GameVersion[])"/> method.</remarks>
-    public static IEnumerable<IEncounterable> GenerateEncounters(PKM pk, ITrainerInfo info, ushort[] moves, params GameVersion[] versions)
+    /// <remarks>When updating, update the sister <see cref="GeneratePKMs(PKM,ITrainerInfo,ReadOnlyMemory{ushort},GameVersion[])"/> method.</remarks>
+    public static IEnumerable<IEncounterable> GenerateEncounters(PKM pk, ITrainerInfo info, ReadOnlyMemory<ushort> moves, params GameVersion[] versions)
     {
-        if (!IsSane(pk, moves))
+        if (!IsSane(pk, moves.Span))
             yield break;
 
         OptimizeCriteria(pk, info);
@@ -99,7 +99,7 @@ public static class EncounterMovesetGenerator
     /// <param name="info">Trainer information of the receiver.</param>
     /// <param name="generation">Specific generation to iterate versions for.</param>
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
-    public static IEnumerable<PKM> GeneratePKMs(PKM pk, ITrainerInfo info, int generation, ushort[] moves)
+    public static IEnumerable<PKM> GeneratePKMs(PKM pk, ITrainerInfo info, int generation, ReadOnlyMemory<ushort> moves)
     {
         var vers = GameUtil.GetVersionsInGeneration(generation, pk.Version);
         return GeneratePKMs(pk, info, moves, vers);
@@ -112,7 +112,7 @@ public static class EncounterMovesetGenerator
     /// <param name="generation">Specific generation to iterate versions for.</param>
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    public static IEnumerable<IEncounterable> GenerateEncounter(PKM pk, int generation, ushort[] moves)
+    public static IEnumerable<IEncounterable> GenerateEncounter(PKM pk, int generation, ReadOnlyMemory<ushort> moves)
     {
         var vers = GameUtil.GetVersionsInGeneration(generation, pk.Version);
         return GenerateEncounters(pk, moves, vers);
@@ -125,9 +125,9 @@ public static class EncounterMovesetGenerator
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
     /// <param name="versions">Any specific version(s) to iterate for. If left blank, all will be checked.</param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    public static IEnumerable<IEncounterable> GenerateEncounters(PKM pk, ushort[] moves, params GameVersion[] versions)
+    public static IEnumerable<IEncounterable> GenerateEncounters(PKM pk, ReadOnlyMemory<ushort> moves, params GameVersion[] versions)
     {
-        if (!IsSane(pk, moves))
+        if (!IsSane(pk, moves.Span))
             yield break;
 
         if (versions.Length > 0)
@@ -152,9 +152,9 @@ public static class EncounterMovesetGenerator
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
     /// <param name="vers">Any specific version(s) to iterate for. If left blank, all will be checked.</param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    public static IEnumerable<IEncounterable> GenerateEncounters(PKM pk, ushort[] moves, IReadOnlyList<GameVersion> vers)
+    public static IEnumerable<IEncounterable> GenerateEncounters(PKM pk, ReadOnlyMemory<ushort> moves, IReadOnlyList<GameVersion> vers)
     {
-        if (!IsSane(pk, moves))
+        if (!IsSane(pk, moves.Span))
             yield break;
 
         foreach (var ver in vers)
@@ -171,17 +171,22 @@ public static class EncounterMovesetGenerator
     /// <param name="moves">Moves that the resulting <see cref="IEncounterable"/> must be able to learn.</param>
     /// <param name="version">Specific version to iterate for.</param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    private static IEnumerable<IEncounterable> GenerateVersionEncounters(PKM pk, ushort[] moves, GameVersion version)
+    private static IEnumerable<IEncounterable> GenerateVersionEncounters(PKM pk, ReadOnlyMemory<ushort> moves, GameVersion version)
     {
         pk.Version = (int)version;
 
-        var context = pk.Context;
-        if (context is EntityContext.Gen2 && version is GameVersion.RD or GameVersion.GN or GameVersion.BU or GameVersion.YW)
-            context = EntityContext.Gen1; // try excluding baby pokemon from our evolution chain, for move learning purposes.
-        var et = EvolutionTree.GetEvolutionTree(context);
-        var chain = et.GetValidPreEvolutions(pk, levelMax: 100, skipChecks: true);
+        var generation = (byte)version.GetGeneration();
+        if (version is GameVersion.GO)
+            generation = (byte)pk.Format;
+        if (pk.Context is EntityContext.Gen2 && version is GameVersion.RD or GameVersion.GN or GameVersion.BU or GameVersion.YW)
+            generation = 1; // try excluding baby pokemon from our evolution chain, for move learning purposes.
 
-        var needs = GetNeededMoves(pk, moves, version);
+        var origin = new EvolutionOrigin(pk.Species, (byte)version, generation, 1, 100, true);
+        var chain = EvolutionChain.GetOriginChain(pk, origin);
+        if (chain.Length == 0)
+            yield break;
+
+        ReadOnlyMemory<ushort> needs = GetNeededMoves(pk, moves.Span, version, generation);
         var generator = EncounterGenerator.GetGenerator(version);
 
         foreach (var type in PriorityList)
@@ -224,27 +229,17 @@ public static class EncounterMovesetGenerator
         return true;
     }
 
-    private static ushort[] GetNeededMoves(PKM pk, ReadOnlySpan<ushort> moves, GameVersion ver)
+    private static ushort[] GetNeededMoves(PKM pk, ReadOnlySpan<ushort> moves, GameVersion ver, int generation)
     {
         if (pk.Species == (int)Species.Smeargle)
             return Array.Empty<ushort>();
 
-        // Roughly determine the generation the PKM is originating from
-        int origin = pk.Generation;
-        if (origin < 0)
-            origin = ver.GetGeneration();
-
-        // Temporarily replace the Version for VC1 transfers, so that they can have VC2 moves if needed.
-        bool vcBump = origin == 1 && pk.Format >= 7;
-        if (vcBump)
-            pk.Version = (int)GameVersion.C;
-
         var length = pk.MaxMoveID + 1;
         var rent = ArrayPool<bool>.Shared.Rent(length);
         var permitted = rent.AsSpan(0, length);
-        var enc = new EvolutionOrigin(0, (byte)ver, (byte)origin, 1, 100, true);
-        var history = EvolutionChain.GetEvolutionChainsSearch(pk, enc);
-        var e = EncounterInvalid.Default with { Generation = origin };
+        var enc = new EvolutionOrigin(pk.Species, (byte)ver, (byte)generation, 1, 100, true);
+        var history = EvolutionChain.GetEvolutionChainsSearch(pk, enc, ver.GetContext(), 0);
+        var e = EncounterInvalid.Default; // default empty
         LearnPossible.Get(pk, e, history, permitted);
 
         int ctr = 0; // count of moves that can be learned
@@ -260,15 +255,12 @@ public static class EncounterMovesetGenerator
         permitted.Clear();
         ArrayPool<bool>.Shared.Return(rent);
 
-        if (vcBump)
-            pk.Version = (int)ver;
-
         if (ctr == 0)
             return Array.Empty<ushort>();
         return result[..ctr].ToArray();
     }
 
-    private static IEnumerable<IEncounterable> GetPossibleOfType(PKM pk, ushort[] needs, GameVersion version, EncounterTypeGroup type, EvoCriteria[] chain, IEncounterGenerator generator)
+    private static IEnumerable<IEncounterable> GetPossibleOfType(PKM pk, ReadOnlyMemory<ushort> needs, GameVersion version, EncounterTypeGroup type, EvoCriteria[] chain, IEncounterGenerator generator)
         => type switch
     {
         Egg => GetEggs(pk, needs, chain, version, generator),
@@ -288,7 +280,7 @@ public static class EncounterMovesetGenerator
     /// <param name="version">Specific version to iterate for. Necessary for retrieving possible Egg Moves.</param>
     /// <param name="generator"></param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    private static IEnumerable<IEncounterable> GetEggs(PKM pk, ushort[] needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
+    private static IEnumerable<IEncounterable> GetEggs(PKM pk, ReadOnlyMemory<ushort> needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
     {
         if (!Breeding.CanGameGenerateEggs(version))
             yield break; // no eggs from these games
@@ -296,7 +288,7 @@ public static class EncounterMovesetGenerator
         var eggs = generator.GetPossible(pk, chain, version, Egg);
         foreach (var egg in eggs)
         {
-            if (needs.Length == 0 || HasAllNeededMovesEgg(needs, egg))
+            if (needs.Length == 0 || HasAllNeededMovesEgg(needs.Span, egg))
                 yield return egg;
         }
     }
@@ -310,7 +302,7 @@ public static class EncounterMovesetGenerator
     /// <param name="version">Specific version to iterate for.</param>
     /// <param name="generator">Generator</param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    private static IEnumerable<IEncounterable> GetGifts(PKM pk, ushort[] needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
+    private static IEnumerable<IEncounterable> GetGifts(PKM pk, ReadOnlyMemory<ushort> needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
     {
         var context = pk.Context;
         var gifts = generator.GetPossible(pk, chain, version, Mystery);
@@ -318,7 +310,7 @@ public static class EncounterMovesetGenerator
         {
             if (!IsSane(chain, enc, context))
                 continue;
-            if (needs.Length == 0 || GetHasAllNeededMoves(needs, enc))
+            if (needs.Length == 0 || GetHasAllNeededMoves(needs.Span, enc))
                 yield return enc;
         }
     }
@@ -332,7 +324,7 @@ public static class EncounterMovesetGenerator
     /// <param name="version">Specific version to iterate for.</param>
     /// <param name="generator"></param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    private static IEnumerable<IEncounterable> GetStatic(PKM pk, ushort[] needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
+    private static IEnumerable<IEncounterable> GetStatic(PKM pk, ReadOnlyMemory<ushort> needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
     {
         var context = pk.Context;
         var encounters = generator.GetPossible(pk, chain, version, Static);
@@ -340,7 +332,7 @@ public static class EncounterMovesetGenerator
         {
             if (!IsSane(chain, enc, context))
                 continue;
-            if (needs.Length == 0 || GetHasAllNeededMovesConsiderGen2(needs, enc))
+            if (needs.Length == 0 || GetHasAllNeededMovesConsiderGen2(needs.Span, enc))
                 yield return enc;
         }
     }
@@ -354,7 +346,7 @@ public static class EncounterMovesetGenerator
     /// <param name="version">Specific version to iterate for.</param>
     /// <param name="generator"></param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    private static IEnumerable<IEncounterable> GetTrades(PKM pk, ushort[] needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
+    private static IEnumerable<IEncounterable> GetTrades(PKM pk, ReadOnlyMemory<ushort> needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
     {
         var context = pk.Context;
         var trades = generator.GetPossible(pk, chain, version, Trade);
@@ -362,7 +354,7 @@ public static class EncounterMovesetGenerator
         {
             if (!IsSane(chain, enc, context))
                 continue;
-            if (needs.Length == 0 || GetHasAllNeededMovesConsiderGen2(needs, enc))
+            if (needs.Length == 0 || GetHasAllNeededMovesConsiderGen2(needs.Span, enc))
                 yield return enc;
         }
     }
@@ -376,7 +368,7 @@ public static class EncounterMovesetGenerator
     /// <param name="version">Origin version</param>
     /// <param name="generator"></param>
     /// <returns>A consumable <see cref="IEncounterable"/> list of possible encounters.</returns>
-    private static IEnumerable<IEncounterable> GetSlots(PKM pk, ushort[] needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
+    private static IEnumerable<IEncounterable> GetSlots(PKM pk, ReadOnlyMemory<ushort> needs, EvoCriteria[] chain, GameVersion version, IEncounterGenerator generator)
     {
         var context = pk.Context;
         var slots = generator.GetPossible(pk, chain, version, Slot);
@@ -384,7 +376,7 @@ public static class EncounterMovesetGenerator
         {
             if (!IsSane(chain, slot, context))
                 continue;
-            if (needs.Length == 0 || HasAllNeededMovesSlot(needs, slot))
+            if (needs.Length == 0 || HasAllNeededMovesSlot(needs.Span, slot))
                 yield return slot;
         }
     }

PKHeX.Core/Legality/Encounters/Information/EncounterSuggestion.cs

@@ -18,11 +18,15 @@ public static class EncounterSuggestion
         if (pk.WasEgg)
             return GetSuggestedEncounterEgg(pk, loc);
 
-        var chain = EvolutionChain.GetValidPreEvolutions(pk, maxLevel: 100, skipChecks: true);
+        Span<EvoCriteria> chain = stackalloc EvoCriteria[EvolutionTree.MaxEvolutions];
+        var origin = new EvolutionOrigin(pk.Species, (byte)pk.Version, (byte)pk.Generation, (byte)pk.CurrentLevel, (byte)pk.CurrentLevel, SkipChecks: true);
+        var count = EvolutionChain.GetOriginChain(chain, pk, origin);
         var ver = (GameVersion)pk.Version;
         var generator = EncounterGenerator.GetGenerator(ver);
-        var w = EncounterUtil.GetMinByLevel(chain, generator.GetPossible(pk, chain, ver, EncounterTypeGroup.Slot));
-        var s = EncounterUtil.GetMinByLevel(chain, generator.GetPossible(pk, chain, ver, EncounterTypeGroup.Static));
+
+        var evos = chain[..count].ToArray();
+        var w = EncounterUtil.GetMinByLevel(evos, generator.GetPossible(pk, evos, ver, EncounterTypeGroup.Slot));
+        var s = EncounterUtil.GetMinByLevel(evos, generator.GetPossible(pk, evos, ver, EncounterTypeGroup.Static));
 
         if (w is null)
             return s is null ? null : GetSuggestedEncounter(pk, s, loc);
@@ -116,22 +120,22 @@ public static class EncounterSuggestion
         if (startLevel >= 100)
             startLevel = 100;
 
-        var table = EvolutionTree.GetEvolutionTree(pk.Context);
-        int count = 1;
-        byte i = 100;
+        int most = 1;
+        Span<EvoCriteria> chain = stackalloc EvoCriteria[EvolutionTree.MaxEvolutions];
+        var origin = new EvolutionOrigin(pk.Species, (byte)pk.Version, (byte)pk.Generation, startLevel, 100, SkipChecks: true);
         while (true)
         {
-            var evos = table.GetValidPreEvolutions(pk, levelMax: i, skipChecks: true, levelMin: startLevel);
-            if (evos.Length < count) // lost an evolution, prior level was minimum current level
-                return GetMaxLevelMax(evos) + 1;
-            count = evos.Length;
-            if (i == startLevel)
+            var count = EvolutionChain.GetOriginChain(chain, pk, origin);
+            if (count < most) // lost an evolution, prior level was minimum current level
+                return GetMaxLevelMax(chain) + 1;
+            most = count;
+            if (origin.LevelMax == origin.LevelMin)
                 return startLevel;
-            --i;
+            origin = origin with { LevelMax = (byte)(origin.LevelMax - 1) };
         }
     }
 
-    private static int GetMaxLevelMax(EvoCriteria[] evos)
+    private static int GetMaxLevelMax(ReadOnlySpan<EvoCriteria> evos)
     {
         int max = 0;
         foreach (var evo in evos)

PKHeX.Core/Legality/Encounters/Verifiers/EvolutionVerifier.cs

@@ -8,6 +8,8 @@ namespace PKHeX.Core;
 /// </summary>
 public static class EvolutionVerifier
 {
+    private static readonly CheckResult VALID = new(CheckIdentifier.Evolution);
+
     /// <summary>
     /// Verifies Evolution scenarios of an <see cref="IEncounterable"/> for an input <see cref="PKM"/> and relevant <see cref="LegalInfo"/>.
     /// </summary>
@@ -16,7 +18,7 @@ public static class EvolutionVerifier
     public static CheckResult VerifyEvolution(PKM pk, LegalInfo info)
     {
         // Check if basic evolution methods are satisfiable with this encounter.
-        if (!IsValidEvolution(pk, info))
+        if (!IsValidEvolution(pk, info.EvoChainsAllGens, info.EncounterOriginal))
             return new CheckResult(Severity.Invalid, CheckIdentifier.Evolution, LEvoInvalid);
 
         // Check if complex evolution methods are satisfiable with this encounter.
@@ -26,36 +28,34 @@ public static class EvolutionVerifier
         return VALID;
     }
 
-    private static readonly CheckResult VALID = new(CheckIdentifier.Evolution);
-
     /// <summary>
     /// Checks if the Evolution from the source <see cref="IEncounterable"/> is valid.
     /// </summary>
     /// <param name="pk">Source data to verify</param>
-    /// <param name="info">Source supporting information to verify with</param>
+    /// <param name="history">Source supporting information to verify with</param>
+    /// <param name="enc">Matched encounter</param>
     /// <returns>Evolution is valid or not</returns>
-    private static bool IsValidEvolution(PKM pk, LegalInfo info)
+    private static bool IsValidEvolution(PKM pk, EvolutionHistory history, IEncounterTemplate enc)
     {
-        var chains = info.EvoChainsAllGens;
-        if (chains.Get(pk.Context).Length == 0)
+        // OK if un-evolved from original encounter
+        var encSpecies = enc.Species;
+        var curSpecies = pk.Species;
+        if (curSpecies == encSpecies)
+            return true; // never evolved
+
+        var current = history.Get(pk.Context);
+        if (!EvolutionUtil.Contains(current, curSpecies))
             return false; // Can't exist as current species
 
-        // OK if un-evolved from original encounter
-        ushort species = pk.Species;
-        var enc = info.EncounterMatch;
-        if (species == enc.Species) // never evolved
-            return true;
-
-        // Bigender->Fixed (non-Genderless) destination species, accounting for PID-Gender relationship
-        if (species == (int)Species.Vespiquen && enc.Generation < 6 && (pk.EncryptionConstant & 0xFF) >= 0x1F) // Combee->Vespiquen Invalid Evolution
+        // Double check that our encounter was able to exist as the encounter species.
+        var original = history.Get(enc.Context);
+        if (!EvolutionUtil.Contains(original, encSpecies))
             return false;
 
-        // Double check that our encounter was able to exist as the encounter species.
-        foreach (var z in chains.Get(enc.Context))
-        {
-            if (z.Species == enc.Species)
-                return true;
-        }
-        return false;
+        // Bigender->Fixed (non-Genderless) destination species, accounting for PID-Gender relationship
+        if (curSpecies == (int)Species.Vespiquen && enc.Generation < 6 && (pk.EncryptionConstant & 0xFF) >= 0x1F) // Combee->Vespiquen Invalid Evolution
+            return false;
+
+        return true;
     }
 }

PKHeX.Core/Legality/Evolutions/EncounterOrigin.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Collections.Generic;
-using static PKHeX.Core.Species;
 
 namespace PKHeX.Core;
 
@@ -13,14 +11,14 @@ public static class EncounterOrigin
     /// Gets possible evolution details for the input <see cref="pk"/>
     /// </summary>
     /// <param name="pk">Current state of the Pokémon</param>
+    /// <param name="generation">Original Generation</param>
     /// <returns>Possible origin species-form-levels to match against encounter data.</returns>
     /// <remarks>Use <see cref="GetOriginChain12"/> if the <see cref="pk"/> originated from Generation 1 or 2.</remarks>
-    public static EvoCriteria[] GetOriginChain(PKM pk)
+    public static EvoCriteria[] GetOriginChain(PKM pk, byte generation)
     {
-        bool hasOriginMet = pk.HasOriginalMetLocation;
-        var maxLevel = GetLevelOriginMax(pk, hasOriginMet);
-        var minLevel = GetLevelOriginMin(pk, hasOriginMet);
-        return GetOriginChain(pk, -1, (byte)maxLevel, (byte)minLevel, hasOriginMet);
+        var (minLevel, maxLevel) = GetMinMax(pk, generation);
+        var origin = new EvolutionOrigin(pk.Species, (byte)pk.Version, generation, minLevel, maxLevel);
+        return EvolutionChain.GetOriginChain(pk, origin);
     }
 
     /// <summary>
@@ -31,150 +29,67 @@ public static class EncounterOrigin
     /// <returns>Possible origin species-form-levels to match against encounter data.</returns>
     public static EvoCriteria[] GetOriginChain12(PKM pk, GameVersion gameSource)
     {
+        var (minLevel, maxLevel) = GetMinMaxGB(pk);
         bool rby = gameSource == GameVersion.RBY;
-        var maxSpecies = rby ? Legal.MaxSpeciesID_1 : Legal.MaxSpeciesID_2;
-
-        bool hasOriginMet;
-        int maxLevel, minLevel;
-        if (pk is ICaughtData2 pk2)
-        {
-            hasOriginMet = pk2.CaughtData != 0;
-            maxLevel = rby && Future_LevelUp2.Contains(pk.Species) ? pk.CurrentLevel - 1 : pk.CurrentLevel;
-            minLevel = !hasOriginMet ? 2 : pk.IsEgg ? 5 : pk2.Met_Level;
-        }
-        else if (pk is PK1 pk1)
-        {
-            hasOriginMet = false;
-            maxLevel = pk1.CurrentLevel;
-            minLevel = 2;
-        }
-        else if (rby)
-        {
-            hasOriginMet = false;
-            maxLevel = Future_LevelUp2.Contains(pk.Species) ? pk.CurrentLevel - 1 : GetLevelOriginMaxTransfer(pk, pk.Met_Level, 1);
-            minLevel = 2;
-        }
-        else // GSC
-        {
-            hasOriginMet = false;
-            maxLevel = GetLevelOriginMaxTransfer(pk, pk.Met_Level, 2);
-            minLevel = 2;
-        }
-
-        return GetOriginChain(pk, maxSpecies, (byte)maxLevel, (byte)minLevel, hasOriginMet);
+        byte ver = rby ? (byte)GameVersion.RBY : (byte)GameVersion.GSC;
+        byte gen = rby ? (byte)1 : (byte)2;
+        var origin = new EvolutionOrigin(pk.Species, ver, gen, minLevel, maxLevel);
+        return EvolutionChain.GetOriginChain(pk, origin);
     }
 
-    private static EvoCriteria[] GetOriginChain(PKM pk, int maxSpecies, byte maxLevel, byte minLevel, bool hasOriginMet)
+    private static (byte minLevel, byte maxLevel) GetMinMax(PKM pk, byte generation)
     {
-        if (maxLevel < minLevel)
-            return Array.Empty<EvoCriteria>();
-
-        if (hasOriginMet)
-            return EvolutionChain.GetValidPreEvolutions(pk, maxSpecies, maxLevel, minLevel);
-
-        // Permit the maximum to be all the way up to Current Level; we'll trim these impossible evolutions out later.
-        var tempMax = pk.CurrentLevel;
-        var chain = EvolutionChain.GetValidPreEvolutions(pk, maxSpecies, tempMax, minLevel);
-
-        for (var i = 0; i < chain.Length; i++)
-            chain[i] = chain[i] with { LevelMax = maxLevel, LevelMin = minLevel };
-
-        return chain;
+        byte maxLevel = (byte)pk.CurrentLevel;
+        byte minLevel = GetLevelOriginMin(pk, generation);
+        return (minLevel, maxLevel);
     }
 
-    private static int GetLevelOriginMin(PKM pk, bool hasMet)
+    private static (byte minLevel, byte maxLevel) GetMinMaxGB(PKM pk)
     {
-        if (pk.Format <= 3)
-        {
-            if (pk.IsEgg)
-                return 5;
-            return Math.Max(2, pk.Met_Level);
-        }
-        if (!hasMet)
-            return 1;
-        return Math.Max(1, pk.Met_Level);
+        byte maxLevel = (byte)pk.CurrentLevel;
+        byte minLevel = GetLevelOriginMinGB(pk);
+        return (minLevel, maxLevel);
     }
 
-    private static int GetLevelOriginMax(PKM pk, bool hasMet)
+    private static byte GetLevelOriginMin(PKM pk, byte generation) => generation switch
     {
-        var met = pk.Met_Level;
-        if (hasMet)
-            return pk.CurrentLevel;
-
-        int generation = pk.Generation;
-        if (generation >= 4)
-            return met;
-
-        var downLevel = GetLevelOriginMaxTransfer(pk, pk.CurrentLevel, generation);
-        return Math.Min(met, downLevel);
-    }
-
-    private static int GetLevelOriginMaxTransfer(PKM pk, int met, int generation)
-    {
-        var species = pk.Species;
-
-        if (Future_LevelUp.TryGetValue((ushort)(species | (pk.Form << 11)), out var delta))
-            return met - delta;
-
-        if (generation < 4 && Future_LevelUp4.Contains(species) && (pk.Format <= 7 || !Future_LevelUp4_Not8.Contains(species)))
-            return met - 1;
-
-        return met;
-    }
-
-    /// <summary>
-    /// Species introduced in Generation 2 that require a level up to evolve into from a specimen that originated in a previous generation.
-    /// </summary>
-    private static readonly HashSet<ushort> Future_LevelUp2 = new()
-    {
-        (int)Crobat,
-        (int)Espeon,
-        (int)Umbreon,
-        (int)Blissey,
+        3 => GetLevelOriginMin3(pk),
+        4 => GetLevelOriginMin4(pk),
+        _ => Math.Max((byte)1, (byte)pk.Met_Level),
     };
 
-    /// <summary>
-    /// Species introduced in Generation 4 that require a level up to evolve into from a specimen that originated in a previous generation.
-    /// </summary>
-    private static readonly HashSet<ushort> Future_LevelUp4 = new()
+    private static bool IsEggLocationNonZero(PKM pk) => pk.Egg_Location != LocationEdits.GetNoneLocation(pk.Context);
+
+    private static byte GetLevelOriginMinGB(PKM pk)
     {
-        (int)Ambipom,
-        (int)Weavile,
-        (int)Magnezone,
-        (int)Lickilicky,
-        (int)Tangrowth,
-        (int)Yanmega,
-        (int)Leafeon,
-        (int)Glaceon,
-        (int)Mamoswine,
-        (int)Gliscor,
-        (int)Probopass,
-    };
+        const byte EggLevel = 5;
+        const byte MinWildLevel = 2;
+        if (pk.IsEgg)
+            return EggLevel;
+        if (pk is not ICaughtData2 { CaughtData: not 0 } pk2)
+            return MinWildLevel;
+        return (byte)pk2.Met_Level;
+    }
 
-    /// <summary>
-    /// Species introduced in Generation 4 that used to require a level up to evolve prior to Generation 8.
-    /// </summary>
-    private static readonly HashSet<ushort> Future_LevelUp4_Not8 = new()
+    private static byte GetLevelOriginMin3(PKM pk)
     {
-        (int)Magnezone, // Thunder Stone
-        (int)Leafeon, // Leaf Stone
-        (int)Glaceon, // Ice Stone
-    };
+        const byte EggLevel = 5;
+        const byte MinWildLevel = 2;
+        if (pk.Format != 3)
+            return MinWildLevel;
+        if (pk.IsEgg)
+            return EggLevel;
+        return (byte)pk.Met_Level;
+    }
 
-    /// <summary>
-    /// Species introduced in Generation 6+ that require a level up to evolve into from a specimen that originated in a previous generation.
-    /// </summary>
-    private static readonly Dictionary<ushort, byte> Future_LevelUp = new()
+    private static byte GetLevelOriginMin4(PKM pk)
     {
-        // Gen6
-        {(int)Sylveon, 1},
-
-        // Gen7
-        {(int)Marowak | (1 << 11), 1},
-
-        // Gen8
-        {(int)Weezing | (1 << 11), 1},
-        {(int)MrMime | (1 << 11), 1},
-        {(int)MrRime, 2},
-    };
+        const byte EggLevel = 1;
+        const byte MinWildLevel = 2;
+        if (pk.Format != 4)
+            return IsEggLocationNonZero(pk) ? EggLevel : MinWildLevel;
+        if (pk.IsEgg || IsEggLocationNonZero(pk))
+            return EggLevel;
+        return (byte)pk.Met_Level;
+    }
 }

PKHeX.Core/Legality/Evolutions/EvolutionChain.cs

@@ -1,7 +1,5 @@
 using System;
 
-using static PKHeX.Core.Legal;
-
 namespace PKHeX.Core;
 
 /// <summary>
@@ -11,29 +9,53 @@ public static class EvolutionChain
 {
     public static EvolutionHistory GetEvolutionChainsAllGens(PKM pk, IEncounterTemplate enc)
     {
-        var origin = new EvolutionOrigin(enc.Species, (byte)enc.Version, (byte)enc.Generation, enc.LevelMin, (byte)pk.CurrentLevel);
+        var min = GetMinLevel(pk, enc);
+        var origin = new EvolutionOrigin(pk.Species, (byte)enc.Version, (byte)enc.Generation, min, (byte)pk.CurrentLevel);
         if (!pk.IsEgg && enc is not EncounterInvalid)
-            return GetEvolutionChainsSearch(pk, origin);
+            return GetEvolutionChainsSearch(pk, origin, enc.Context, enc.Species);
 
-        var history = new EvolutionHistory();
-        var group = EvolutionGroupUtil.GetCurrentGroup(pk);
-        var chain = group.GetInitialChain(pk, origin, pk.Species, pk.Form);
-        history.Set(pk.Context, chain);
-        return history;
+        return GetEvolutionChainsSearch(pk, origin, pk.Context, enc.Species);
     }
 
-    public static EvolutionHistory GetEvolutionChainsSearch(PKM pk, EvolutionOrigin enc)
+    private static byte GetMinLevel(PKM pk, IEncounterTemplate enc) => enc.Generation switch
     {
-        var group = EvolutionGroupUtil.GetCurrentGroup(pk);
-        ReadOnlySpan<EvoCriteria> chain = group.GetInitialChain(pk, enc, pk.Species, pk.Form);
+        2 => pk is ICaughtData2 c2 ? Math.Max((byte)c2.Met_Level, enc.LevelMin) : enc.LevelMin,
+        <= 4 when pk.Format != enc.Generation => enc.LevelMin,
+        _ => Math.Max((byte)pk.Met_Level, enc.LevelMin),
+    };
 
+    public static EvolutionHistory GetEvolutionChainsSearch(PKM pk, EvolutionOrigin enc, EntityContext context, ushort encSpecies)
+    {
+        Span<EvoCriteria> chain = stackalloc EvoCriteria[EvolutionTree.MaxEvolutions];
+        return EvolutionChainsSearch(pk, enc, context, encSpecies, chain);
+    }
+
+    private static EvolutionHistory EvolutionChainsSearch(PKM pk, EvolutionOrigin enc, EntityContext context, ushort encSpecies, Span<EvoCriteria> chain)
+    {
         var history = new EvolutionHistory();
+        var length = GetOriginChain(chain, pk, enc, encSpecies, false);
+        if (length == 0)
+            return history;
+        chain = chain[..length];
+
+        // Update the chain to only include the current species, leave future evolutions as unknown
+        if (encSpecies != 0)
+            EvolutionUtil.ConditionBaseChainForward(chain, encSpecies);
+        if (context == EntityContext.Gen2)
+        {
+            EvolutionGroup2.Instance.Evolve(chain, pk, enc, history);
+            EvolutionGroup1.Instance.Evolve(chain, pk, enc, history);
+            if (pk.Format > 2)
+                context = EntityContext.Gen7;
+            else
+                return history;
+        }
+
+        var group = EvolutionGroupUtil.GetGroup(context);
         while (true)
         {
-            var any = group.Append(pk, history, ref chain, enc);
-            if (!any)
-                break;
-            var previous = group.GetPrevious(pk, enc);
+            group.Evolve(chain, pk, enc, history);
+            var previous = group.GetNext(pk, enc);
             if (previous is null)
                 break;
             group = previous;
@@ -41,18 +63,65 @@ public static class EvolutionChain
         return history;
     }
 
-    public static EvoCriteria[] GetValidPreEvolutions(PKM pk, int maxspeciesorigin = -1, int maxLevel = -1, int minLevel = 1, bool skipChecks = false)
+    public static EvoCriteria[] GetOriginChain(PKM pk, EvolutionOrigin enc, ushort encSpecies = 0, bool discard = true)
     {
-        if (maxLevel < 0)
-            maxLevel = pk.CurrentLevel;
+        Span<EvoCriteria> result = stackalloc EvoCriteria[EvolutionTree.MaxEvolutions];
+        int count = GetOriginChain(result, pk, enc, encSpecies, discard);
+        if (count == 0)
+            return Array.Empty<EvoCriteria>();
 
-        if (maxspeciesorigin == -1 && ParseSettings.AllowGen1Tradeback && pk is { Format: <= 2, Generation: 1 })
-            maxspeciesorigin = MaxSpeciesID_2;
+        var chain = result[..count];
+        return chain.ToArray();
+    }
 
-        var context = pk.Context;
-        if (context < EntityContext.Gen2)
-            context = EntityContext.Gen2;
-        var et = EvolutionTree.GetEvolutionTree(context);
-        return et.GetValidPreEvolutions(pk, levelMax: (byte)maxLevel, maxSpeciesOrigin: maxspeciesorigin, skipChecks: skipChecks, levelMin: (byte)minLevel);
+    public static int GetOriginChain(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, ushort encSpecies = 0, bool discard = true)
+    {
+        ushort species = enc.Species;
+        byte form = pk.Form;
+        if (pk.IsEgg && !enc.SkipChecks)
+        {
+            result[0] = new EvoCriteria { Species = species, Form = form, LevelMax = enc.LevelMax, LevelMin = enc.LevelMax };
+            return 1;
+        }
+
+        result[0] = new EvoCriteria { Species = species, Form = form, LevelMax = enc.LevelMax };
+        var count = DevolveFrom(result, pk, enc, pk.Context, encSpecies, discard);
+
+        var chain = result[..count];
+        EvolutionUtil.CleanDevolve(chain, enc.LevelMin);
+        return count;
+    }
+
+    private static int DevolveFrom(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EntityContext context, ushort encSpecies, bool discard)
+    {
+        var group = EvolutionGroupUtil.GetGroup(context);
+        while (true)
+        {
+            group.Devolve(result, pk, enc);
+            var previous = group.GetPrevious(pk, enc);
+            if (previous is null)
+                break;
+            group = previous;
+        }
+
+        if (discard)
+            group.DiscardForOrigin(result, pk);
+        if (encSpecies != 0)
+            return EvolutionUtil.IndexOf(result, encSpecies) + 1;
+        return GetCount(result);
+    }
+
+    private static int GetCount(Span<EvoCriteria> result)
+    {
+        // return the count of species != 0
+        int count = 0;
+        foreach (var evo in result)
+        {
+            if (evo.Species == 0)
+                break;
+            count++;
+        }
+
+        return count;
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup1.cs

@@ -2,60 +2,76 @@ using System;
 
 namespace PKHeX.Core;
 
-public sealed class EvolutionGroup1 : IEvolutionGroup
+public sealed class EvolutionGroup1 : IEvolutionGroup, IEvolutionEnvironment
 {
     public static readonly EvolutionGroup1 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves1;
-    private const int MaxSpecies = Legal.MaxSpeciesID_1;
-    private static PersonalTable1 Personal => PersonalTable.RB;
 
     public IEvolutionGroup GetNext(PKM pk, EvolutionOrigin enc) => EvolutionGroup2.Instance;
-    public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => pk.Format == 1 ? EvolutionGroup2.Instance : null;
+    public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => pk.Format == 1 && ParseSettings.AllowGen1Tradeback ? EvolutionGroup2.Instance : null;
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
+        if (!ParseSettings.AllowGen1Tradeback)
+            return; // no other groups were iterated, so no need to discard
 
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = enc.LevelMin };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen1 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
+        EvolutionUtil.Discard(result, PersonalTable.C);
     }
 
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        if (pk.Format >= 7 && !enc.SkipChecks)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
+            var max = pk.Met_Level;
+            enc = enc with { LevelMin = 2, LevelMax = (byte)max };
+        }
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
+        {
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
                 continue;
 
-            result = evo;
-            return true;
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
         }
+        return present;
+    }
 
-        result = default;
-        return false;
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (pk.Format > 2)
+            enc = enc with { LevelMax = (byte)pk.Met_Level };
+
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen1 = EvolutionUtil.SetHistory(result, PersonalTable.RB);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup2.cs

@@ -6,64 +6,69 @@ public sealed class EvolutionGroup2 : IEvolutionGroup
 {
     public static readonly EvolutionGroup2 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves2;
-    private const int MaxSpecies = Legal.MaxSpeciesID_2;
     private const int Generation = 2;
     private static PersonalTable2 Personal => PersonalTable.C;
 
     public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup7.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => pk.Format != 1 ? EvolutionGroup1.Instance : null;
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        byte min;
-        if (pk.Format > Generation)
-            min = enc.LevelMin;
-        else if (pk is ICaughtData2 { CaughtData: not 0 } c)
-            min = (byte)c.Met_Level;
-        else
-            min = enc.LevelMin;
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = min };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen2 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        if (pk.Format > Generation && !enc.SkipChecks)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
-                continue;
-
-            result = evo;
-            return true;
+            var max = pk.Met_Level;
+            EvolutionUtil.UpdateCeiling(result, max);
+            enc = enc with { LevelMin = 2, LevelMax = (byte)max };
         }
 
-        result = default;
-        return false;
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
+        {
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
+                continue;
+
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
+        }
+        return present;
+    }
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (pk.Format > Generation)
+            enc = enc with { LevelMax = (byte)pk.Met_Level };
+
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen2 = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup3.cs

@@ -6,58 +6,69 @@ public sealed class EvolutionGroup3 : IEvolutionGroup
 {
     public static readonly EvolutionGroup3 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves3;
-    private const int MaxSpecies = Legal.MaxSpeciesID_3;
     private const int Generation = 3;
     private static PersonalTable3 Personal => PersonalTable.E;
 
     public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup4.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => null;
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var min = pk.Format > Generation ? enc.LevelMin : (byte)pk.Met_Level;
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = min };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen3 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        if (pk.Format == 4 && !enc.SkipChecks) // 5+ already have been revised
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
-                continue;
-
-            result = evo;
-            return true;
+            var max = pk.Met_Level;
+            EvolutionUtil.UpdateCeiling(result, max);
+            enc = enc with { LevelMin = 1, LevelMax = (byte)max };
         }
 
-        result = default;
-        return false;
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
+        {
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
+                continue;
+
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
+        }
+        return present;
+    }
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (pk.Format > Generation)
+            enc = enc with { LevelMax = (byte)pk.Met_Level };
+
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen3 = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup4.cs

@@ -6,58 +6,71 @@ public sealed class EvolutionGroup4 : IEvolutionGroup
 {
     public static readonly EvolutionGroup4 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves4;
-    private const int MaxSpecies = Legal.MaxSpeciesID_4;
     private const int Generation = 4;
     private static PersonalTable4 Personal => PersonalTable.HGSS;
 
     public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup5.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => enc.Generation == 3 ? EvolutionGroup3.Instance : null;
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var min = pk.Format > Generation ? enc.LevelMin : (byte)pk.Met_Level;
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = min };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen4 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        if (pk.Format > Generation && !enc.SkipChecks)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
-                continue;
-
-            result = evo;
-            return true;
+            var max = pk.Met_Level;
+            EvolutionUtil.UpdateCeiling(result, max);
+            enc = enc with { LevelMin = 1, LevelMax = (byte)max };
         }
 
-        result = default;
-        return false;
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
+        {
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
+                continue;
+
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
+        }
+        return present;
+    }
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (pk.Format > Generation)
+            enc = enc with { LevelMax = (byte)pk.Met_Level };
+        else if (enc.Generation < Generation)
+            EvolutionUtil.UpdateFloor(result, pk.Met_Level);
+
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen4 = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup5.cs

@@ -6,57 +6,62 @@ public sealed class EvolutionGroup5 : IEvolutionGroup
 {
     public static readonly EvolutionGroup5 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves5;
-    private const int MaxSpecies = Legal.MaxSpeciesID_5;
     private const int Generation = 5;
     private static PersonalTable5B2W2 Personal => PersonalTable.B2W2;
 
     public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup6.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => enc.Generation < Generation ? EvolutionGroup4.Instance : null;
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = (byte)pk.Met_Level };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen5 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
                 continue;
 
-            result = evo;
-            return true;
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
         }
+        return present;
+    }
 
-        result = default;
-        return false;
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (enc.Generation < Generation)
+            EvolutionUtil.UpdateFloor(result, pk.Met_Level);
+
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen5 = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup6.cs

@@ -6,57 +6,59 @@ public sealed class EvolutionGroup6 : IEvolutionGroup
 {
     public static readonly EvolutionGroup6 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves6;
-    private const int MaxSpecies = Legal.MaxSpeciesID_6;
     private const int Generation = 6;
     private static PersonalTable6AO Personal => PersonalTable.AO;
 
     public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup7.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => enc.Generation < Generation ? EvolutionGroup5.Instance : null;
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = (byte)pk.Met_Level };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen6 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
                 continue;
 
-            result = evo;
-            return true;
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
         }
+        return present;
+    }
 
-        result = default;
-        return false;
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen6 = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup7.cs

@@ -6,64 +6,90 @@ public sealed class EvolutionGroup7 : IEvolutionGroup
 {
     public static readonly EvolutionGroup7 Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves7;
-    private const int MaxSpecies = Legal.MaxSpeciesID_7_USUM;
     private const int Generation = 7;
     private static PersonalTable7 Personal => PersonalTable.USUM;
 
-    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup8.Instance : null;
+    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroupHOME.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc)
     {
-        if (enc.Generation <= 2)
+        if (enc.Generation is 1 or 2)
             return EvolutionGroup2.Instance;
         if (enc.Generation < Generation)
             return EvolutionGroup6.Instance;
         return null;
     }
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
+
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = (byte)pk.Met_Level };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen7 = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
+            ref var prev = ref result[i - 1];
+            RevertMutatedForms(ref prev);
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
                 continue;
 
-            result = evo;
-            return true;
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
         }
-
-        result = default;
-        return false;
+        return present;
     }
+
+    private static void RevertMutatedForms(ref EvoCriteria evo)
+    {
+        // Zygarde's 10% Form and 50% Form can be changed with the help of external tools: the Reassembly Unit and the Zygarde Cube.
+        if (evo is { Species: (ushort)Species.Zygarde, Form: not (0 or 1) })
+            evo = evo with { Form = evo.LevelMax == 63 ? (byte)1 : (byte)0 }; // 50% Forme
+        else if (evo is { Species: (ushort)Species.Silvally, Form: not 0 })
+            evo = evo with { Form = 0 }; // Normal
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (enc.Generation <= 2) // VC Transfer
+            EvolutionUtil.UpdateFloor(result, pk.Met_Level);
+
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen7 = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        var b = Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
+        return b && !IsEvolutionBanned(pk, result);
+    }
+
+    // Kanto Evolutions are not accessible unless it visits US/UM.
+    private static bool IsEvolutionBanned(PKM pk, in ISpeciesForm dest) => pk is PK7 { SM: true, IsUntraded: true } && dest switch
+    {
+        { Species: (ushort)Species.Raichu, Form: 0 } => true,
+        { Species: (ushort)Species.Marowak, Form: 0 } => true,
+        { Species: (ushort)Species.Exeggutor, Form: 0 } => true,
+        _ => false,
+    };
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup7b.cs

@@ -6,57 +6,59 @@ public sealed class EvolutionGroup7b : IEvolutionGroup
 {
     public static readonly EvolutionGroup7b Instance = new();
     private static readonly EvolutionTree Tree = EvolutionTree.Evolves7b;
-    private const int MaxSpecies = Legal.MaxSpeciesID_7b;
     private const int Generation = 7;
     private static PersonalTable7GG Personal => PersonalTable.GG;
 
-    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroup8.Instance : null;
+    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => pk.Format > Generation ? EvolutionGroupHOME.Instance : null;
     public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc) => null;
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk) => EvolutionUtil.Discard(result, Personal);
 
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
     {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = (byte)pk.Met_Level };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        history.Gen7b = revised;
-
-        // Retain a reference to the current chain for future appending as we step backwards.
-        chain = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        return Tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private static bool GetFirstEvolution(ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result)
-    {
-        var pt = Personal;
-        foreach (var evo in chain)
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
         {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
+            var prev = result[i - 1];
+            if (!TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
                 continue;
 
-            result = evo;
-            return true;
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
         }
+        return present;
+    }
 
-        result = default;
-        return false;
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+        history.Gen7b = EvolutionUtil.SetHistory(result, Personal);
+        return present;
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup8.cs

@@ -1,232 +0,0 @@
-using System;
-using static PKHeX.Core.GameVersion;
-
-namespace PKHeX.Core;
-
-public sealed class EvolutionGroup8 : IEvolutionGroup
-{
-    public static readonly EvolutionGroup8 Instance = new();
-    private static readonly EvolutionTree Tree8  = EvolutionTree.Evolves8;
-    private static readonly EvolutionTree Tree8a = EvolutionTree.Evolves8a;
-    private static readonly EvolutionTree Tree8b = EvolutionTree.Evolves8b;
-    private const int MaxSpecies = Legal.MaxSpeciesID_8a;
-    private const int Generation = 8;
-
-    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => EvolutionGroup9.Instance;
-    public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc)
-    {
-        if ((GameVersion)enc.Version is GP or GE or GG or GO)
-            return EvolutionGroup7b.Instance;
-        if (enc.Generation >= Generation)
-            return null;
-        return EvolutionGroup7.Instance;
-    }
-
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
-    {
-        if (chain.Length == 0)
-            return false;
-
-        var swsh = Append(pk, chain, enc, PersonalTable.SWSH, Tree8 , ref history.Gen8 );
-        var pla  = Append(pk, chain, enc, PersonalTable.LA  , Tree8a, ref history.Gen8a);
-        var bdsp = Append(pk, chain, enc, PersonalTable.BDSP, Tree8b, ref history.Gen8b);
-
-        if (!(swsh || pla || bdsp))
-            return false;
-
-        // Block BD/SP transfers that are impossible
-        BlockBDSP(history, enc);
-
-        if (!pk.IsUntraded && !(ParseSettings.IgnoreTransferIfNoTracker && pk is IHomeTrack { HasTracker: false }))
-        {
-            CrossPropagate(history);
-        }
-        else
-        {
-            DeleteAdjacent(pk, history);
-
-            if (!HasVisited(history))
-                return false;
-        }
-
-        chain = GetMaxChain(history);
-
-        return chain.Length != 0;
-    }
-
-    private static bool HasVisited(EvolutionHistory history)
-    {
-        return history.Gen8.Length != 0 || history.Gen8a.Length != 0 || history.Gen8b.Length != 0;
-    }
-
-    private static void DeleteAdjacent(PKM pk, EvolutionHistory history)
-    {
-        if (pk is not PK8)
-            history.Gen8 = Array.Empty<EvoCriteria>();
-        if (pk is not PA8)
-            history.Gen8a = Array.Empty<EvoCriteria>();
-        if (pk is not PB8)
-            history.Gen8b = Array.Empty<EvoCriteria>();
-    }
-
-    private static void BlockBDSP(EvolutionHistory history, EvolutionOrigin enc)
-    {
-        var bdsp = history.Gen8b;
-        if (bdsp.Length == 0)
-            return;
-
-        // Spinda and Nincada cannot transfer in or out as the current species.
-        // Remove them from their non-origin game evolution chains.
-        var last = bdsp[^1];
-        if (last.Species == (int)Species.Nincada)
-            RemoveIfSpecies(history, enc);
-        else if (last.Species == (int)Species.Spinda)
-            RemoveIfSpecies(history, enc);
-
-        static void RemoveIfSpecies(EvolutionHistory history, EvolutionOrigin enc)
-        {
-            var wasBDSP = BDSP.Contains(enc.Version);
-            ref var evos = ref wasBDSP ? ref history.Gen8 : ref history.Gen8b;
-            evos = evos.Length < 2 ? Array.Empty<EvoCriteria>() : evos.AsSpan(0, evos.Length - 1).ToArray();
-        }
-    }
-
-    private static ReadOnlySpan<EvoCriteria> GetMaxChain(EvolutionHistory history)
-    {
-        var arr0 = history.Gen8;
-        var arr1 = history.Gen8a;
-        var arr2 = history.Gen8b;
-        if (arr0.Length >= arr1.Length && arr0.Length >= arr2.Length)
-            return arr0;
-        if (arr1.Length >= arr2.Length)
-            return arr1;
-        return arr2;
-    }
-
-    private static void CrossPropagate(EvolutionHistory history)
-    {
-        var arr0 = history.Gen8;
-        var arr1 = history.Gen8a;
-        var arr2 = history.Gen8b;
-
-        ReplaceIfBetter(arr0, arr1, arr2);
-        ReplaceIfBetter(arr1, arr0, arr2);
-        ReplaceIfBetter(arr2, arr0, arr1);
-    }
-
-    private static void ReplaceIfBetter(Span<EvoCriteria> local, ReadOnlySpan<EvoCriteria> other1, ReadOnlySpan<EvoCriteria> other2)
-    {
-        for (int i = 0; i < local.Length; i++)
-        {
-            ReplaceIfBetter(local, other1, i);
-            ReplaceIfBetter(local, other2, i);
-        }
-    }
-
-    private static void ReplaceIfBetter(Span<EvoCriteria> local, ReadOnlySpan<EvoCriteria> other, int parentIndex)
-    {
-        // Replace the evolution entry if another connected game has a better evolution method (different min/max).
-        var native = local[parentIndex];
-
-        // Check if the evo is in the other game; if not, we're done here.
-        var index = IndexOfSpecies(other, native.Species);
-        if (index == -1)
-            return;
-
-        var alt = other[index];
-        if (alt.LevelMin < native.LevelMin || alt.LevelMax > native.LevelMax)
-            local[parentIndex] = alt;
-    }
-
-    private static int IndexOfSpecies(ReadOnlySpan<EvoCriteria> evos, ushort species)
-    {
-        // Returns the index of the first evo that matches the species
-        for (int i = 0; i < evos.Length; i++)
-        {
-            if (evos[i].Species == species)
-                return i;
-        }
-        return -1;
-    }
-
-    private static bool Append<T>(PKM pk, ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc, T pt, EvolutionTree tree, ref EvoCriteria[] dest) where T : IPersonalTable
-    {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(pt, chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = (byte)pk.Met_Level };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form, tree);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        dest = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        if (!GetPreferredGroup(species, form, out var group))
-            return Array.Empty<EvoCriteria>();
-        var tree = GetTree(group);
-        return GetInitialChain(pk, enc, species, form, tree);
-    }
-
-    private static EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form, EvolutionTree tree)
-    {
-        if (species is (int)Species.Dialga or (int)Species.Palkia or (int)Species.Arceus)
-            form = 0; // PLA forms; play nice for yielding SW/SH context
-        return tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvolutionTree GetTree(PreferredGroup group) => group switch
-    {
-        PreferredGroup.BDSP => Tree8b,
-        PreferredGroup.LA => Tree8a,
-        _ => Tree8,
-    };
-
-    private static bool GetPreferredGroup(ushort species, byte form, out PreferredGroup result)
-    {
-        if (PersonalTable.LA.IsPresentInGame(species, form))
-            result = PreferredGroup.LA;
-        else if (PersonalTable.SWSH.IsPresentInGame(species, form))
-            result = PreferredGroup.SWSH;
-        else if (PersonalTable.BDSP.IsPresentInGame(species, form))
-            result = PreferredGroup.BDSP;
-        else
-            result = PreferredGroup.None;
-        return result != 0;
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private enum PreferredGroup
-    {
-        None,
-        LA,
-        SWSH,
-        BDSP,
-    }
-
-    private static bool GetFirstEvolution<T>(T pt, ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result) where T : IPersonalTable
-    {
-        foreach (var evo in chain)
-        {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
-                continue;
-
-            result = evo;
-            return true;
-        }
-
-        result = default;
-        return false;
-    }
-}

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroup9.cs

@@ -1,98 +0,0 @@
-using System;
-
-namespace PKHeX.Core;
-
-public sealed class EvolutionGroup9 : IEvolutionGroup
-{
-    public static readonly EvolutionGroup9 Instance = new();
-    private static readonly EvolutionTree Tree9  = EvolutionTree.Evolves9;
-    private const int MaxSpecies = Legal.MaxSpeciesID_9;
-
-    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => null;
-    public IEvolutionGroup GetPrevious(PKM pk, EvolutionOrigin enc) => EvolutionGroup8.Instance;
-
-    public bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc)
-    {
-        if (chain.Length == 0)
-            return false;
-
-        var sv = Append(pk, chain, enc, PersonalTable.SV, Tree9, ref history.Gen9);
-
-        if (!sv)
-            return false;
-
-        chain = history.Gen9;
-        return chain.Length != 0;
-    }
-
-    private static bool Append<T>(PKM pk, ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc, T pt, EvolutionTree tree, ref EvoCriteria[] dest) where T : IPersonalTable
-    {
-        // Get the first evolution in the chain that can be present in this group
-        var any = GetFirstEvolution(pt, chain, out var evo);
-        if (!any)
-            return false;
-
-        // Get the evolution tree from this group and get the new chain from it.
-        var criteria = enc with { LevelMax = evo.LevelMax, LevelMin = (byte)pk.Met_Level };
-        var local = GetInitialChain(pk, criteria, evo.Species, evo.Form, tree);
-
-        // Revise the tree
-        var revised = Prune(local);
-
-        // Set the tree to the history field
-        dest = revised;
-
-        return revised.Length != 0;
-    }
-
-    public EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form)
-    {
-        if (!GetPreferredGroup(species, form, out var group))
-            return Array.Empty<EvoCriteria>();
-        var tree = GetTree(group);
-        return GetInitialChain(pk, enc, species, form, tree);
-    }
-
-    private static EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form, EvolutionTree tree)
-    {
-        return tree.GetExplicitLineage(species, form, pk, enc.LevelMin, enc.LevelMax, MaxSpecies, enc.SkipChecks, enc.Species);
-    }
-
-    private static EvolutionTree GetTree(PreferredGroup group) => group switch
-    {
-        _ => Tree9,
-    };
-
-    private static bool GetPreferredGroup(ushort species, byte form, out PreferredGroup result)
-    {
-        if (PersonalTable.SV.IsPresentInGame(species, form))
-            result = PreferredGroup.SV;
-        else
-            result = PreferredGroup.None;
-        return result != 0;
-    }
-
-    private static EvoCriteria[] Prune(EvoCriteria[] chain) => chain;
-
-    private enum PreferredGroup
-    {
-        None,
-        SV,
-    }
-
-    private static bool GetFirstEvolution<T>(T pt, ReadOnlySpan<EvoCriteria> chain, out EvoCriteria result) where T : IPersonalTable
-    {
-        foreach (var evo in chain)
-        {
-            // If the evo can't exist in the game, it must be a future evolution.
-            if (!pt.IsPresentInGame(evo.Species, evo.Form))
-                continue;
-
-            result = evo;
-            return true;
-        }
-
-        result = default;
-        return false;
-    }
-}

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroupHOME.cs

@@ -0,0 +1,258 @@
+using System;
+using System.Runtime.CompilerServices;
+using static PKHeX.Core.GameVersion;
+using static PKHeX.Core.EvolutionUtil;
+
+namespace PKHeX.Core;
+
+public sealed class EvolutionGroupHOME : IEvolutionGroup
+{
+    public static readonly EvolutionGroupHOME Instance = new();
+
+    private static readonly EvolutionEnvironment8 SWSH = new();
+    private static readonly EvolutionEnvironment8a LA = new();
+    private static readonly EvolutionEnvironment8b BDSP = new();
+    private static readonly EvolutionEnvironment9 SV = new();
+
+    public IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc) => null;
+
+    public IEvolutionGroup? GetPrevious(PKM pk, EvolutionOrigin enc)
+    {
+        if (enc.Generation >= 8)
+            return null;
+        if ((GameVersion)enc.Version is GP or GE or GG or GO)
+            return EvolutionGroup7b.Instance;
+        return EvolutionGroup7.Instance;
+    }
+
+    public void DiscardForOrigin(Span<EvoCriteria> result, PKM pk)
+    {
+        if (pk.SV)
+            Discard(result, PersonalTable.SV);
+        else if (pk.LA)
+            Discard(result, PersonalTable.LA);
+        else if (pk.BDSP)
+            Discard(result, PersonalTable.BDSP);
+        else if (pk.SWSH)
+            Discard(result, PersonalTable.SWSH);
+        // GO can be otherwise, don't discard any.
+    }
+
+    /// <summary>
+    /// Checks if we should check all adjacent evolution sources in addition to the current one.
+    /// </summary>
+    /// <returns>True if we should check all adjacent evolution sources.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static bool CheckAllAdjacent(PKM pk, EvolutionOrigin enc) => enc.SkipChecks || pk is IHomeTrack { HasTracker: true } || !ParseSettings.IgnoreTransferIfNoTracker;
+
+    public int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc)
+    {
+        if (CheckAllAdjacent(pk, enc))
+            return DevolveMulti(result, pk, enc);
+        return DevolveSingle(result, pk, enc);
+    }
+
+    public int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history)
+    {
+        if (CheckAllAdjacent(pk, enc))
+            return EvolveMulti(result, pk, enc, history);
+        return EvolveSingle(result, pk, enc, history);
+    }
+
+    private int DevolveMulti(Span<EvoCriteria> result, PKM pk, in EvolutionOrigin enc)
+    {
+        int present = 1;
+        for (int i = 1; i < result.Length; i++)
+        {
+            ref var prev = ref result[i - 1];
+            RevertMutatedForms(ref prev);
+            ref var reference = ref result[i];
+
+            bool devolvedAny = false;
+            if (SWSH.TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
+                devolvedAny = UpdateIfBetter(ref reference, evo);
+            if (LA  .TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out evo))
+                devolvedAny = UpdateIfBetter(ref reference, evo);
+            if (BDSP.TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out evo))
+                devolvedAny = UpdateIfBetter(ref reference, evo);
+            if (SV  .TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out evo))
+                devolvedAny = UpdateIfBetter(ref reference, evo);
+
+            if (devolvedAny)
+                present++;
+
+            static bool UpdateIfBetter(ref EvoCriteria reference, in EvoCriteria evo)
+            {
+                if (evo.IsBetterDevolution(reference))
+                    reference = evo;
+                return true;
+            }
+        }
+
+        return present;
+    }
+
+    private int EvolveMulti(Span<EvoCriteria> result, PKM pk, in EvolutionOrigin enc, EvolutionHistory history)
+    {
+        int present = 1;
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+
+            bool devolvedAny = false;
+            if (SWSH.TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+                devolvedAny = UpdateIfBetter(ref dest, evo);
+            if (LA  .TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out evo))
+                devolvedAny = UpdateIfBetter(ref dest, evo);
+            if (BDSP.TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out evo))
+                devolvedAny = UpdateIfBetter(ref dest, evo);
+            if (SV  .TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out evo))
+                devolvedAny = UpdateIfBetter(ref dest, evo);
+
+            if (devolvedAny)
+                present++;
+            else if (dest.Method == EvoCriteria.SentinelNotReached)
+                break; // Don't continue for higher evolutions.
+
+            static bool UpdateIfBetter(ref EvoCriteria reference, in EvoCriteria evo)
+            {
+                if (evo.IsBetterEvolution(reference))
+                    reference = evo;
+                return true;
+            }
+        }
+
+        history.Gen8  = SetHistory(result, PersonalTable.SWSH);
+        history.Gen8a = SetHistory(result, PersonalTable.LA);
+        history.Gen8b = SetHistory(result, PersonalTable.BDSP);
+        history.Gen9  = SetHistory(result, PersonalTable.SV);
+
+        return present;
+    }
+
+    private static int DevolveSingle(Span<EvoCriteria> result, PKM pk, in EvolutionOrigin enc)
+    {
+        int present = 1;
+        var env = GetSingleEnv(pk);
+        for (int i = 1; i < result.Length; i++)
+        {
+            ref var prev = ref result[i - 1];
+            RevertMutatedForms(ref prev);
+            if (!env.TryDevolve(prev, pk, prev.LevelMax, enc.LevelMin, enc.SkipChecks, out var evo))
+                continue;
+
+            ref var reference = ref result[i];
+            if (evo.IsBetterDevolution(reference))
+                reference = evo;
+            present++;
+        }
+
+        return present;
+    }
+
+    private static void RevertMutatedForms(ref EvoCriteria evo)
+    {
+        if (evo is { Species: (ushort)Species.Dialga, Form: not 0 })
+            evo = evo with { Form = 0 }; // Normal
+        else if (evo is { Species: (ushort)Species.Palkia, Form: not 0 })
+            evo = evo with { Form = 0 }; // Normal
+        else if (evo is { Species: (ushort)Species.Silvally, Form: not 0 })
+            evo = evo with { Form = 0 }; // Normal
+    }
+
+    private int EvolveSingle(Span<EvoCriteria> result, PKM pk, in EvolutionOrigin enc, EvolutionHistory history)
+    {
+        int present = 1;
+        var env = GetSingleEnv(pk);
+        for (int i = result.Length - 1; i >= 1; i--)
+        {
+            ref var dest = ref result[i - 1];
+            var devolved = result[i];
+            if (!env.TryEvolve(devolved, dest, pk, enc.LevelMax, devolved.LevelMin, enc.SkipChecks, out var evo))
+            {
+                if (dest.Method == EvoCriteria.SentinelNotReached)
+                    break; // Don't continue for higher evolutions.
+                continue;
+            }
+
+            if (evo.IsBetterEvolution(dest))
+                dest = evo;
+            present++;
+        }
+
+        if      (pk is PK8) history.Gen8  = SetHistory(result, PersonalTable.SWSH);
+        else if (pk is PA8) history.Gen8a = SetHistory(result, PersonalTable.LA);
+        else if (pk is PB8) history.Gen8b = SetHistory(result, PersonalTable.BDSP);
+        else if (pk is PK9) history.Gen9  = SetHistory(result, PersonalTable.SV);
+
+        return present;
+    }
+
+    private static IEvolutionEnvironment GetSingleEnv(PKM pk) => pk switch
+    {
+        PK8 => SWSH,
+        PA8 => LA,
+        PB8 => BDSP,
+        PK9 => SV,
+        _ => throw new ArgumentOutOfRangeException(nameof(pk), pk, null),
+    };
+}
+
+public sealed class EvolutionEnvironment8 : IEvolutionEnvironment
+{
+    private static readonly EvolutionTree Tree = EvolutionTree.Evolves8;
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        return Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        var b = Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
+        return b && !IsEvolutionBanned(pk, head);
+    }
+
+    // Gigantamax Pikachu, Meowth, and Eevee are prevented from evolving.
+    private static bool IsEvolutionBanned(PKM pk, in ISpeciesForm head) => head.Species switch
+    {
+        (int)Species.Pikachu => pk is PK8 { CanGigantamax: true },
+        (int)Species.Meowth => pk is PK8 { CanGigantamax: true },
+        (int)Species.Eevee => pk is PK8 { CanGigantamax: true },
+        _ => false,
+    };
+}
+
+public sealed class EvolutionEnvironment8a : IEvolutionEnvironment
+{
+    private static readonly EvolutionTree Tree = EvolutionTree.Evolves8a;
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+        => Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+        => Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
+}
+
+public sealed class EvolutionEnvironment8b : IEvolutionEnvironment
+{
+    private static readonly EvolutionTree Tree = EvolutionTree.Evolves8b;
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+        => Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+        => Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
+}
+
+public sealed class EvolutionEnvironment9 : IEvolutionEnvironment
+{
+    private static readonly EvolutionTree Tree = EvolutionTree.Evolves9;
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+        => Tree.Reverse.TryDevolve(head, pk, currentMaxLevel, levelMin, skipChecks, out result);
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+        => Tree.Forward.TryEvolve(head, next, pk, currentMaxLevel, levelMin, skipChecks, out result);
+}

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionGroupUtil.cs

@@ -1,4 +1,3 @@
-using System;
 using static PKHeX.Core.EntityContext;
 
 namespace PKHeX.Core;
@@ -8,15 +7,10 @@ namespace PKHeX.Core;
 /// </summary>
 public static class EvolutionGroupUtil
 {
-    /// <summary>
-    /// Gets the <see cref="IEvolutionGroup"/> for the <see cref="pk"/>.
-    /// </summary>
-    public static IEvolutionGroup GetCurrentGroup(PKM pk) => GetCurrentGroup(pk.Context);
-
     /// <summary>
     /// Gets the <see cref="IEvolutionGroup"/> for the <see cref="EntityContext"/>.
     /// </summary>
-    public static IEvolutionGroup GetCurrentGroup(EntityContext context) => context switch
+    public static IEvolutionGroup GetGroup(EntityContext context) => context switch
     {
         Gen1 => EvolutionGroup1.Instance,
         Gen2 => EvolutionGroup2.Instance,
@@ -25,13 +19,8 @@ public static class EvolutionGroupUtil
         Gen5 => EvolutionGroup5.Instance,
         Gen6 => EvolutionGroup6.Instance,
         Gen7 => EvolutionGroup7.Instance,
-        Gen8 => EvolutionGroup8.Instance,
-        Gen9 => EvolutionGroup9.Instance,
-
         Gen7b => EvolutionGroup7b.Instance,
-        Gen8a => EvolutionGroup8.Instance,
-        Gen8b => EvolutionGroup8.Instance,
 
-        _ => throw new ArgumentOutOfRangeException(nameof(context), context, null),
+        _ => EvolutionGroupHOME.Instance,
     };
 }

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionOrigin.cs

@@ -0,0 +1,12 @@
+namespace PKHeX.Core;
+
+/// <summary>
+/// Details about the original encounter.
+/// </summary>
+/// <param name="Species">Species the encounter originated as</param>
+/// <param name="Version">Version the encounter originated on</param>
+/// <param name="Generation">Generation the encounter originated in</param>
+/// <param name="LevelMin">Minimum level the encounter originated at</param>
+/// <param name="LevelMax">Maximum level in final state</param>
+/// <param name="SkipChecks">Skip enforcement of legality for evolution criteria</param>
+public readonly record struct EvolutionOrigin(ushort Species, byte Version, byte Generation, byte LevelMin, byte LevelMax, bool SkipChecks = false);

PKHeX.Core/Legality/Evolutions/EvolutionGroup/EvolutionUtil.cs

@@ -0,0 +1,172 @@
+using System;
+
+namespace PKHeX.Core;
+
+internal static class EvolutionUtil
+{
+    public static EvoCriteria[] SetHistory<T>(Span<EvoCriteria> result, T pt) where T : IPersonalTable
+    {
+        // Get the range of {result} that is present within pt.
+        int start = 0;
+        int count = 0;
+
+        // Find first index that exists in the game, and take while they do.
+        foreach (ref readonly var evo in result)
+        {
+            if (evo.Method == EvoCriteria.SentinelNotReached)
+            {
+                // Unable to evolve to this stage, skip everything before it
+                start += count + 1;
+                count = 0;
+            }
+            else if (pt.IsPresentInGame(evo.Species, evo.Form))
+            {
+                // Found a valid entry, increment count.
+                count++;
+            }
+            else if (count == 0)
+            {
+                // No valid entries found yet, increment start.
+                start++;
+            }
+            else
+            {
+                // Found an invalid entry, stop.
+                break;
+            }
+        }
+
+        return GetLocalEvolutionArray(result.Slice(start, count));
+    }
+
+    private static EvoCriteria[] GetLocalEvolutionArray(Span<EvoCriteria> result)
+    {
+        if (result.Length == 0)
+            return Array.Empty<EvoCriteria>();
+
+        var array = result.ToArray();
+        var length = CleanEvolve(array);
+        if (length != array.Length)
+            Array.Resize(ref array, length);
+        return array;
+    }
+
+    public static void Discard<T>(Span<EvoCriteria> result, T pt) where T : IPersonalTable
+    {
+        // Iterate through the result, and if an entry is not present in the game, shift other entries down and zero out the last entry.
+        for (int i = 0; i < result.Length; i++)
+        {
+            var evo = result[i];
+            if (evo.Species == 0)
+                break;
+            if (pt.IsPresentInGame(evo.Species, evo.Form))
+                continue;
+
+            ShiftDown(result[i..]);
+        }
+    }
+
+    private static void ShiftDown(Span<EvoCriteria> result)
+    {
+        for (int i = 1; i < result.Length; i++)
+            result[i - 1] = result[i];
+        result[^1] = default; // zero out the last entry
+    }
+
+    public static int IndexOf(Span<EvoCriteria> result, ushort species)
+    {
+        for (int i = 0; i < result.Length; i++)
+        {
+            if (result[i].Species == species)
+                return i;
+        }
+        return -1;
+    }
+
+    public static bool Contains(ReadOnlySpan<EvoCriteria> result, ushort species)
+    {
+        foreach (ref readonly var z in result)
+        {
+            if (z.Species == species)
+                return true;
+        }
+        return false;
+    }
+
+    /// <summary>
+    /// Revises the <see cref="result"/> to account for a new maximum <see cref="level"/>.
+    /// </summary>
+    public static void UpdateCeiling(Span<EvoCriteria> result, int level)
+    {
+        foreach (ref var evo in result)
+        {
+            if (evo.Species == 0)
+                break;
+            evo = evo with { LevelMax = (byte)Math.Min(evo.LevelMax, level) };
+        }
+    }
+
+    /// <summary>
+    /// Revises the <see cref="result"/> to account for a new minimum <see cref="level"/>.
+    /// </summary>
+    public static void UpdateFloor(Span<EvoCriteria> result, int level)
+    {
+        foreach (ref var evo in result)
+        {
+            if (evo.Species == 0)
+                break;
+            evo = evo with { LevelMin = (byte)Math.Max(evo.LevelMin, level) };
+        }
+    }
+
+    /// <summary>
+    /// Mutates the result to leave placeholder data for the species that the <see cref="encSpecies"/> evolves into.
+    /// </summary>
+    /// <param name="result">All evolutions for the species.</param>
+    /// <param name="encSpecies">Species encountered as.</param>
+    public static void ConditionBaseChainForward(Span<EvoCriteria> result, ushort encSpecies)
+    {
+        foreach (ref var evo in result)
+        {
+            if (evo.Species == encSpecies)
+                break;
+            evo = evo with { Method = EvoCriteria.SentinelNotReached };
+        }
+    }
+
+    public static void CleanDevolve(Span<EvoCriteria> result, byte levelMin)
+    {
+        // Rectify minimum levels.
+        // trickle our two temp variables up the chain (essentially evolving from min).
+        byte req = 0;
+        EvolutionType method = EvolutionType.None;
+        for (int i = result.Length - 1; i >= 0; i--)
+        {
+            ref var evo = ref result[i];
+            var nextMin = evo.LevelMin; // to evolve
+            var nextReq = evo.LevelUpRequired;
+            var nextMethod = evo.Method;
+            evo = evo with { LevelMin = Math.Min(evo.LevelMax, levelMin), LevelUpRequired = req, Method = method };
+            levelMin = Math.Max(nextMin, levelMin);
+            req = nextReq;
+            method = nextMethod;
+        }
+    }
+
+    private static int CleanEvolve(Span<EvoCriteria> result)
+    {
+        // Rectify maximum levels.
+        int i = 1;
+        for (; i < result.Length; i++)
+        {
+            var next = result[i - 1];
+            // Ignore LevelUp byte as it can learn moves prior to evolving.
+            var newMax = next.LevelMax;
+            ref var evo = ref result[i];
+            if (evo.LevelMin > newMax - next.LevelUpRequired)
+                break;
+            evo = evo with { LevelMax = newMax };
+        }
+        return i;
+    }
+}

PKHeX.Core/Legality/Evolutions/EvolutionGroup/IEvolutionGroup.cs

@@ -17,18 +17,45 @@ public interface IEvolutionGroup
     /// </summary>
     IEvolutionGroup? GetNext(PKM pk, EvolutionOrigin enc);
 
-    bool Append(PKM pk, EvolutionHistory history, ref ReadOnlySpan<EvoCriteria> chain, EvolutionOrigin enc);
+    /// <summary>
+    /// Walks down the evolution chain to find previous evolutions.
+    /// </summary>
+    /// <param name="result">Array to store results in.</param>
+    /// <param name="pk">PKM to check.</param>
+    /// <param name="enc">Cached metadata about the PKM.</param>
+    /// <returns>Number of results found.</returns>
+    int Devolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc);
 
-    EvoCriteria[] GetInitialChain(PKM pk, EvolutionOrigin enc, ushort species, byte form);
+    /// <summary>
+    /// Walks up the evolution chain to find the evolution path.
+    /// </summary>
+    /// <param name="result">Array to store best results in.</param>
+    /// <param name="pk">PKM to check.</param>
+    /// <param name="enc">Cached metadata about the PKM.</param>
+    /// <param name="history">History of evolutions to cache arrays for individual contexts.</param>
+    /// <returns>Number of results found.</returns>
+    int Evolve(Span<EvoCriteria> result, PKM pk, EvolutionOrigin enc, EvolutionHistory history);
+
+    /// <summary>
+    /// Discards all entries that do not exist in the group.
+    /// </summary>
+    void DiscardForOrigin(Span<EvoCriteria> result, PKM pk);
 }
 
 /// <summary>
-/// Details about the original encounter.
+/// Provides information about how to evolve to the next evolution stage.
 /// </summary>
-/// <param name="Species">Species the encounter originated as</param>
-/// <param name="Version">Version the encounter originated on</param>
-/// <param name="Generation">Generation the encounter originated in</param>
-/// <param name="LevelMin">Minimum level the encounter originated at</param>
-/// <param name="LevelMax">Maximum level in final state</param>
-/// <param name="SkipChecks">Skip enforcement of legality for evolution criteria</param>
-public readonly record struct EvolutionOrigin(ushort Species, byte Version, byte Generation, byte LevelMin, byte LevelMax, bool SkipChecks = false);
+public interface IEvolutionEnvironment
+{
+    /// <summary>
+    /// Attempts to devolve the provided <see cref="head"/> to the previous evolution.
+    /// </summary>
+    /// <returns>True if the de-evolution was successful and <see cref="result"/> should be used.</returns>
+    bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result);
+
+    /// <summary>
+    /// Attempts to evolve the provided <see cref="head"/> to the next evolution.
+    /// </summary>
+    /// <returns>True if the evolution was successful and <see cref="result"/> should be used.</returns>
+    bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result);
+}

PKHeX.Core/Legality/Evolutions/EvolutionHistory.cs

@@ -37,28 +37,26 @@ public sealed class EvolutionHistory
     public bool HasVisitedPLA => Gen8a.Length != 0;
     public bool HasVisitedBDSP => Gen8b.Length != 0;
 
-    public ref EvoCriteria[] Get(EntityContext context)
+    public ReadOnlySpan<EvoCriteria> Get(EntityContext context) => context switch
     {
-        if (context == EntityContext.Gen1) return ref Gen1;
-        if (context == EntityContext.Gen2) return ref Gen2;
-        if (context == EntityContext.Gen3) return ref Gen3;
-        if (context == EntityContext.Gen4) return ref Gen4;
-        if (context == EntityContext.Gen5) return ref Gen5;
-        if (context == EntityContext.Gen6) return ref Gen6;
-        if (context == EntityContext.Gen7) return ref Gen7;
-        if (context == EntityContext.Gen8) return ref Gen8;
-        if (context == EntityContext.Gen9) return ref Gen9;
+        EntityContext.Gen1 => Gen1,
+        EntityContext.Gen2 => Gen2,
+        EntityContext.Gen3 => Gen3,
+        EntityContext.Gen4 => Gen4,
+        EntityContext.Gen5 => Gen5,
+        EntityContext.Gen6 => Gen6,
+        EntityContext.Gen7 => Gen7,
+        EntityContext.Gen8 => Gen8,
+        EntityContext.Gen9 => Gen9,
 
-        if (context == EntityContext.Gen7b) return ref Gen7b;
-        if (context == EntityContext.Gen8a) return ref Gen8a;
-        if (context == EntityContext.Gen8b) return ref Gen8b;
+        EntityContext.Gen7b => Gen7b,
+        EntityContext.Gen8a => Gen8a,
+        EntityContext.Gen8b => Gen8b,
+        _ => throw new ArgumentOutOfRangeException(nameof(context), context, null),
+    };
 
-        throw new ArgumentOutOfRangeException(nameof(context));
-    }
-
-    public bool HasVisited(EntityContext context, ushort species)
+    public static bool HasVisited(ReadOnlySpan<EvoCriteria> evos, ushort species)
     {
-        var evos = Get(context);
         foreach (var evo in evos)
         {
             if (evo.Species == species)
@@ -66,10 +64,4 @@ public sealed class EvolutionHistory
         }
         return false;
     }
-
-    public void Set(EntityContext context, EvoCriteria[] chain)
-    {
-        ref var arr = ref Get(context);
-        arr = chain;
-    }
 }

PKHeX.Core/Legality/Evolutions/EvolutionLegality.cs

@@ -1,35 +0,0 @@
-using System.Collections.Generic;
-
-namespace PKHeX.Core;
-
-internal static class EvolutionLegality
-{
-    internal static readonly HashSet<ushort> FutureEvolutionsGen1 = new()
-    {
-        (int)Species.Crobat,
-        (int)Species.Bellossom,
-        (int)Species.Politoed,
-        (int)Species.Espeon,
-        (int)Species.Umbreon,
-        (int)Species.Slowking,
-        (int)Species.Steelix,
-        (int)Species.Scizor,
-        (int)Species.Kingdra,
-        (int)Species.Porygon2,
-        (int)Species.Blissey,
-
-        (int)Species.Magnezone,
-        (int)Species.Lickilicky,
-        (int)Species.Rhyperior,
-        (int)Species.Tangrowth,
-        (int)Species.Electivire,
-        (int)Species.Magmortar,
-        (int)Species.Leafeon,
-        (int)Species.Glaceon,
-        (int)Species.PorygonZ,
-
-        (int)Species.Sylveon,
-
-        (int)Species.Kleavor,
-    };
-}

PKHeX.Core/Legality/Evolutions/EvolutionReversal.cs

@@ -1,160 +0,0 @@
-using System;
-
-namespace PKHeX.Core;
-
-/// <summary>
-/// Evolution Reversal logic
-/// </summary>
-public static class EvolutionReversal
-{
-    /// <summary>
-    /// Reverses from current state to see what evolutions the <see cref="pk"/> may have existed as.
-    /// </summary>
-    /// <param name="lineage">Evolution Method lineage reversal object</param>
-    /// <param name="species">Species to devolve from</param>
-    /// <param name="form">Form to devolve from</param>
-    /// <param name="pk">Entity reference to sanity check evolutions with</param>
-    /// <param name="levelMin">Minimum level the entity may exist as</param>
-    /// <param name="levelMax">Maximum the entity may exist as</param>
-    /// <param name="maxSpeciesID">Maximum species ID that may exist</param>
-    /// <param name="skipChecks">Option to bypass some evolution criteria</param>
-    /// <param name="stopSpecies">Species ID that should be the last node, if at all.</param>
-    /// <returns>Reversed evolution lineage, with the lowest index being the current species-form.</returns>
-    public static EvoCriteria[] Reverse(this IEvolutionLookup lineage, ushort species, byte form, PKM pk, byte levelMin, byte levelMax, int maxSpeciesID, bool skipChecks, int stopSpecies)
-    {
-        // Sometimes we have to sanitize the inputs.
-        switch (species)
-        {
-            case (int)Species.Silvally:
-                form = 0;
-                break;
-        }
-
-        // Store our results -- trim at the end when we place it on the heap.
-        const int maxEvolutions = 3;
-        Span<EvoCriteria> evos = stackalloc EvoCriteria[maxEvolutions];
-
-        var lvl = levelMax; // highest level, any level-up method will decrease.
-        evos[0] = new EvoCriteria { Species = species, Form = form, LevelMax = lvl }; // current species-form
-
-        // There aren't any circular evolution paths, and all lineages have at most 3 evolutions total.
-        // There aren't any convergent evolution paths, so only yield the first connection.
-        int ctr = 1; // count in the 'evos' span
-        while (species != stopSpecies)
-        {
-            var key = EvolutionTree.GetLookupKey(species, form);
-            var node = lineage[key];
-
-            bool oneValid = false;
-            for (int i = 0; i < 2; i++)
-            {
-                ref var link = ref i == 0 ? ref node.First : ref node.Second;
-                if (link.IsEmpty)
-                    break;
-
-                if (link.IsEvolutionBanned(pk) && !skipChecks)
-                    continue;
-
-                var evo = link.Method;
-                if (!evo.Valid(pk, lvl, skipChecks))
-                    continue;
-
-                if (evo.RequiresLevelUp && levelMin >= lvl)
-                    break; // impossible evolution
-
-                UpdateMinValues(evos[..ctr], evo, levelMin);
-
-                species = link.Species;
-                form = link.Form;
-                evos[ctr++] = evo.GetEvoCriteria(species, form, lvl);
-                if (evo.RequiresLevelUp)
-                    lvl--;
-
-                oneValid = true;
-                break;
-            }
-            if (!oneValid)
-                break;
-        }
-
-        // Remove future gen pre-evolutions; no Munchlax from a Gen3 Snorlax, no Pichu from a Gen1-only Raichu, etc
-        ref var last = ref evos[ctr - 1];
-        if (last.Species > maxSpeciesID)
-        {
-            for (int i = 0; i < ctr; i++)
-            {
-                if (evos[i].Species > maxSpeciesID)
-                    continue;
-                ctr--;
-                break;
-            }
-        }
-
-        // Last species is the wild/hatched species, the minimum level is because it has not evolved from previous species
-        var result = evos[..ctr];
-        last = ref result[^1];
-        last = last with { LevelMin = levelMin, LevelUpRequired = 0 };
-
-        // Rectify minimum levels
-        RectifyMinimumLevels(result);
-
-        return result.ToArray();
-    }
-
-    private static void RectifyMinimumLevels(Span<EvoCriteria> result)
-    {
-        for (int i = result.Length - 2; i >= 0; i--)
-        {
-            ref var evo = ref result[i];
-            var prev = result[i + 1];
-            var min = (byte)Math.Max(prev.LevelMin + evo.LevelUpRequired, evo.LevelMin);
-            evo = evo with { LevelMin = min };
-        }
-    }
-
-    private static void UpdateMinValues(Span<EvoCriteria> evos, EvolutionMethod evo, byte minLevel)
-    {
-        ref var last = ref evos[^1];
-        if (!evo.RequiresLevelUp)
-        {
-            // Evolutions like elemental stones, trade, etc
-            last = last with { LevelMin = minLevel };
-            return;
-        }
-        if (evo.Level == 0)
-        {
-            // Friendship based Level Up Evolutions, Pichu -> Pikachu, Eevee -> Umbreon, etc
-            last = last with { LevelMin = (byte)(minLevel + 1) };
-
-            // Raichu from Pikachu would have a minimum level of 1; accounting for Pichu (level up required) results in a minimum level of 2
-            if (evos.Length > 1)
-            {
-                ref var first = ref evos[0];
-                if (!first.RequiresLvlUp)
-                    first = first with { LevelMin = (byte)(minLevel + 1) };
-            }
-        }
-        else // level up evolutions
-        {
-            last = last with { LevelMin = evo.Level };
-
-            if (evos.Length > 1)
-            {
-                ref var first = ref evos[0];
-                if (first.RequiresLvlUp)
-                {
-                    // Pokemon like Crobat, its minimum level is Golbat minimum level + 1
-                    if (first.LevelMin <= evo.Level)
-                        first = first with { LevelMin = (byte)(evo.Level + 1) };
-                }
-                else
-                {
-                    // Pokemon like Nidoqueen who evolve with an evolution stone, minimum level is prior evolution minimum level
-                    if (first.LevelMin < evo.Level)
-                        first = first with { LevelMin = evo.Level };
-                }
-            }
-        }
-        last = last with { LevelUpRequired = evo.RequiresLevelUp ? (byte)1 : (byte)0 };
-    }
-}

PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet1.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 
 namespace PKHeX.Core;
 
@@ -19,28 +18,24 @@ public static class EvolutionSet1
             : new EvolutionMethod((EvolutionType)method, species, Argument: arg);
     }
 
-    public static IReadOnlyList<EvolutionMethod[]> GetArray(ReadOnlySpan<byte> data, int maxSpecies)
+    public static EvolutionMethod[][] GetArray(ReadOnlySpan<byte> data, int maxSpecies)
     {
-        var evos = new EvolutionMethod[maxSpecies + 1][];
-        int ofs = 0;
+        var result = new EvolutionMethod[maxSpecies + 1][];
+        for (int i = 0, offset = 0; i < result.Length; i++)
+            result[i] = GetEntry(data, ref offset);
+        return result;
+    }
+
+    private static EvolutionMethod[] GetEntry(ReadOnlySpan<byte> data, ref int offset)
+    {
+        var count = data[offset++];
+        if (count == 0)
+            return Array.Empty<EvolutionMethod>();
+
         const int bpe = 3;
-        for (int i = 0; i < evos.Length; i++)
-        {
-            int count = data[ofs];
-            ofs++;
-            if (count == 0)
-            {
-                evos[i] = Array.Empty<EvolutionMethod>();
-                continue;
-            }
-            var m = new EvolutionMethod[count];
-            for (int j = 0; j < m.Length; j++)
-            {
-                m[j] = GetMethod(data.Slice(ofs, bpe));
-                ofs += bpe;
-            }
-            evos[i] = m;
-        }
-        return evos;
+        var result = new EvolutionMethod[count];
+        for (int i = 0; i < result.Length; i++, offset += bpe)
+            result[i] = GetMethod(data.Slice(offset, bpe));
+        return result;
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet3.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using static System.Buffers.Binary.BinaryPrimitives;
 
 namespace PKHeX.Core;
@@ -45,37 +44,43 @@ public static class EvolutionSet3
         }
     }
 
-    public static IReadOnlyList<EvolutionMethod[]> GetArray(ReadOnlySpan<byte> data)
+    public static EvolutionMethod[][] GetArray(ReadOnlySpan<byte> data)
     {
-        var evos = new EvolutionMethod[Legal.MaxSpeciesID_3 + 1][];
-        evos[0] = Array.Empty<EvolutionMethod>();
+        var result = new EvolutionMethod[Legal.MaxSpeciesID_3 + 1][];
+        result[0] = Array.Empty<EvolutionMethod>();
         for (ushort i = 1; i <= Legal.MaxSpeciesIndex_3; i++)
         {
             int g4species = SpeciesConverter.GetNational3(i);
-            if (g4species == 0)
-                continue;
-
-            const int maxCount = 5;
-            const int size = 8;
-
-            int offset = i * (maxCount * size);
-            int count = 0;
-            for (; count < maxCount; count++)
-            {
-                if (data[offset + (count * size)] == 0)
-                    break;
-            }
-            if (count == 0)
-            {
-                evos[g4species] = Array.Empty<EvolutionMethod>();
-                continue;
-            }
-
-            var set = new EvolutionMethod[count];
-            for (int j = 0; j < set.Length; j++)
-                set[j] = GetMethod(data.Slice(offset + (j * size), size));
-            evos[g4species] = set;
+            if (g4species != 0)
+                result[g4species] = GetEntry(data, i);
         }
-        return evos;
+        return result;
+    }
+
+    private const int maxCount = 5;
+    private const int size = 8;
+
+    private static EvolutionMethod[] GetEntry(ReadOnlySpan<byte> data, ushort index)
+    {
+        var span = data.Slice(index * (maxCount * size), maxCount * size);
+        int count = ScanCountEvolutions(span);
+
+        if (count == 0)
+            return Array.Empty<EvolutionMethod>();
+
+        var result = new EvolutionMethod[count];
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += size)
+            result[i] = GetMethod(span.Slice(offset, size));
+        return result;
+    }
+
+    private static int ScanCountEvolutions(ReadOnlySpan<byte> span)
+    {
+        for (int count = 0; count < maxCount; count++)
+        {
+            if (span[count * size] == 0)
+                return count;
+        }
+        return maxCount;
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet4.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using static System.Buffers.Binary.BinaryPrimitives;
 
 namespace PKHeX.Core;
@@ -29,35 +28,39 @@ public static class EvolutionSet4
         return new EvolutionMethod(type, species, Argument: arg, Level: (byte)lvl, LevelUp: lvlup);
     }
 
-    public static IReadOnlyList<EvolutionMethod[]> GetArray(ReadOnlySpan<byte> data)
+    private const int bpe = 6; // bytes per evolution entry
+    private const int entries = 7; // amount of entries per species
+    private const int size = (entries * bpe) + 2; // bytes per species entry, + 2 alignment bytes
+
+    public static EvolutionMethod[][] GetArray(ReadOnlySpan<byte> data)
     {
-        const int bpe = 6; // bytes per evolution entry
-        const int entries = 7; // amount of entries per species
-        const int size = (entries * bpe) + 2; // bytes per species entry, + 2 alignment bytes
+        var result = new EvolutionMethod[data.Length / size][];
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += size)
+            result[i] = GetEntry(data.Slice(offset, size));
+        return result;
+    }
 
-        var evos = new EvolutionMethod[data.Length / size][];
-        for (int i = 0; i < evos.Length; i++)
+    private static EvolutionMethod[] GetEntry(ReadOnlySpan<byte> data)
+    {
+        int count = ScanCountEvolutions(data);
+        if (count == 0)
+            return Array.Empty<EvolutionMethod>();
+
+        var result = new EvolutionMethod[count];
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += bpe)
+            result[i] = GetMethod(data.Slice(offset, bpe));
+        return result;
+    }
+
+    private static int ScanCountEvolutions(ReadOnlySpan<byte> data)
+    {
+        for (int count = 0; count < entries; count++)
         {
-            int offset = i * size;
-            int count = 0;
-            for (; count < entries; count++)
-            {
-                var methodOffset = offset + (count * bpe);
-                var method = data[methodOffset];
-                if (method == 0)
-                    break;
-            }
-            if (count == 0)
-            {
-                evos[i] = Array.Empty<EvolutionMethod>();
-                continue;
-            }
-
-            var set = new EvolutionMethod[count];
-            for (int j = 0; j < set.Length; j++)
-                set[j] = GetMethod(data.Slice(offset + (j * bpe), bpe));
-            evos[i] = set;
+            var methodOffset = count * bpe;
+            var method = data[methodOffset];
+            if (method == 0)
+                return count;
         }
-        return evos;
+        return entries;
     }
 }

PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet5.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using static System.Buffers.Binary.BinaryPrimitives;
 
 namespace PKHeX.Core;
@@ -28,26 +27,24 @@ public static class EvolutionSet5
     private const int entries = 7; // amount of entries per species
     private const int size = entries * bpe; // bytes per species entry
 
-    public static IReadOnlyList<EvolutionMethod[]> GetArray(ReadOnlySpan<byte> data)
+    public static EvolutionMethod[][] GetArray(ReadOnlySpan<byte> data)
     {
-        var evos = new EvolutionMethod[data.Length / size][];
-        for (int i = 0; i < evos.Length; i++)
-        {
-            int offset = i * size;
-            var rawEntries = data.Slice(offset, size);
-            var count = ScanCountEvolutions(rawEntries);
-            if (count == 0)
-            {
-                evos[i] = Array.Empty<EvolutionMethod>();
-                continue;
-            }
+        var result = new EvolutionMethod[data.Length / size][];
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += size)
+            result[i] = GetEntry(data.Slice(offset, size));
+        return result;
+    }
 
-            var set = new EvolutionMethod[count];
-            for (int j = 0; j < set.Length; j++)
-                set[j] = GetMethod(rawEntries.Slice(j * bpe, bpe));
-            evos[i] = set;
-        }
-        return evos;
+    private static EvolutionMethod[] GetEntry(ReadOnlySpan<byte> data)
+    {
+        var count = ScanCountEvolutions(data);
+        if (count == 0)
+            return Array.Empty<EvolutionMethod>();
+
+        var result = new EvolutionMethod[count];
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += bpe)
+            result[i] = GetMethod(data.Slice(offset, bpe));
+        return result;
     }
 
     private static int ScanCountEvolutions(ReadOnlySpan<byte> data)

PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet6.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using static System.Buffers.Binary.BinaryPrimitives;
 
 namespace PKHeX.Core;
@@ -13,15 +12,20 @@ public static class EvolutionSet6
     internal static bool IsMethodWithArg(byte method) => (0b100000011111110000000101000000 & (1 << method)) != 0;
     private const int SIZE = 6;
 
-    private static EvolutionMethod[] GetMethods(ReadOnlySpan<byte> data)
+    public static EvolutionMethod[][] GetArray(BinLinkerAccessor data)
     {
-        var evos = new EvolutionMethod[data.Length / SIZE];
-        for (int i = 0; i < data.Length; i += SIZE)
-        {
-            var entry = data.Slice(i, SIZE);
-            evos[i/SIZE] = GetMethod(entry);
-        }
-        return evos;
+        var result = new EvolutionMethod[data.Length][];
+        for (int i = 0; i < result.Length; i++)
+            result[i] = GetEntry(data[i]);
+        return result;
+    }
+
+    private static EvolutionMethod[] GetEntry(ReadOnlySpan<byte> data)
+    {
+        var result = new EvolutionMethod[data.Length / SIZE];
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += SIZE)
+            result[i] = GetMethod(data.Slice(offset, SIZE));
+        return result;
     }
 
     private static EvolutionMethod GetMethod(ReadOnlySpan<byte> entry)
@@ -36,12 +40,4 @@ public static class EvolutionSet6
         var lvlup = type.IsLevelUpRequired() ? (byte)1 : (byte)0;
         return new EvolutionMethod(type, species, Argument: arg, Level: (byte)lvl, LevelUp: lvlup);
     }
-
-    public static IReadOnlyList<EvolutionMethod[]> GetArray(BinLinkerAccessor data)
-    {
-        var evos = new EvolutionMethod[data.Length][];
-        for (int i = 0; i < evos.Length; i++)
-            evos[i] = GetMethods(data[i]);
-        return evos;
-    }
 }

PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet7.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using static System.Buffers.Binary.BinaryPrimitives;
 
 namespace PKHeX.Core;
@@ -11,40 +10,33 @@ public static class EvolutionSet7
 {
     private const int SIZE = 8;
 
-    private static EvolutionMethod[] GetMethods(ReadOnlySpan<byte> data, bool LevelUpBypass)
+    public static EvolutionMethod[][] GetArray(BinLinkerAccessor data, byte levelUp = 1)
+    {
+        var result = new EvolutionMethod[data.Length][];
+        for (int i = 0; i < result.Length; i++)
+            result[i] = GetEntry(data[i], levelUp);
+        return result;
+    }
+
+    private static EvolutionMethod[] GetEntry(ReadOnlySpan<byte> data, byte levelUp)
     {
         if (data.Length == 0)
             return Array.Empty<EvolutionMethod>();
 
         var result = new EvolutionMethod[data.Length / SIZE];
-        int i = 0, offset = 0;
-        while (true)
-        {
-            var entry = data.Slice(offset, SIZE);
-            result[i] = ReadEvolution(entry, LevelUpBypass);
-            offset += SIZE;
-            if (offset >= data.Length)
-                return result;
-            i++;
-        }
+        for (int i = 0, offset = 0; i < result.Length; i++, offset += SIZE)
+            result[i] = GetMethod(data.Slice(offset, SIZE), levelUp);
+        return result;
     }
 
-    private static EvolutionMethod ReadEvolution(ReadOnlySpan<byte> entry, bool levelUpBypass)
+    private static EvolutionMethod GetMethod(ReadOnlySpan<byte> entry, byte levelUp)
     {
         var type = (EvolutionType)entry[0];
         var arg = ReadUInt16LittleEndian(entry[2..]);
         var species = ReadUInt16LittleEndian(entry[4..]);
         var form = entry[6];
         var level = entry[7];
-        var lvlup = !levelUpBypass && type.IsLevelUpRequired() ? (byte)1 : (byte)0;
+        var lvlup = type.IsLevelUpRequired() ? levelUp : (byte)0;
         return new EvolutionMethod(type, species, form, arg, level, lvlup);
     }
-
-    public static IReadOnlyList<EvolutionMethod[]> GetArray(BinLinkerAccessor data, bool LevelUpBypass)
-    {
-        var evos = new EvolutionMethod[data.Length][];
-        for (int i = 0; i < evos.Length; i++)
-            evos[i] = GetMethods(data[i], LevelUpBypass);
-        return evos;
-    }
 }

PKHeX.Core/Legality/Evolutions/EvolutionTree.cs

@@ -1,7 +1,4 @@
 using System;
-using System.Collections.Generic;
-using static PKHeX.Core.GameVersion;
-using static PKHeX.Core.Legal;
 
 namespace PKHeX.Core;
 
@@ -11,30 +8,38 @@ namespace PKHeX.Core;
 /// <remarks>
 /// Used to determine if a <see cref="PKM.Species"/> can evolve from prior steps in its evolution branch.
 /// </remarks>
-public sealed class EvolutionTree
+public sealed class EvolutionTree : EvolutionNetwork
 {
-    public static readonly EvolutionTree Evolves1 = new(GetResource("rby"), Gen1, PersonalTable.Y, MaxSpeciesID_1);
-    public static readonly EvolutionTree Evolves2 = new(GetResource("gsc"), Gen2, PersonalTable.C, MaxSpeciesID_2);
-    public static readonly EvolutionTree Evolves3 = new(GetResource("g3"), Gen3, PersonalTable.RS, MaxSpeciesID_3);
-    public static readonly EvolutionTree Evolves4 = new(GetResource("g4"), Gen4, PersonalTable.DP, MaxSpeciesID_4);
-    public static readonly EvolutionTree Evolves5 = new(GetResource("g5"), Gen5, PersonalTable.BW, MaxSpeciesID_5);
-    public static readonly EvolutionTree Evolves6 = new(GetReader("ao"), Gen6, PersonalTable.AO, MaxSpeciesID_6);
-    public static readonly EvolutionTree Evolves7 = new(GetReader("uu"), Gen7, PersonalTable.USUM, MaxSpeciesID_7_USUM);
-    public static readonly EvolutionTree Evolves7b = new(GetReader("gg"), Gen7, PersonalTable.GG, MaxSpeciesID_7b);
-    public static readonly EvolutionTree Evolves8 = new(GetReader("ss"), Gen8, PersonalTable.SWSH, MaxSpeciesID_8);
-    public static readonly EvolutionTree Evolves8a = new(GetReader("la"), PLA, PersonalTable.LA, MaxSpeciesID_8a);
-    public static readonly EvolutionTree Evolves8b = new(GetReader("bs"), BDSP, PersonalTable.BDSP, MaxSpeciesID_8b);
-    public static readonly EvolutionTree Evolves9 = new(GetReader("sv"), Gen9, PersonalTable.SV, MaxSpeciesID_9);
+    public const int MaxEvolutions = 3;
+    public static readonly EvolutionTree Evolves1  = GetViaSpecies (PersonalTable.Y,    EvolutionSet1.GetArray(GetResource("rby"), 151));
+    public static readonly EvolutionTree Evolves2  = GetViaSpecies (PersonalTable.C,    EvolutionSet1.GetArray(GetResource("gsc"), 251));
+    public static readonly EvolutionTree Evolves3  = GetViaSpecies (PersonalTable.RS,   EvolutionSet3.GetArray(GetResource("g3")));
+    public static readonly EvolutionTree Evolves4  = GetViaSpecies (PersonalTable.DP,   EvolutionSet4.GetArray(GetResource("g4")));
+    public static readonly EvolutionTree Evolves5  = GetViaSpecies (PersonalTable.BW,   EvolutionSet5.GetArray(GetResource("g5")));
+    public static readonly EvolutionTree Evolves6  = GetViaSpecies (PersonalTable.AO,   EvolutionSet6.GetArray(GetReader("ao")));
+    public static readonly EvolutionTree Evolves7  = GetViaPersonal(PersonalTable.USUM, EvolutionSet7.GetArray(GetReader("uu")));
+    public static readonly EvolutionTree Evolves7b = GetViaPersonal(PersonalTable.GG,   EvolutionSet7.GetArray(GetReader("gg")));
+    public static readonly EvolutionTree Evolves8  = GetViaPersonal(PersonalTable.SWSH, EvolutionSet7.GetArray(GetReader("ss")));
+    public static readonly EvolutionTree Evolves8a = GetViaPersonal(PersonalTable.LA,   EvolutionSet7.GetArray(GetReader("la"), 0));
+    public static readonly EvolutionTree Evolves8b = GetViaPersonal(PersonalTable.BDSP, EvolutionSet7.GetArray(GetReader("bs")));
+    public static readonly EvolutionTree Evolves9  = GetViaPersonal(PersonalTable.SV,   EvolutionSet7.GetArray(GetReader("sv")));
 
     private static ReadOnlySpan<byte> GetResource(string resource) => Util.GetBinaryResource($"evos_{resource}.pkl");
     private static BinLinkerAccessor GetReader(string resource) => BinLinkerAccessor.Get(GetResource(resource), resource);
+    private EvolutionTree(IEvolutionForward forward, IEvolutionReverse reverse) : base(forward, reverse) { }
 
-    static EvolutionTree()
+    private static EvolutionTree GetViaSpecies(IPersonalTable t, EvolutionMethod[][] entries)
     {
-        // Add in banned evolution data!
-        Evolves7.FixEvoTreeSM();
-        Evolves8.FixEvoTreeSS();
-        Evolves8b.FixEvoTreeBS();
+        var forward = new EvolutionForwardSpecies(entries);
+        var reverse = new EvolutionReverseSpecies(entries, t);
+        return new EvolutionTree(forward, reverse);
+    }
+
+    private static EvolutionTree GetViaPersonal(IPersonalTable t, EvolutionMethod[][] entries)
+    {
+        var forward = new EvolutionForwardPersonal(entries, t);
+        var reverse = new EvolutionReversePersonal(entries, t);
+        return new EvolutionTree(forward, reverse);
     }
 
     public static EvolutionTree GetEvolutionTree(EntityContext context) => context switch
@@ -53,297 +58,4 @@ public sealed class EvolutionTree
         EntityContext.Gen8b => Evolves8b,
         _ => throw new ArgumentOutOfRangeException(nameof(context), context, null),
     };
-
-    private readonly IReadOnlyList<EvolutionMethod[]> Entries;
-    private readonly IPersonalTable Personal;
-    private readonly int MaxSpeciesTree;
-    private readonly EvolutionReverseLookup Lineage;
-    private bool FormSeparateEvoData => MaxSpeciesTree > MaxSpeciesID_6;
-
-    internal static int GetLookupKey(ushort species, byte form) => species | (form << 11);
-
-    #region Constructor
-
-    private EvolutionTree(ReadOnlySpan<byte> data, GameVersion game, IPersonalTable personal, int maxSpeciesTree)
-    {
-        Personal = personal;
-        MaxSpeciesTree = maxSpeciesTree;
-        Entries = GetEntries(data, game);
-        var connections = CreateTreeOld();
-        Lineage = new(connections, maxSpeciesTree);
-    }
-
-    private EvolutionTree(BinLinkerAccessor data, GameVersion game, IPersonalTable personal, int maxSpeciesTree)
-    {
-        Personal = personal;
-        MaxSpeciesTree = maxSpeciesTree;
-        Entries = GetEntries(data, game);
-
-        // Starting in Generation 7, forms have separate evolution data.
-        var oldStyle = !FormSeparateEvoData;
-        var connections = oldStyle ? CreateTreeOld() : CreateTree();
-        Lineage = new(connections, maxSpeciesTree);
-    }
-
-    private IEnumerable<(int Key, EvolutionLink Value)> CreateTreeOld()
-    {
-        for (ushort sSpecies = 1; sSpecies <= MaxSpeciesTree; sSpecies++)
-        {
-            var fc = Personal[sSpecies].FormCount;
-            for (byte sForm = 0; sForm < fc; sForm++)
-            {
-                var index = sSpecies;
-                var evos = Entries[index];
-                foreach (var evo in evos)
-                {
-                    var dSpecies = evo.Species;
-                    if (dSpecies == 0)
-                        continue;
-
-                    var dForm = sSpecies == (int)Species.Espurr && evo.Method == EvolutionType.LevelUpFormFemale1 ? (byte)1 : sForm;
-                    var key = GetLookupKey(dSpecies, dForm);
-
-                    var link = new EvolutionLink(sSpecies, sForm, evo);
-                    yield return (key, link);
-                }
-            }
-        }
-    }
-
-    private IEnumerable<(int Key, EvolutionLink Value)> CreateTree()
-    {
-        for (ushort sSpecies = 1; sSpecies <= MaxSpeciesTree; sSpecies++)
-        {
-            var fc = Personal[sSpecies].FormCount;
-            for (byte sForm = 0; sForm < fc; sForm++)
-            {
-                var index = Personal.GetFormIndex(sSpecies, sForm);
-                var evos = Entries[index];
-                foreach (var evo in evos)
-                {
-                    var dSpecies = evo.Species;
-                    if (dSpecies == 0)
-                        break;
-
-                    var dForm = evo.GetDestinationForm(sForm);
-                    var key = GetLookupKey(dSpecies, dForm);
-
-                    var link = new EvolutionLink(sSpecies, sForm, evo);
-                    yield return (key, link);
-                }
-            }
-        }
-    }
-
-    private static IReadOnlyList<EvolutionMethod[]> GetEntries(ReadOnlySpan<byte> data, GameVersion game) => game switch
-    {
-        Gen1 => EvolutionSet1.GetArray(data, 151),
-        Gen2 => EvolutionSet1.GetArray(data, 251),
-        Gen3 => EvolutionSet3.GetArray(data),
-        Gen4 => EvolutionSet4.GetArray(data),
-        Gen5 => EvolutionSet5.GetArray(data),
-        _ => throw new ArgumentOutOfRangeException(nameof(game)),
-    };
-
-    private static IReadOnlyList<EvolutionMethod[]> GetEntries(BinLinkerAccessor data, GameVersion game) => game switch
-    {
-        Gen6 => EvolutionSet6.GetArray(data),
-        Gen7 or Gen8 or BDSP or Gen9 => EvolutionSet7.GetArray(data, false),
-        PLA => EvolutionSet7.GetArray(data, true),
-        _ => throw new ArgumentOutOfRangeException(nameof(game)),
-    };
-
-    private void FixEvoTreeSM()
-    {
-        // Sun/Moon lack Ultra's Kantonian evolution methods.
-        static bool BanOnlySM(PKM pk) => pk is { IsUntraded: true, SM: true };
-        BanEvo((int)Species.Raichu, 0, BanOnlySM);
-        BanEvo((int)Species.Marowak, 0, BanOnlySM);
-        BanEvo((int)Species.Exeggutor, 0, BanOnlySM);
-    }
-
-    private void FixEvoTreeSS()
-    {
-        // Gigantamax Pikachu, Meowth-0, and Eevee are prevented from evolving.
-        // Raichu cannot be evolved to the Alolan variant at this time.
-        static bool BanGmax(PKM pk) => pk is IGigantamax { CanGigantamax: true };
-        BanEvo((int)Species.Raichu, 0, BanGmax);
-        BanEvo((int)Species.Raichu, 1, pk => (pk is IGigantamax {CanGigantamax: true}) || pk.Version is (int)GO or >= (int)GP);
-        BanEvo((int)Species.Persian, 0, BanGmax);
-        BanEvo((int)Species.Persian, 1, BanGmax);
-        BanEvo((int)Species.Perrserker, 0, BanGmax);
-
-        BanEvo((int)Species.Exeggutor, 1, pk => pk.Version is (int)GO or >= (int)GP);
-        BanEvo((int)Species.Marowak, 1, pk => pk.Version is (int)GO or >= (int)GP);
-        BanEvo((int)Species.Weezing, 0, pk => pk.Version >= (int)SW);
-        BanEvo((int)Species.MrMime, 0, pk => pk.Version >= (int)SW);
-
-        foreach (var s in GetEvolutions((int)Species.Eevee, 0)) // Eeveelutions
-            BanEvo(s.Species, s.Form, BanGmax);
-    }
-
-    private void FixEvoTreeBS()
-    {
-        BanEvo((int)Species.Glaceon, 0, pk => pk.CurrentLevel == pk.Met_Level); // Ice Stone is unreleased, requires Route 217 Ice Rock Level Up instead
-        BanEvo((int)Species.Milotic, 0, pk => pk is IContestStatsReadOnly { CNT_Beauty: < 170 } || pk.CurrentLevel == pk.Met_Level); // Prism Scale is unreleased, requires 170 Beauty Level Up instead
-    }
-
-    private void BanEvo(ushort species, byte form, Func<PKM, bool> func)
-    {
-        var key = GetLookupKey(species, form);
-        ref var node = ref Lineage[key];
-        node.Ban(func);
-    }
-
-    #endregion
-
-    /// <summary>
-    /// Gets a list of evolutions for the input <see cref="PKM"/> by checking each evolution in the chain.
-    /// </summary>
-    /// <param name="pk">Pokémon data to check with.</param>
-    /// <param name="levelMax">Maximum level to permit before the chain breaks.</param>
-    /// <param name="maxSpeciesOrigin">Maximum species ID to permit within the chain.</param>
-    /// <param name="skipChecks">Ignores an evolution's criteria, causing the returned list to have all possible evolutions.</param>
-    /// <param name="levelMin">Minimum level to permit before the chain breaks.</param>
-    /// <param name="stopSpecies">Final species to stop at, if known</param>
-    public EvoCriteria[] GetValidPreEvolutions(PKM pk, byte levelMax, int maxSpeciesOrigin = -1, bool skipChecks = false, byte levelMin = 1, int stopSpecies = 0)
-    {
-        if (maxSpeciesOrigin <= 0)
-            maxSpeciesOrigin = GetMaxSpeciesOrigin(pk);
-
-        ushort species = pk.Species;
-        byte form = pk.Form;
-
-        return GetExplicitLineage(species, form, pk, levelMin, levelMax, maxSpeciesOrigin, skipChecks, stopSpecies);
-    }
-
-    private static int GetMaxSpeciesOrigin(PKM pk)
-    {
-        if (pk.Format == 1)
-            return MaxSpeciesID_1;
-        if (pk.Format == 2 || pk.VC)
-            return MaxSpeciesID_2;
-        return Legal.GetMaxSpeciesOrigin(pk.Generation);
-    }
-
-    public bool IsSpeciesDerivedFrom(ushort species, byte form, int otherSpecies, int otherForm, bool ignoreForm = true)
-    {
-        var evos = GetEvolutionsAndPreEvolutions(species, form);
-        foreach (var (s, f) in evos)
-        {
-            if (s != otherSpecies)
-                continue;
-            if (ignoreForm)
-                return true;
-            return f == otherForm;
-        }
-        return false;
-    }
-
-    /// <summary>
-    /// Gets all species the <see cref="species"/>-<see cref="form"/> can evolve to &amp; from, yielded in order of increasing evolution stage.
-    /// </summary>
-    /// <param name="species">Species ID</param>
-    /// <param name="form">Form ID</param>
-    /// <returns>Enumerable of species IDs (with the Form IDs included, left shifted by 11).</returns>
-    public IEnumerable<(ushort Species, byte Form)> GetEvolutionsAndPreEvolutions(ushort species, byte form)
-    {
-        foreach (var s in GetPreEvolutions(species, form))
-            yield return s;
-        yield return (species, form);
-        foreach (var s in GetEvolutions(species, form))
-            yield return s;
-    }
-
-    public (ushort Species, byte Form) GetBaseSpeciesForm(ushort species, byte form, int skip = 0)
-    {
-        var chain = GetEvolutionsAndPreEvolutions(species, form);
-        foreach (var c in chain)
-        {
-            if (skip == 0)
-                return c;
-            skip--;
-        }
-        return (species, form);
-    }
-
-    /// <summary>
-    /// Gets all species the <see cref="species"/>-<see cref="form"/> can evolve from, yielded in order of increasing evolution stage.
-    /// </summary>
-    /// <param name="species">Species ID</param>
-    /// <param name="form">Form ID</param>
-    /// <returns>Enumerable of species IDs (with the Form IDs included, left shifted by 11).</returns>
-    public IEnumerable<(ushort Species, byte Form)> GetPreEvolutions(ushort species, byte form)
-    {
-        int index = GetLookupKey(species, form);
-        var node = Lineage[index];
-        {
-            // No convergent evolutions; first method is enough.
-            var s = node.First.Tuple;
-            if (s.Species == 0)
-                yield break;
-
-            var preEvolutions = GetPreEvolutions(s.Species, s.Form);
-            foreach (var preEvo in preEvolutions)
-                yield return preEvo;
-            yield return s;
-        }
-    }
-
-    /// <summary>
-    /// Gets all species the <see cref="species"/>-<see cref="form"/> can evolve to, yielded in order of increasing evolution stage.
-    /// </summary>
-    /// <param name="species">Species ID</param>
-    /// <param name="form">Form ID</param>
-    /// <returns>Enumerable of species IDs (with the Form IDs included, left shifted by 11).</returns>
-    public IEnumerable<(ushort Species, byte Form)> GetEvolutions(ushort species, byte form)
-    {
-        int index = !FormSeparateEvoData ? species : Personal.GetFormIndex(species, form);
-        var evos = Entries[index];
-        foreach (var method in evos)
-        {
-            var s = method.Species;
-            if (s == 0)
-                continue;
-            var f = method.GetDestinationForm(form);
-            yield return (s, f);
-            var nextEvolutions = GetEvolutions(s, f);
-            foreach (var nextEvo in nextEvolutions)
-                yield return nextEvo;
-        }
-    }
-
-    /// <summary>
-    /// Generates the reverse evolution path for the input <see cref="pk"/>.
-    /// </summary>
-    /// <param name="species">Entity Species to begin the chain</param>
-    /// <param name="form">Entity Form to begin the chain</param>
-    /// <param name="pk">Entity data</param>
-    /// <param name="levelMin">Minimum level</param>
-    /// <param name="levelMax">Maximum level</param>
-    /// <param name="maxSpeciesID">Clamp for maximum species ID</param>
-    /// <param name="skipChecks">Skip the secondary checks that validate the evolution</param>
-    /// <param name="stopSpecies">Final species to stop at, if known</param>
-    public EvoCriteria[] GetExplicitLineage(ushort species, byte form, PKM pk, byte levelMin, byte levelMax, int maxSpeciesID, bool skipChecks, int stopSpecies)
-    {
-        if (pk.IsEgg && !skipChecks)
-        {
-            return new[]
-            {
-                new EvoCriteria{ Species = species, Form = form, LevelMax = levelMax, LevelMin = levelMax },
-            };
-        }
-
-        // Shedinja's evolution case can be a little tricky; hard-code handling.
-        if (species == (int)Species.Shedinja && levelMax >= 20 && (!pk.HasOriginalMetLocation || levelMin < levelMax))
-        {
-            var min = Math.Max(levelMin, (byte)20);
-            return new[]
-            {
-                new EvoCriteria { Species = (ushort)Species.Shedinja, LevelMax = levelMax, LevelMin = min, Method = EvolutionType.LevelUp },
-                new EvoCriteria { Species = (ushort)Species.Nincada, LevelMax = levelMax, LevelMin = levelMin },
-            };
-        }
-        return Lineage.Reverse(species, form, pk, levelMin, levelMax, maxSpeciesID, skipChecks, stopSpecies);
-    }
 }

PKHeX.Core/Legality/Evolutions/Forward/EvolutionForwardPersonal.cs

@@ -0,0 +1,79 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public sealed class EvolutionForwardPersonal : IEvolutionForward
+{
+    private readonly IPersonalTable Personal;
+    private readonly EvolutionMethod[][] Entries;
+
+    public EvolutionForwardPersonal(EvolutionMethod[][] entries, IPersonalTable personal)
+    {
+        Entries = entries;
+        Personal = personal;
+    }
+
+    public ReadOnlyMemory<EvolutionMethod> GetForward(ushort species, byte form)
+    {
+        int index = Personal.GetFormIndex(species, form);
+        return Entries[index];
+    }
+
+    public IEnumerable<(ushort Species, byte Form)> GetEvolutions(ushort species, byte form)
+    {
+        var methods  = GetForward(species, form);
+        return GetEvolutions(methods, form);
+    }
+
+    private IEnumerable<(ushort Species, byte Form)> GetEvolutions(ReadOnlyMemory<EvolutionMethod> evos, byte form)
+    {
+        for (int i = 0; i < evos.Length; i++)
+        {
+            var method = evos.Span[i];
+            var s = method.Species;
+            if (s == 0)
+                continue;
+            var f = method.GetDestinationForm(form);
+            yield return (s, f);
+            var nextEvolutions = GetEvolutions(s, f);
+            foreach (var nextEvo in nextEvolutions)
+                yield return nextEvo;
+        }
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        var methods = GetForward(head.Species, head.Form);
+        foreach (var method in methods.Span)
+        {
+            if (method.Species != next.Species)
+                continue;
+            var expectForm = method.GetDestinationForm(head.Form);
+            if (next.Form != expectForm)
+                continue;
+
+            var chk = method.Check(pk, currentMaxLevel, levelMin, skipChecks);
+            if (chk != EvolutionCheckResult.Valid)
+                continue;
+
+            result = Create(next.Species, next.Form, method, currentMaxLevel, levelMin);
+            return true;
+        }
+
+        result = default;
+        return false;
+    }
+
+    private static EvoCriteria Create(ushort species, byte form, EvolutionMethod method, byte currentMaxLevel, byte min) => new()
+    {
+        Species = species,
+        Form = form,
+        LevelMax = currentMaxLevel,
+        Method = method.Method,
+
+        // Temporarily store these and overwrite them when we clean the list.
+        LevelMin = Math.Max(min, method.Level),
+        LevelUpRequired = method.RequiresLevelUp ? (byte)1 : (byte)0,
+    };
+}

PKHeX.Core/Legality/Evolutions/Forward/EvolutionForwardSpecies.cs

@@ -0,0 +1,76 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public sealed class EvolutionForwardSpecies : IEvolutionForward
+{
+    private readonly EvolutionMethod[][] Entries;
+
+    public EvolutionForwardSpecies(EvolutionMethod[][] entries) => Entries = entries;
+
+    public IEnumerable<(ushort Species, byte Form)> GetEvolutions(ushort species, byte form)
+    {
+        var methods = GetForward(species, form);
+        return GetEvolutions(methods, form);
+    }
+
+    public ReadOnlyMemory<EvolutionMethod> GetForward(ushort species, byte form)
+    {
+        var arr = Entries;
+        if (species >= arr.Length)
+            return Array.Empty<EvolutionMethod>();
+        return arr[species];
+    }
+
+    private IEnumerable<(ushort Species, byte Form)> GetEvolutions(ReadOnlyMemory<EvolutionMethod> evos, byte form)
+    {
+        for (int i = 0; i < evos.Length; i++)
+        {
+            var method = evos.Span[i];
+            var s = method.Species;
+            if (s == 0)
+                continue;
+            var f = method.GetDestinationForm(form);
+            yield return (s, f);
+            var nextEvolutions = GetEvolutions(s, f);
+            foreach (var nextEvo in nextEvolutions)
+                yield return nextEvo;
+        }
+    }
+
+    public bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        var methods = GetForward(head.Species, head.Form);
+        foreach (var method in methods.Span)
+        {
+            if (method.Species != next.Species)
+                continue;
+            var expectForm = method.GetDestinationForm(head.Form);
+            if (next.Form != expectForm)
+                continue;
+
+            var chk = method.Check(pk, currentMaxLevel, levelMin, skipChecks);
+            if (chk != EvolutionCheckResult.Valid)
+                continue;
+
+            result = Create(next.Species, next.Form, method, currentMaxLevel, levelMin);
+            return true;
+        }
+
+        result = default;
+        return false;
+    }
+
+    private static EvoCriteria Create(ushort species, byte form, EvolutionMethod method, byte currentMaxLevel, byte min) => new()
+    {
+        Species = species,
+        Form = form,
+        LevelMax = currentMaxLevel,
+        Method = method.Method,
+
+        // Temporarily store these and overwrite them when we clean the list.
+        LevelMin = Math.Max(min, method.Level),
+        LevelUpRequired = method.RequiresLevelUp ? (byte)1 : (byte)0,
+    };
+}

PKHeX.Core/Legality/Evolutions/Forward/IEvolutionForward.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public interface IEvolutionForward
+{
+    ReadOnlyMemory<EvolutionMethod> GetForward(ushort species, byte form);
+
+    /// <summary>
+    /// Gets all species the <see cref="species"/>-<see cref="form"/> can evolve to, yielded in order of increasing evolution stage.
+    /// </summary>
+    /// <param name="species">Species ID</param>
+    /// <param name="form">Form ID</param>
+    /// <returns>Enumerable of species IDs (with the Form IDs included, left shifted by 11).</returns>
+    IEnumerable<(ushort Species, byte Form)> GetEvolutions(ushort species, byte form);
+
+    bool TryEvolve(ISpeciesForm head, ISpeciesForm next, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result);
+}

PKHeX.Core/Legality/Evolutions/IEvolutionNetwork.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public interface IEvolutionNetwork
+{
+    IEvolutionForward Forward { get; }
+    IEvolutionReverse Reverse { get; }
+}
+
+/// <summary>
+/// Base abstraction for <see cref="EvolutionTree"/>
+/// </summary>
+public abstract class EvolutionNetwork : IEvolutionNetwork
+{
+    public IEvolutionForward Forward { get; }
+    public IEvolutionReverse Reverse { get; }
+
+    protected EvolutionNetwork(IEvolutionForward forward, IEvolutionReverse reverse)
+    {
+        Forward = forward;
+        Reverse = reverse;
+    }
+
+    /// <summary>
+    /// Gets all species the <see cref="species"/>-<see cref="form"/> can evolve to &amp; from, yielded in order of increasing evolution stage.
+    /// </summary>
+    /// <param name="species">Species ID</param>
+    /// <param name="form">Form ID</param>
+    /// <returns>Enumerable of species IDs (with the Form IDs included, left shifted by 11).</returns>
+    public IEnumerable<(ushort Species, byte Form)> GetEvolutionsAndPreEvolutions(ushort species, byte form)
+    {
+        foreach (var s in Reverse.GetPreEvolutions(species, form))
+            yield return s;
+        yield return (species, form);
+        foreach (var s in Forward.GetEvolutions(species, form))
+            yield return s;
+    }
+
+    public bool IsSpeciesDerivedFrom(ushort species, byte form, int otherSpecies, int otherForm, bool ignoreForm = true)
+    {
+        var evos = GetEvolutionsAndPreEvolutions(species, form);
+        foreach (var (s, f) in evos)
+        {
+            if (s != otherSpecies)
+                continue;
+            if (ignoreForm)
+                return true;
+            return f == otherForm;
+        }
+        return false;
+    }
+
+    public (ushort Species, byte Form) GetBaseSpeciesForm(ushort species, byte form)
+    {
+        var chain = Reverse.GetPreEvolutions(species, form);
+        foreach (var evo in chain)
+            return evo;
+        return (species, form);
+    }
+
+    public int Devolve(Span<EvoCriteria> result, ushort species, byte form, PKM pk, byte levelMin, byte levelMax, ushort stopSpecies,
+        bool skipChecks)
+    {
+        return Reverse.Devolve(result, species, form, pk, levelMin, levelMax, stopSpecies, skipChecks);
+    }
+}

PKHeX.Core/Legality/Evolutions/Methods/EvoCriteria.cs

@@ -19,4 +19,25 @@ public readonly record struct EvoCriteria : ISpeciesForm
     public bool InsideLevelRange(int level) => LevelMin <= level && level <= LevelMax;
 
     public override string ToString() => $"{(Species) Species}{(Form != 0 ? $"-{Form}" : "")}}} [{LevelMin},{LevelMax}] via {Method}";
+
+    internal const EvolutionType SentinelNotReached = EvolutionType.Invalid;
+
+    public bool IsBetterDevolution(EvoCriteria reference)
+    {
+        if (reference.Species == 0)
+            return true;
+
+        return LevelMin + LevelUpRequired < reference.LevelMin + reference.LevelUpRequired;
+    }
+
+    public bool IsBetterEvolution(EvoCriteria reference)
+    {
+        if (reference.Method == SentinelNotReached)
+            return true;
+
+        if (LevelMin + LevelUpRequired > reference.LevelMin + reference.LevelUpRequired)
+            return false;
+
+        return LevelMax > reference.LevelMax;
+    }
 }

PKHeX.Core/Legality/Evolutions/Methods/EvolutionCheckResult.cs

@@ -0,0 +1,13 @@
+namespace PKHeX.Core;
+
+public enum EvolutionCheckResult
+{
+    Valid = 0,
+    InsufficientLevel,
+    BadGender,
+    BadForm,
+    WrongEC,
+    VisitVersion,
+    LowContestStat,
+    Untraded,
+}

PKHeX.Core/Legality/Evolutions/Methods/EvolutionMethod.cs

@@ -1,4 +1,5 @@
 using static PKHeX.Core.EvolutionType;
+using static PKHeX.Core.EvolutionCheckResult;
 
 namespace PKHeX.Core;
 
@@ -11,7 +12,7 @@ namespace PKHeX.Core;
 /// <param name="Argument">Conditional Argument (different from <see cref="Level"/>)</param>
 /// <param name="Level">Conditional Argument (different from <see cref="Argument"/>)</param>
 /// <param name="LevelUp">Indicates if a level up is required to trigger evolution.</param>
-public readonly record struct EvolutionMethod(EvolutionType Method, ushort Species, byte Form = 0, ushort Argument = 0, byte Level = 0, byte LevelUp = 0) : ISpeciesForm
+public readonly record struct EvolutionMethod(EvolutionType Method, ushort Species, byte Form = EvolutionMethod.AnyForm, ushort Argument = 0, byte Level = 0, byte LevelUp = 0) : ISpeciesForm
 {
     /// <summary>Evolve to Species</summary>
     public ushort Species { get; } = Species;
@@ -56,88 +57,61 @@ public readonly record struct EvolutionMethod(EvolutionType Method, ushort Speci
     /// </summary>
     /// <param name="pk">Entity to check</param>
     /// <param name="lvl">Current level</param>
+    /// <param name="levelMin">Minimum level to sanity check with</param>
     /// <param name="skipChecks">Option to skip some comparisons to return a 'possible' evolution.</param>
     /// <returns>True if a evolution criteria is valid.</returns>
-    public bool Valid(PKM pk, byte lvl, bool skipChecks)
+    public EvolutionCheckResult Check(PKM pk, byte lvl, byte levelMin, bool skipChecks)
     {
         if (!Method.IsLevelUpRequired())
             return ValidNotLevelUp(pk, skipChecks);
 
-        if (!IsLevelUpMethodSecondarySatisfied(pk, skipChecks))
-            return false;
+        var chk = IsLevelUpMethodSecondarySatisfied(pk, skipChecks);
+        if (chk != Valid)
+            return chk;
 
         // Level Up (any); the above Level Up (with condition) cases will reach here if they were valid
-        if (!RequiresLevelUp)
-            return lvl >= Level;
-
-        if (Level == 0 && lvl < 2)
-            return false;
         if (lvl < Level)
-            return false;
+            return InsufficientLevel;
+        if (!RequiresLevelUp)
+            return Valid;
+        if (Level == 0 && lvl < 2)
+            return InsufficientLevel;
+        if (lvl < levelMin + LevelUp && !skipChecks)
+            return InsufficientLevel;
 
-        if (skipChecks)
-            return lvl >= Level;
-
-        // Check Met Level for extra validity
-        return HasMetLevelIncreased(pk, lvl);
+        return Valid;
     }
 
-    private bool IsLevelUpMethodSecondarySatisfied(PKM pk, bool skipChecks) => Method switch
+    private EvolutionCheckResult IsLevelUpMethodSecondarySatisfied(PKM pk, bool skipChecks) => Method switch
     {
         // Special Level Up Cases -- return false if invalid
-        LevelUpMale when pk.Gender != 0 => false,
-        LevelUpFemale when pk.Gender != 1 => false,
-        LevelUpFormFemale1 when pk.Gender != 1 || pk.Form != 1 => false,
+        LevelUpMale when pk.Gender != 0 => BadGender,
+        LevelUpFemale when pk.Gender != 1 => BadGender,
+        LevelUpFormFemale1 when pk.Gender != 1 => BadGender,
+        LevelUpFormFemale1 when pk.Form != 1 => BadForm,
 
         // Permit the evolution if we're exploring for mistakes.
-        LevelUpBeauty when pk is IContestStatsReadOnly s && s.CNT_Beauty < Argument => skipChecks,
-        LevelUpNatureAmped or LevelUpNatureLowKey when GetAmpLowKeyResult(pk.Nature) != pk.Form => skipChecks,
+        LevelUpBeauty when pk is IContestStatsReadOnly s && s.CNT_Beauty < Argument => skipChecks ? Valid : LowContestStat,
+        LevelUpNatureAmped or LevelUpNatureLowKey when GetAmpLowKeyResult(pk.Nature) != pk.Form => skipChecks ? Valid : BadForm,
 
         // Version checks come in pairs, check for any pair match
-        LevelUpVersion or LevelUpVersionDay or LevelUpVersionNight when ((pk.Version & 1) != (Argument & 1) && pk.IsUntraded) => skipChecks,
+        LevelUpVersion or LevelUpVersionDay or LevelUpVersionNight when ((pk.Version & 1) != (Argument & 1) && pk.IsUntraded) => skipChecks ? Valid : VisitVersion,
 
-        LevelUpKnowMoveEC100  when pk.EncryptionConstant % 100 != 0 => skipChecks,
-        LevelUpKnowMoveECElse when pk.EncryptionConstant % 100 == 0 => skipChecks,
-        LevelUpInBattleEC100  when pk.EncryptionConstant % 100 != 0 => skipChecks,
-        LevelUpInBattleECElse when pk.EncryptionConstant % 100 == 0 => skipChecks,
+        LevelUpKnowMoveEC100  when pk.EncryptionConstant % 100 != 0 => skipChecks ? Valid : WrongEC,
+        LevelUpKnowMoveECElse when pk.EncryptionConstant % 100 == 0 => skipChecks ? Valid : WrongEC,
+        LevelUpInBattleEC100  when pk.EncryptionConstant % 100 != 0 => skipChecks ? Valid : WrongEC,
+        LevelUpInBattleECElse when pk.EncryptionConstant % 100 == 0 => skipChecks ? Valid : WrongEC,
 
-        _ => true,
+        _ => Valid,
     };
 
-    private bool ValidNotLevelUp(PKM pk, bool skipChecks) => Method switch
+    private EvolutionCheckResult ValidNotLevelUp(PKM pk, bool skipChecks) => Method switch
     {
-        UseItemMale or LevelUpRecoilDamageMale => pk.Gender == 0,
-        UseItemFemale or LevelUpRecoilDamageFemale => pk.Gender == 1,
+        UseItemMale or LevelUpRecoilDamageMale => pk.Gender == 0 ? Valid : BadGender,
+        UseItemFemale or LevelUpRecoilDamageFemale => pk.Gender == 1 ? Valid : BadGender,
 
-        Trade or TradeHeldItem or TradeShelmetKarrablast => !pk.IsUntraded || skipChecks,
-        _ => true, // no conditions
-    };
-
-    private bool HasMetLevelIncreased(PKM pk, int lvl)
-    {
-        int origin = pk.Generation;
-        return origin switch
-        {
-            // No met data in RBY; No met data in GS, Crystal met data can be reset
-            1 or 2 => true,
-
-            // Pal Park / PokeTransfer updates Met Level
-            3 or 4 => pk.Format > origin || lvl > pk.Met_Level,
-
-            // 5=>6 and later transfers keep current level
-            >=5 => lvl >= Level && (!pk.IsNative || lvl > pk.Met_Level),
-
-            _ => false,
-        };
-    }
-
-    public EvoCriteria GetEvoCriteria(ushort species, byte form, byte lvl) => new()
-    {
-        Species = species,
-        Form = form,
-        LevelMax = lvl,
-        LevelMin = 0,
-        Method = Method,
+        Trade or TradeHeldItem or TradeShelmetKarrablast => !pk.IsUntraded || skipChecks ? Valid : Untraded,
+        _ => Valid, // no conditions
     };
 
     public static int GetAmpLowKeyResult(int n)

PKHeX.Core/Legality/Evolutions/Methods/EvolutionType.cs

@@ -78,6 +78,11 @@ public enum EvolutionType : byte
     UseItemFullMoon = 90, // Ursaluna
     UseMoveAgileStyle = 91, // Wyrdeer
     UseMoveStrongStyle = 92, // Overqwil
+
+    /// <summary>
+    /// Used as a placeholder sentinel for internal logic.
+    /// </summary>
+    Invalid = unchecked((byte)-1),
 }
 
 /// <summary>

PKHeX.Core/Legality/Evolutions/Reversal/EvolutionLink.cs

@@ -1,34 +1,9 @@
-using System;
-
 namespace PKHeX.Core;
 
 /// <summary>
 /// Links a <see cref="EvolutionMethod"/> to the source <see cref="Species"/> and <see cref="Form"/> that the method can be triggered from.
 /// </summary>
-public struct EvolutionLink
+public readonly record struct EvolutionLink(EvolutionMethod Method, ushort Species, byte Form)
 {
-    private Func<PKM, bool>? IsBanned = null;
-    public readonly EvolutionMethod Method;
-    public readonly ushort Species;
-    public readonly byte Form;
-
-    public EvolutionLink(ushort species, byte form, EvolutionMethod method)
-    {
-        Species = species;
-        Form = form;
-        Method = method;
-    }
-
     public bool IsEmpty => Species == 0;
-
-    public (ushort Species, byte Form) Tuple => (Species, Form);
-
-    public void Ban(Func<PKM, bool> check) => IsBanned = check;
-
-    /// <summary>
-    /// Checks if the <see cref="Method"/> is allowed.
-    /// </summary>
-    /// <param name="pk">Entity to check</param>
-    /// <returns>True if banned, false if allowed.</returns>
-    public bool IsEvolutionBanned(PKM pk) => IsBanned != null && IsBanned(pk);
 }

PKHeX.Core/Legality/Evolutions/Reversal/EvolutionNode.cs

@@ -26,20 +26,4 @@ public struct EvolutionNode
         else
             throw new InvalidOperationException($"{nameof(EvolutionNode)} already has two links.");
     }
-
-    /// <summary>
-    /// Registers a function that disallows the reverse evolution link from being valid if the <see cref="func"/> is not satisfied.
-    /// </summary>
-    /// <param name="func">Function that checks if the link should be allowed as an evolution path.</param>
-    public void Ban(Func<PKM, bool> func)
-    {
-        ref var first = ref First;
-        if (first.IsEmpty)
-            return;
-        first.Ban(func);
-        ref var second = ref Second;
-        if (second.IsEmpty)
-            return;
-        second.Ban(func);
-    }
 }

PKHeX.Core/Legality/Evolutions/Reversal/EvolutionReversal.cs

@@ -0,0 +1,84 @@
+using System;
+
+namespace PKHeX.Core;
+
+/// <summary>
+/// Evolution Reversal logic
+/// </summary>
+public static class EvolutionReversal
+{
+    /// <summary>
+    /// Reverses from current state to see what evolutions the <see cref="pk"/> may have existed as.
+    /// </summary>
+    /// <param name="lineage">Evolution Method lineage reversal object</param>
+    /// <param name="result">Result storage</param>
+    /// <param name="species">Species to devolve from</param>
+    /// <param name="form">Form to devolve from</param>
+    /// <param name="pk">Entity reference to sanity check evolutions with</param>
+    /// <param name="levelMin">Minimum level the entity may exist as</param>
+    /// <param name="levelMax">Maximum the entity may exist as</param>
+    /// <param name="stopSpecies">Species ID that should be the last node, if at all. Provide 0 to fully devolve.</param>
+    /// <param name="skipChecks">Option to bypass some evolution criteria</param>
+    /// <returns>Reversed evolution lineage, with the lowest index being the current species-form.</returns>
+    public static int Devolve(this IEvolutionLookup lineage, Span<EvoCriteria> result, ushort species, byte form,
+        PKM pk, byte levelMin, byte levelMax, ushort stopSpecies, bool skipChecks)
+    {
+        // Store our results -- trim at the end when we place it on the heap.
+        var head = result[0] = new EvoCriteria { Species = species, Form = form, LevelMax = levelMax };
+        int ctr = Devolve(lineage, result, head, pk, levelMax, levelMin, skipChecks, stopSpecies);
+        EvolutionUtil.CleanDevolve(result[..ctr], levelMin);
+        return ctr;
+    }
+
+    private static int Devolve(IEvolutionLookup lineage, Span<EvoCriteria> result, EvoCriteria head,
+        PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, ushort stopSpecies)
+    {
+        // There aren't any circular evolution paths, and all lineages have at most 3 evolutions total.
+        // There aren't any convergent evolution paths, so only yield the first connection.
+        int ctr = 1; // count in the 'evos' span
+        while (head.Species != stopSpecies)
+        {
+            var node = lineage[head.Species, head.Form];
+            if (!node.TryDevolve(pk, currentMaxLevel, levelMin, skipChecks, out var x))
+                return ctr;
+
+            result[ctr++] = x;
+            currentMaxLevel -= x.LevelUpRequired;
+        }
+        return ctr;
+    }
+
+    public static bool TryDevolve(this EvolutionNode node, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        // Multiple methods can exist to devolve to the same species-form.
+        // The first method is less restrictive (no LevelUp req), if two {level/other} methods exist.
+        for (int i = 0; i < 2; i++)
+        {
+            ref var link = ref i == 0 ? ref node.First : ref node.Second;
+            if (link.IsEmpty)
+                break;
+
+            var chk = link.Method.Check(pk, currentMaxLevel, levelMin, skipChecks);
+            if (chk != EvolutionCheckResult.Valid)
+                continue;
+
+            result = Create(link, currentMaxLevel);
+            return true;
+        }
+
+        result = default;
+        return false;
+    }
+
+    private static EvoCriteria Create(EvolutionLink link, byte currentMaxLevel) => new()
+    {
+        Species = link.Species,
+        Form = link.Form,
+        LevelMax = currentMaxLevel,
+        Method = link.Method.Method,
+
+        // Temporarily store these and overwrite them when we clean the list.
+        LevelMin = link.Method.Level,
+        LevelUpRequired = link.Method.RequiresLevelUp ? (byte)1 : (byte)0,
+    };
+}

PKHeX.Core/Legality/Evolutions/Reversal/EvolutionReverseLookup.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using System.Diagnostics;
 
 namespace PKHeX.Core;
 
@@ -9,36 +8,56 @@ namespace PKHeX.Core;
 public sealed class EvolutionReverseLookup : IEvolutionLookup
 {
     private readonly EvolutionNode[] Nodes;
-    private readonly int MaxSpecies;
     private readonly Dictionary<int, int> KeyLookup;
+    private readonly ushort MaxSpecies;
 
-    public EvolutionReverseLookup(IEnumerable<(int Key, EvolutionLink Value)> links, int maxSpecies)
+    public EvolutionReverseLookup(ushort maxSpecies)
     {
-        MaxSpecies = maxSpecies;
+        Nodes = new EvolutionNode[maxSpecies * 2];
         KeyLookup = new Dictionary<int, int>(maxSpecies);
-        var nodes = new EvolutionNode[maxSpecies * 2];
-        int ctr = maxSpecies + 1;
-        foreach (var (key, value) in links)
-        {
-            var index = key <= MaxSpecies ? key : KeyLookup.TryGetValue(key, out var x) ? x : KeyLookup[key] = ctr++;
-            ref var node = ref nodes[index];
-            node.Add(value);
-        }
-        Nodes = nodes;
-        Debug.Assert(KeyLookup.Count < maxSpecies);
+        MaxSpecies = maxSpecies;
     }
 
-    private int GetIndex(int key)
+    private void Register(EvolutionLink link, ushort species)
     {
-        if (key <= MaxSpecies)
-            return key;
+        ref var node = ref Nodes[species];
+        node.Add(link);
+    }
+
+    public void Register(EvolutionLink link, ushort species, byte form)
+    {
+        if (form == 0)
+        {
+            Register(link, species);
+            return;
+        }
+
+        int key = GetKey(species, form);
+        if (!KeyLookup.TryGetValue(key, out var index))
+        {
+            index = Nodes.Length - KeyLookup.Count - 1;
+            KeyLookup.Add(key, index);
+        }
+
+        ref var node = ref Nodes[index];
+        node.Add(link);
+    }
+
+    private int GetIndex(ushort species, byte form)
+    {
+        if (species > MaxSpecies)
+            return 0;
+        if (form == 0)
+            return species;
+        int key = GetKey(species, form);
         return KeyLookup.TryGetValue(key, out var index) ? index : 0;
     }
 
-    public ref EvolutionNode this[int key] => ref Nodes[GetIndex(key)];
+    private static int GetKey(ushort species, byte form) => species | form << 11;
+    public ref EvolutionNode this[ushort species, byte form] => ref Nodes[GetIndex(species, form)];
 }
 
 public interface IEvolutionLookup
 {
-    ref EvolutionNode this[int key] { get; }
+    ref EvolutionNode this[ushort species, byte form] { get; }
 }

PKHeX.Core/Legality/Evolutions/Reversal/EvolutionReversePersonal.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public sealed class EvolutionReversePersonal : IEvolutionReverse
+{
+    public IEvolutionLookup Lineage { get; }
+
+    public EvolutionReversePersonal(EvolutionMethod[][] entries, IPersonalTable t)
+    {
+        Lineage = GetLineage(t, entries);
+    }
+
+    private static EvolutionReverseLookup GetLineage(IPersonalTable t, EvolutionMethod[][] entries)
+    {
+        var maxSpecies = t.MaxSpeciesID;
+        var lineage = new EvolutionReverseLookup(maxSpecies);
+        for (ushort sSpecies = 1; sSpecies <= maxSpecies; sSpecies++)
+        {
+            var fc = t[sSpecies].FormCount;
+            for (byte sForm = 0; sForm < fc; sForm++)
+            {
+                var index = t.GetFormIndex(sSpecies, sForm);
+                foreach (var evo in entries[index])
+                {
+                    var dSpecies = evo.Species;
+                    if (dSpecies == 0)
+                        break;
+
+                    var dForm = evo.GetDestinationForm(sForm);
+                    var link = new EvolutionLink(evo, sSpecies, sForm);
+                    lineage.Register(link, dSpecies, dForm);
+                }
+            }
+        }
+        return lineage;
+    }
+
+    public EvolutionNode GetReverse(ushort species, byte form) => Lineage[species, form];
+
+    public IEnumerable<(ushort Species, byte Form)> GetPreEvolutions(ushort species, byte form)
+    {
+        var node = Lineage[species, form];
+
+        // No convergent evolutions; first method is enough.
+        var s = node.First;
+        if (s.Species == 0)
+            yield break;
+
+        var preEvolutions = GetPreEvolutions(s.Species, s.Form);
+        foreach (var preEvo in preEvolutions)
+            yield return preEvo;
+        yield return (s.Species, s.Form);
+    }
+
+    public int Devolve(Span<EvoCriteria> result, ushort species, byte form, PKM pk, byte levelMin, byte levelMax, ushort stopSpecies,
+        bool skipChecks)
+    {
+        return Lineage.Devolve(result, species, form, pk, levelMin, levelMax, stopSpecies, skipChecks);
+    }
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        var node = Lineage[head.Species, head.Form];
+        return node.TryDevolve(pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+}

PKHeX.Core/Legality/Evolutions/Reversal/EvolutionReverseSpecies.cs

@@ -0,0 +1,70 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public sealed class EvolutionReverseSpecies : IEvolutionReverse
+{
+    public EvolutionReverseLookup Lineage { get; }
+
+    public EvolutionReverseSpecies(EvolutionMethod[][] entries, IPersonalTable t)
+    {
+        Lineage = GetLineage(t, entries);
+    }
+
+    private static EvolutionReverseLookup GetLineage(IPersonalTable t, EvolutionMethod[][] entries)
+    {
+        var maxSpecies = t.MaxSpeciesID;
+        var lineage = new EvolutionReverseLookup(maxSpecies);
+        for (ushort sSpecies = 1; sSpecies <= maxSpecies; sSpecies++)
+        {
+            var fc = t[sSpecies].FormCount;
+            for (byte sForm = 0; sForm < fc; sForm++)
+            {
+                foreach (var evo in entries[sSpecies])
+                {
+                    var dSpecies = evo.Species;
+                    if (dSpecies == 0)
+                        break;
+
+                    var dForm = sSpecies == (int)Species.Espurr && evo.Method == EvolutionType.LevelUpFormFemale1
+                        ? (byte)1
+                        : sForm;
+                    var link = new EvolutionLink(evo, sSpecies, sForm);
+                    lineage.Register(link, dSpecies, dForm);
+                }
+            }
+        }
+
+        return lineage;
+    }
+
+    public EvolutionNode GetReverse(ushort species, byte form) => Lineage[species, form];
+
+    public IEnumerable<(ushort Species, byte Form)> GetPreEvolutions(ushort species, byte form)
+    {
+        var node = Lineage[species, form];
+
+        // No convergent evolutions; first method is enough.
+        var s = node.First;
+        if (s.Species == 0)
+            yield break;
+
+        var preEvolutions = GetPreEvolutions(s.Species, s.Form);
+        foreach (var preEvo in preEvolutions)
+            yield return preEvo;
+        yield return (s.Species, s.Form);
+    }
+
+    public int Devolve(Span<EvoCriteria> result, ushort species, byte form, PKM pk, byte levelMin, byte levelMax, ushort stopSpecies,
+        bool skipChecks)
+    {
+        return Lineage.Devolve(result, species, form, pk, levelMin, levelMax, stopSpecies, skipChecks);
+    }
+
+    public bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result)
+    {
+        var node = Lineage[head.Species, head.Form];
+        return node.TryDevolve(pk, currentMaxLevel, levelMin, skipChecks, out result);
+    }
+}

PKHeX.Core/Legality/Evolutions/Reversal/IEvolutionReverse.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Collections.Generic;
+
+namespace PKHeX.Core;
+
+public interface IEvolutionReverse
+{
+    EvolutionNode GetReverse(ushort species, byte form);
+
+    /// <summary>
+    /// Gets all species the <see cref="species"/>-<see cref="form"/> can evolve from, yielded in order of increasing evolution stage.
+    /// </summary>
+    /// <param name="species">Species ID</param>
+    /// <param name="form">Form ID</param>
+    /// <returns>Enumerable of species IDs (with the Form IDs included, left shifted by 11).</returns>
+    IEnumerable<(ushort Species, byte Form)> GetPreEvolutions(ushort species, byte form);
+
+    int Devolve(Span<EvoCriteria> result, ushort species, byte form, PKM pk, byte levelMin, byte levelMax, ushort stopSpecies, bool skipChecks);
+
+    bool TryDevolve(ISpeciesForm head, PKM pk, byte currentMaxLevel, byte levelMin, bool skipChecks, out EvoCriteria result);
+}

PKHeX.Core/Legality/LearnSource/Group/ILearnGroup.cs

@@ -7,6 +7,11 @@ namespace PKHeX.Core;
 /// </summary>
 public interface ILearnGroup
 {
+    /// <summary>
+    /// Gets the maximum move ID that this group can learn.
+    /// </summary>
+    ushort MaxMoveID { get; }
+
     /// <summary>
     /// Gets the next group to traverse to continue checking moves.
     /// </summary>

PKHeX.Core/Legality/LearnSource/Group/LearnGroup1.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup1 : ILearnGroup
 {
     public static readonly LearnGroup1 Instance = new();
     private const int Generation = 1;
+    public ushort MaxMoveID => Legal.MaxMoveID_1;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => pk.Context switch
     {

PKHeX.Core/Legality/LearnSource/Group/LearnGroup2.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup2 : ILearnGroup
 {
     public static readonly LearnGroup2 Instance = new();
     private const int Generation = 2;
+    public ushort MaxMoveID => Legal.MaxMoveID_2;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => pk.Context switch
     {

PKHeX.Core/Legality/LearnSource/Group/LearnGroup3.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup3 : ILearnGroup
 {
     public static readonly LearnGroup3 Instance = new();
     private const int Generation = 3;
+    public ushort MaxMoveID => Legal.MaxMoveID_3;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => null; // Gen3 is the end of the line!
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedGen3;

PKHeX.Core/Legality/LearnSource/Group/LearnGroup4.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup4 : ILearnGroup
 {
     public static readonly LearnGroup4 Instance = new();
     private const int Generation = 4;
+    public ushort MaxMoveID => Legal.MaxMoveID_4;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => enc.Generation is Generation ? null : LearnGroup3.Instance;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedGen4;

PKHeX.Core/Legality/LearnSource/Group/LearnGroup5.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup5 : ILearnGroup
 {
     public static readonly LearnGroup5 Instance = new();
     private const int Generation = 5;
+    public ushort MaxMoveID => Legal.MaxMoveID_5;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => enc.Generation is Generation ? null : LearnGroup4.Instance;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedGen5;

PKHeX.Core/Legality/LearnSource/Group/LearnGroup6.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup6 : ILearnGroup
 {
     public static readonly LearnGroup6 Instance = new();
     private const int Generation = 6;
+    public ushort MaxMoveID => Legal.MaxMoveID_6_AO;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => enc.Generation is Generation ? null : LearnGroup5.Instance;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedGen6;

PKHeX.Core/Legality/LearnSource/Group/LearnGroup7.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup7 : ILearnGroup
 {
     public static readonly LearnGroup7 Instance = new();
     private const int Generation = 7;
+    public ushort MaxMoveID => Legal.MaxMoveID_7;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => enc.Generation switch
     {
@@ -167,7 +168,7 @@ public sealed class LearnGroup7 : ILearnGroup
         }
 
         // Check all forms
-        var inst = LearnSource6AO.Instance;
+        var inst = LearnSource7USUM.Instance;
         if (!inst.TryGetPersonal(evo.Species, evo.Form, out var pi))
             return;
 

PKHeX.Core/Legality/LearnSource/Group/LearnGroup7b.cs

@@ -9,6 +9,7 @@ public sealed class LearnGroup7b : ILearnGroup
 {
     public static readonly LearnGroup7b Instance = new();
     private const int Generation = 7;
+    public ushort MaxMoveID => Legal.MaxMoveID_7b;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => null;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedLGPE;

PKHeX.Core/Legality/LearnSource/Group/LearnGroup8.cs

@@ -10,6 +10,7 @@ public sealed class LearnGroup8 : ILearnGroup
     public static readonly LearnGroup8 Instance = new();
     private const int Generation = 8;
     private const EntityContext Context = EntityContext.Gen8;
+    public ushort MaxMoveID => Legal.MaxMoveID_8;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option)
     {
@@ -48,7 +49,13 @@ public sealed class LearnGroup8 : ILearnGroup
         if (option is not LearnOption.Current && types.HasFlag(MoveSourceType.Encounter) && pk.IsOriginalMovesetDeleted() && enc is EncounterEgg { Generation: Generation } egg)
             CheckEncounterMoves(result, current, egg);
 
-        return MoveResult.AllParsed(result);
+        if (MoveResult.AllParsed(result))
+            return true;
+
+        var home = LearnGroupHOME.Instance;
+        if (option != LearnOption.HOME && home.HasVisited(pk, history))
+            return home.Check(result, current, pk, history, enc, types);
+        return false;
     }
 
     private static void CheckSharedMoves(Span<MoveResult> result, ReadOnlySpan<ushort> current, EvoCriteria evo)

PKHeX.Core/Legality/LearnSource/Group/LearnGroup8a.cs

@@ -10,6 +10,7 @@ public sealed class LearnGroup8a : ILearnGroup
     public static readonly LearnGroup8a Instance = new();
     private const int Generation = 8;
     private const EntityContext Context = EntityContext.Gen8a;
+    public ushort MaxMoveID => Legal.MaxMoveID_8a;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => null;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedPLA;
@@ -21,7 +22,13 @@ public sealed class LearnGroup8a : ILearnGroup
         for (var i = 0; i < evos.Length; i++)
             Check(result, current, pk, evos[i], i);
 
-        return MoveResult.AllParsed(result);
+        if (MoveResult.AllParsed(result))
+            return true;
+
+        var home = LearnGroupHOME.Instance;
+        if (option != LearnOption.HOME && home.HasVisited(pk, history))
+            return home.Check(result, current, pk, history, enc, types);
+        return false;
     }
 
     private static void Check(Span<MoveResult> result, ReadOnlySpan<ushort> current, PKM pk, EvoCriteria evo, int stage)

PKHeX.Core/Legality/LearnSource/Group/LearnGroup8b.cs

@@ -10,6 +10,7 @@ public sealed class LearnGroup8b : ILearnGroup
     public static readonly LearnGroup8b Instance = new();
     private const int Generation = 8;
     private const EntityContext Context = EntityContext.Gen8b;
+    public ushort MaxMoveID => Legal.MaxMoveID_8b;
 
     public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => null;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedBDSP;
@@ -26,7 +27,13 @@ public sealed class LearnGroup8b : ILearnGroup
 
         CheckSharedMoves(result, current, evos[0]);
 
-        return MoveResult.AllParsed(result);
+        if (MoveResult.AllParsed(result))
+            return true;
+
+        var home = LearnGroupHOME.Instance;
+        if (option != LearnOption.HOME && home.HasVisited(pk, history))
+            return home.Check(result, current, pk, history, enc, types);
+        return false;
     }
 
     private static void CheckSharedMoves(Span<MoveResult> result, ReadOnlySpan<ushort> current, EvoCriteria evo)

PKHeX.Core/Legality/LearnSource/Group/LearnGroup9.cs

@@ -10,12 +10,9 @@ public sealed class LearnGroup9 : ILearnGroup
     public static readonly LearnGroup9 Instance = new();
     private const int Generation = 9;
     private const EntityContext Context = EntityContext.Gen9;
+    public ushort MaxMoveID => Legal.MaxMoveID_9;
 
-    public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option)
-    {
-        return null;
-    }
-
+    public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => null;
     public bool HasVisited(PKM pk, EvolutionHistory history) => history.HasVisitedGen9;
 
     public bool Check(Span<MoveResult> result, ReadOnlySpan<ushort> current, PKM pk, EvolutionHistory history,
@@ -33,7 +30,13 @@ public sealed class LearnGroup9 : ILearnGroup
         if (option is not LearnOption.Current && types.HasFlag(MoveSourceType.Encounter) && pk.IsOriginalMovesetDeleted() && enc is EncounterEgg { Generation: Generation } egg)
             CheckEncounterMoves(result, current, egg);
 
-        return MoveResult.AllParsed(result);
+        if (MoveResult.AllParsed(result))
+            return true;
+
+        var home = LearnGroupHOME.Instance;
+        if (option != LearnOption.HOME && home.HasVisited(pk, history))
+            return home.Check(result, current, pk, history, enc, types);
+        return false;
     }
 
     private static void CheckSharedMoves(Span<MoveResult> result, ReadOnlySpan<ushort> current, EvoCriteria evo)
@@ -129,7 +132,7 @@ public sealed class LearnGroup9 : ILearnGroup
         }
 
         // Check all forms
-        var inst = LearnSource6AO.Instance;
+        var inst = LearnSource9SV.Instance;
         if (!inst.TryGetPersonal(evo.Species, evo.Form, out var pi))
             return;
 

PKHeX.Core/Legality/LearnSource/Group/LearnGroupHOME.cs

@@ -0,0 +1,239 @@
+using System;
+using System.Buffers;
+
+namespace PKHeX.Core;
+
+/// <summary>
+/// Encapsulates logic for HOME's Move Relearner feature.
+/// </summary>
+/// <remarks>
+/// If the Entity knew a move at any point in its history, it can be relearned if the current format can learn it.
+/// </remarks>
+public class LearnGroupHOME : ILearnGroup
+{
+    public static readonly LearnGroupHOME Instance = new();
+    public ushort MaxMoveID => 0;
+
+    public ILearnGroup? GetPrevious(PKM pk, EvolutionHistory history, IEncounterTemplate enc, LearnOption option) => null;
+    public bool HasVisited(PKM pk, EvolutionHistory history) => pk is IHomeTrack { HasTracker: true } || !ParseSettings.IgnoreTransferIfNoTracker;
+
+    public bool Check(Span<MoveResult> result, ReadOnlySpan<ushort> current, PKM pk, EvolutionHistory history,
+        IEncounterTemplate enc, MoveSourceType types = MoveSourceType.All, LearnOption option = LearnOption.HOME)
+    {
+        var context = pk.Context;
+        if (context == EntityContext.None)
+            return false;
+
+        var local = GetCurrent(context);
+        var evos = history.Get(context);
+        if (history.HasVisitedGen9 && pk is not PK9)
+        {
+            var instance = LearnGroup9.Instance;
+            instance.Check(result, current, pk, history, enc, types, option);
+            if (CleanPurge(result, current, pk, types, local, evos))
+                return true;
+        }
+        if (history.HasVisitedSWSH && pk is not PK8)
+        {
+            var instance = LearnGroup8.Instance;
+            instance.Check(result, current, pk, history, enc, types, option);
+            if (CleanPurge(result, current, pk, types, local, evos))
+                return true;
+        }
+        if (history.HasVisitedPLA && pk is not PA8)
+        {
+            var instance = LearnGroup8a.Instance;
+            instance.Check(result, current, pk, history, enc, types, option);
+            if (CleanPurge(result, current, pk, types, local, evos))
+                return true;
+        }
+        if (history.HasVisitedBDSP && pk is not PB8)
+        {
+            var instance = LearnGroup8b.Instance;
+            instance.Check(result, current, pk, history, enc, types, option);
+            if (CleanPurge(result, current, pk, types, local, evos))
+                return true;
+        }
+
+        if (TryAddSpecialCaseMoves(pk.Species, result, current))
+            return true;
+
+        if (history.HasVisitedLGPE)
+        {
+            var instance = LearnGroup7b.Instance;
+            instance.Check(result, current, pk, history, enc, types, option);
+            if (CleanPurge(result, current, pk, types, local, evos))
+                return true;
+        }
+        else if (history.HasVisitedGen7)
+        {
+            ILearnGroup instance = LearnGroup7.Instance;
+            while (true)
+            {
+                instance.Check(result, current, pk, history, enc, types, option);
+                if (CleanPurge(result, current, pk, types, local, evos))
+                    return true;
+                var prev = instance.GetPrevious(pk, history, enc, option);
+                if (prev is null)
+                    break;
+                instance = prev;
+            }
+        }
+        return false;
+    }
+
+    /// <summary>
+    /// Scan the results and remove any that are not valid for the game <see cref="local"/> game.
+    /// </summary>
+    /// <returns>True if all results are valid.</returns>
+    private static bool CleanPurge(Span<MoveResult> result, ReadOnlySpan<ushort> current, PKM pk, MoveSourceType types, IHomeSource local, ReadOnlySpan<EvoCriteria> evos)
+    {
+        // The logic used to update the results did not check if the move was actually able to be learned in the local game.
+        // Double check the results and remove any that are not valid for the local game.
+        // SW/SH will continue to iterate downwards to previous groups after HOME is checked, so we can exactly check via Environment.
+        for (int i = 0; i < result.Length; i++)
+        {
+            ref var r = ref result[i];
+            if (!r.Valid || r.Generation == 0)
+                continue;
+
+            if (r.Info.Environment == local.Environment)
+                continue;
+
+            // Check if any evolution in the local context can learn the move via HOME instruction. If none can, the move is invalid.
+            var move = current[i];
+            if (move == 0)
+                continue;
+
+            bool valid = false;
+            foreach (var evo in evos)
+            {
+                var chk = local.GetCanLearnHOME(pk, evo, move, types);
+                if (chk.Method != LearnMethod.None)
+                    valid = true;
+            }
+            if (!valid)
+                r = default;
+        }
+
+        return MoveResult.AllParsed(result);
+    }
+
+    /// <summary>
+    /// Get the current HOME source for the given context.
+    /// </summary>
+    /// <exception cref="ArgumentOutOfRangeException"></exception>
+    private static IHomeSource GetCurrent(EntityContext context) => context switch
+    {
+        EntityContext.Gen8 => LearnSource8SWSH.Instance,
+        EntityContext.Gen8a => LearnSource8LA.Instance,
+        EntityContext.Gen8b => LearnSource8BDSP.Instance,
+        EntityContext.Gen9 => LearnSource9SV.Instance,
+        _ => throw new ArgumentOutOfRangeException(nameof(context), context, null),
+    };
+
+    public void GetAllMoves(Span<bool> result, PKM pk, EvolutionHistory history, IEncounterTemplate enc,
+        MoveSourceType types = MoveSourceType.All, LearnOption option = LearnOption.HOME)
+    {
+        option = LearnOption.HOME;
+        var local = GetCurrent(pk.Context);
+        var evos = history.Get(pk.Context);
+
+        // Check all adjacent games
+        if (history.HasVisitedGen9 && pk is not PK9)
+            RentLoopGetAll(LearnGroup9. Instance, result, pk, history, enc, types, option, evos, local);
+        if (history.HasVisitedSWSH && pk is not PK8)
+            RentLoopGetAll(LearnGroup8. Instance, result, pk, history, enc, types, option, evos, local);
+        if (history.HasVisitedPLA && pk is not PA8)
+            RentLoopGetAll(LearnGroup8a.Instance, result, pk, history, enc, types, option, evos, local);
+        if (history.HasVisitedBDSP && pk is not PB8)
+            RentLoopGetAll(LearnGroup8b.Instance, result, pk, history, enc, types, option, evos, local);
+
+        AddSpecialCaseMoves(pk.Species, result);
+
+        // Looking backwards before HOME
+        if (history.HasVisitedLGPE)
+        {
+            RentLoopGetAll(LearnGroup7b.Instance, result, pk, history, enc, types, option, evos, local);
+        }
+        else if (history.HasVisitedGen7)
+        {
+            ILearnGroup instance = LearnGroup7.Instance;
+            while (true)
+            {
+                RentLoopGetAll(instance, result, pk, history, enc, types, option, evos, local);
+                var prev = instance.GetPrevious(pk, history, enc, option);
+                if (prev is null)
+                    break;
+                instance = prev;
+            }
+        }
+    }
+
+    private static void RentLoopGetAll<T>(T instance, Span<bool> result, PKM pk, EvolutionHistory history,
+        IEncounterTemplate enc,
+        MoveSourceType types, LearnOption option, ReadOnlySpan<EvoCriteria> evos, IHomeSource local) where T : ILearnGroup
+    {
+        var length = instance.MaxMoveID;
+        var rent = ArrayPool<bool>.Shared.Rent(length);
+        var temp = rent.AsSpan(0, length);
+        instance.GetAllMoves(temp, pk, history, enc, types, option);
+        LoopMerge(result, pk, evos, types, local, temp);
+        temp.Clear();
+        ArrayPool<bool>.Shared.Return(rent);
+    }
+
+    /// <summary>
+    /// For each move that is possible to learn in another game, check if it is possible to learn in the current game.
+    /// </summary>
+    /// <param name="result">Resulting array of moves that are possible to learn in the current game.</param>
+    /// <param name="pk">Entity to check.</param>
+    /// <param name="evos">Evolutions to check.</param>
+    /// <param name="types">Move source types to check.</param>
+    /// <param name="dest">Destination game to check.</param>
+    /// <param name="temp">Temporary array of moves that are possible to learn in the checked game.</param>
+    private static void LoopMerge(Span<bool> result, PKM pk, ReadOnlySpan<EvoCriteria> evos, MoveSourceType types, IHomeSource dest, Span<bool> temp)
+    {
+        var length = Math.Min(result.Length, temp.Length);
+        for (ushort move = 0; move < length; move++)
+        {
+            if (!temp[move])
+                continue; // not possible to learn in other game
+            if (result[move])
+                continue; // already possible to learn in current game
+
+            foreach (var evo in evos)
+            {
+                var chk = dest.GetCanLearnHOME(pk, evo, move, types);
+                if (chk.Method == LearnMethod.None)
+                    continue;
+                result[move] = true;
+                break;
+            }
+        }
+    }
+
+    private static bool TryAddSpecialCaseMoves(ushort species, Span<MoveResult> result, ReadOnlySpan<ushort> current)
+    {
+        if (IsPikachuLine(species))
+        {
+            var index = current.IndexOf((ushort)Move.VoltTackle);
+            if (index == -1)
+                return false;
+            ref var move = ref result[index];
+            if (move.Valid)
+                return false;
+            move = new MoveResult(LearnMethod.Shared, LearnEnvironment.HOME);
+            return MoveResult.AllValid(result);
+        }
+        return false;
+    }
+
+    private static void AddSpecialCaseMoves(ushort species, Span<bool> result)
+    {
+        if (IsPikachuLine(species))
+            result[(int)Move.VoltTackle] = true;
+    }
+
+    private static bool IsPikachuLine(ushort species) => species is (int)Species.Raichu or (int)Species.Pikachu or (int)Species.Pichu;
+}

PKHeX.Core/Legality/LearnSource/LearnEnvironment.cs

@@ -19,6 +19,7 @@ public enum LearnEnvironment : byte
     /* Gen7 */ SM, USUM, GG,
     /* Gen8 */ SWSH, BDSP, PLA,
     /* Gen9 */ SV,
+    HOME,
 }
 
 /// <summary>

PKHeX.Core/Legality/LearnSource/Sources/IHomeSource.cs

@@ -0,0 +1,7 @@
+namespace PKHeX.Core;
+
+public interface IHomeSource
+{
+    LearnEnvironment Environment { get; }
+    MoveLearnInfo GetCanLearnHOME(PKM pk, EvoCriteria evo, ushort move, MoveSourceType types = MoveSourceType.All);
+}

PKHeX.Core/Legality/LearnSource/Sources/LearnSource4HGSS.cs

@@ -77,11 +77,11 @@ public sealed class LearnSource4HGSS : ILearnSource<PersonalInfo4>, IEggSource
 
     private static bool GetIsEnhancedTutor(EvoCriteria evo, ISpeciesForm current, ushort move, LearnOption option) => evo.Species is (int)Species.Rotom && move switch
     {
-        (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-        (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-        (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-        (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-        (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+        (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+        (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+        (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+        (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+        (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
         _ => false,
     };
 

PKHeX.Core/Legality/LearnSource/Sources/LearnSource4Pt.cs

@@ -75,11 +75,11 @@ public sealed class LearnSource4Pt : LearnSource4, ILearnSource<PersonalInfo4>,
 
     private static bool GetIsEnhancedTutor(EvoCriteria evo, ISpeciesForm current, ushort move, LearnOption option) => evo.Species is (int)Species.Rotom && move switch
     {
-        (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-        (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-        (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-        (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-        (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+        (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+        (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+        (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+        (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+        (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
         _ => false,
     };
 

PKHeX.Core/Legality/LearnSource/Sources/LearnSource5B2W2.cs

@@ -73,11 +73,11 @@ public sealed class LearnSource5B2W2 : LearnSource5, ILearnSource<PersonalInfo5B
         (int)Species.Meloetta => move is (int)Move.RelicSong,
         (int)Species.Rotom => move switch
         {
-            (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-            (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-            (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-            (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-            (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+            (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+            (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+            (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+            (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+            (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
             _ => false,
         },
         _ => false,

PKHeX.Core/Legality/LearnSource/Sources/LearnSource5BW.cs

@@ -70,11 +70,11 @@ public sealed class LearnSource5BW : LearnSource5, ILearnSource<PersonalInfo5BW>
         (int)Species.Meloetta => move is (int)Move.RelicSong,
         (int)Species.Rotom => move switch
         {
-            (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-            (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-            (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-            (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-            (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+            (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+            (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+            (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+            (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+            (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
             _ => false,
         },
         _ => false,

PKHeX.Core/Legality/LearnSource/Sources/LearnSource6AO.cs

@@ -74,11 +74,11 @@ public sealed class LearnSource6AO : ILearnSource<PersonalInfo6AO>, IEggSource
         (int)Species.Meloetta => move is (int)Move.RelicSong,
         (int)Species.Rotom => move switch
         {
-            (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-            (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-            (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-            (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-            (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+            (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+            (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+            (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+            (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+            (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
             _ => false,
         },
         _ => false,

PKHeX.Core/Legality/LearnSource/Sources/LearnSource6XY.cs

@@ -71,11 +71,11 @@ public sealed class LearnSource6XY : ILearnSource<PersonalInfo6XY>, IEggSource
         (int)Species.Meloetta => move is (int)Move.RelicSong,
         (int)Species.Rotom => move switch
         {
-            (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-            (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-            (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-            (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-            (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+            (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+            (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+            (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+            (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+            (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
             _ => false,
         },
         _ => false,

PKHeX.Core/Legality/LearnSource/Sources/LearnSource7SM.cs

@@ -72,19 +72,19 @@ public sealed class LearnSource7SM : ILearnSource<PersonalInfo7>, IEggSource
         (int)Species.Pikachu or (int)Species.Raichu => move is (int)Move.VoltTackle,
         (int)Species.Necrozma => move switch
         {
-            (int)Move.SunsteelStrike => (option == LearnOption.AtAnyTime || current.Form == 1), // Sun w/ Solgaleo
-            (int)Move.MoongeistBeam  => (option == LearnOption.AtAnyTime || current.Form == 2), // Moon w/ Lunala
+            (int)Move.SunsteelStrike => option.IsPast() || current.Form == 1, // Sun w/ Solgaleo
+            (int)Move.MoongeistBeam  => option.IsPast() || current.Form == 2, // Moon w/ Lunala
             _ => false,
         },
         (int)Species.Keldeo   => move is (int)Move.SecretSword,
         (int)Species.Meloetta => move is (int)Move.RelicSong,
         (int)Species.Rotom => move switch
         {
-            (int)Move.Overheat  => option == LearnOption.AtAnyTime || current.Form == 1,
-            (int)Move.HydroPump => option == LearnOption.AtAnyTime || current.Form == 2,
-            (int)Move.Blizzard  => option == LearnOption.AtAnyTime || current.Form == 3,
-            (int)Move.AirSlash  => option == LearnOption.AtAnyTime || current.Form == 4,
-            (int)Move.LeafStorm => option == LearnOption.AtAnyTime || current.Form == 5,
+            (int)Move.Overheat  => option.IsPast() || current.Form == 1,
+            (int)Move.HydroPump => option.IsPast() || current.Form == 2,
+            (int)Move.Blizzard  => option.IsPast() || current.Form == 3,
+            (int)Move.AirSlash  => option.IsPast() || current.Form == 4,
+            (int)Move.LeafStorm => option.IsPast() || current.Form == 5,
             _ => false,
         },
         (int)Species.Zygarde => move is (int)Move.ExtremeSpeed or (int)Move.DragonDance or (int)Move.ThousandArrows or (int)Move.ThousandWaves or (int)Move.CoreEnforcer,