using System;
using System.Collections.Generic;
using static System.Buffers.Binary.BinaryPrimitives;

namespace PKHeX.Core;

/// <inheritdoc cref="EncounterArea" />
/// <summary>
/// <see cref="GameVersion.Gen4"/> encounter area
/// </summary>
public sealed record EncounterArea4 : EncounterArea
{
    public readonly int Rate;
    public readonly GroundTileAllowed GroundTile;
    public readonly EncounterSlot4[] Slots;

    protected override IReadOnlyList<EncounterSlot4> Raw => Slots;

    public static EncounterArea4[] GetAreas(BinLinkerAccessor input, GameVersion game)
    {
        var result = new EncounterArea4[input.Length];
        for (int i = 0; i < result.Length; i++)
            result[i] = new EncounterArea4(input[i], game);
        return result;
    }

    private EncounterArea4(ReadOnlySpan<byte> data, GameVersion game) : base(game)
    {
        Location = ReadUInt16LittleEndian(data);
        Type = (SlotType)data[2];
        Rate = data[3];
        // although GroundTilePermission flags are 32bit, none have values > 16bit.
        GroundTile = (GroundTileAllowed)ReadUInt16LittleEndian(data[4..]);

        Slots = ReadRegularSlots(data);
    }

    private EncounterSlot4[] ReadRegularSlots(ReadOnlySpan<byte> data)
    {
        const int size = 10;
        int count = (data.Length - 6) / size;
        var slots = new EncounterSlot4[count];
        for (int i = 0; i < slots.Length; i++)
        {
            int offset = 6 + (size * i);
            var entry = data.Slice(offset, size);
            slots[i] = ReadRegularSlot(entry);
        }

        return slots;
    }

    private EncounterSlot4 ReadRegularSlot(ReadOnlySpan<byte> entry)
    {
        ushort species = ReadUInt16LittleEndian(entry);
        byte form = entry[2];
        byte slotNum = entry[3];
        byte min = entry[4];
        byte max = entry[5];
        byte mpi = entry[6];
        byte mpc = entry[7];
        byte sti = entry[8];
        byte stc = entry[9];
        return new EncounterSlot4(this, species, form, min, max, slotNum, mpi, mpc, sti, stc);
    }

    public override IEnumerable<EncounterSlot4> GetMatchingSlots(PKM pk, EvoCriteria[] chain)
    {
        if (pk.Format != 4) // Met Location and Met Level are changed on PK4->PK5
            return GetSlotsFuzzy(chain);
        if (pk.Met_Location != Location)
            return Array.Empty<EncounterSlot4>();
        return GetSlotsMatching(chain, pk.Met_Level, pk);
    }

    private IEnumerable<EncounterSlot4> GetSlotsMatching(EvoCriteria[] chain, int lvl, PKM pk)
    {
        foreach (var slot in Slots)
        {
            foreach (var evo in chain)
            {
                if (slot.Species != evo.Species)
                    continue;

                if (slot.Form != evo.Form && slot.Species is not (int)Species.Burmy)
                {
                    // Unown forms are random, not specific form IDs
                    if (!slot.IsRandomUnspecificForm)
                        break;
                }
                if (!slot.IsLevelWithinRange(lvl))
                    break;

                if (Type is SlotType.HoneyTree && IsInaccessibleHoneySlotLocation(slot, pk))
                    break;

                yield return slot;
                break;
            }
        }
    }

    private static bool IsInaccessibleHoneySlotLocation(EncounterSlot4 slot, PKM pk)
    {
        // A/B/C tables, only Munchlax is a 'C' encounter, and A/B are accessible from any tree.
        // C table encounters are only available from 4 trees, which are determined by TID16/SID16 of the save file.
        if (slot.Species is not (int)Species.Munchlax)
            return false;

        // We didn't encode the honey tree index to the encounter slot resource.
        // Check if any of the slot's location doesn't match any of the groupC trees' area location ID.
        var location = pk.Met_Location;
        var trees = SAV4Sinnoh.CalculateMunchlaxTrees(pk.TID16, pk.SID16);
        return LocationID_HoneyTree[trees.Tree1] != location
               && LocationID_HoneyTree[trees.Tree2] != location
               && LocationID_HoneyTree[trees.Tree3] != location
               && LocationID_HoneyTree[trees.Tree4] != location;
    }

    private static ReadOnlySpan<byte> LocationID_HoneyTree => new byte[]
    {
        20, // 00 Route 205 Floaroma
        20, // 01 Route 205 Eterna
        21, // 02 Route 206
        22, // 03 Route 207
        23, // 04 Route 208
        24, // 05 Route 209
        25, // 06 Route 210 Solaceon
        25, // 07 Route 210 Celestic
        26, // 08 Route 211
        27, // 09 Route 212 Hearthome
        27, // 10 Route 212 Pastoria
        28, // 11 Route 213
        29, // 12 Route 214
        30, // 13 Route 215
        33, // 14 Route 218
        36, // 15 Route 221
        37, // 16 Route 222
        47, // 17 Valley Windworks
        48, // 18 Eterna Forest
        49, // 19 Fuego Ironworks
        58, // 20 Floaroma Meadow
    };

    // original met level cannot be inferred
    private IEnumerable<EncounterSlot4> GetSlotsFuzzy(EvoCriteria[] chain)
    {
        foreach (var slot in Slots)
        {
            foreach (var evo in chain)
            {
                if (slot.Species != evo.Species)
                    continue;

                if (slot.Form != evo.Form && slot.Species is not (int)Species.Burmy)
                {
                    // Unown forms are random, not specific form IDs
                    if (!slot.IsRandomUnspecificForm)
                        break;
                }
                if (slot.LevelMin > evo.LevelMax)
                    break;

                yield return slot;
                break;
            }
        }
    }
}