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

namespace PKHeX.Core;

/// <inheritdoc cref="EncounterArea" />
/// <summary>
/// <see cref="GameVersion.BDSP"/> encounter area
/// </summary>
public sealed record EncounterArea8b : EncounterArea
{
    public readonly EncounterSlot8b[] Slots;

    protected override IReadOnlyList<EncounterSlot> Raw => Slots;

    public static EncounterArea8b[] GetAreas(BinLinkerAccessor input, GameVersion game)
    {
        var result = new EncounterArea8b[input.Length];
        for (int i = 0; i < result.Length; i++)
            result[i] = new EncounterArea8b(input[i], game);
        return result;
    }

    private EncounterArea8b(ReadOnlySpan<byte> data, GameVersion game) : base(game)
    {
        Location = ReadInt16LittleEndian(data);
        Type = (SlotType)data[2];

        Slots = ReadSlots(data);
    }

    private EncounterSlot8b[] ReadSlots(ReadOnlySpan<byte> data)
    {
        const int size = 4;
        int count = (data.Length - 4) / size;
        var slots = new EncounterSlot8b[count];
        for (int i = 0; i < slots.Length; i++)
        {
            int offset = 4 + (size * i);
            var entry = data.Slice(offset, size);
            slots[i] = ReadSlot(entry);
        }
        return slots;
    }

    private EncounterSlot8b ReadSlot(ReadOnlySpan<byte> data)
    {
        ushort species = ReadUInt16LittleEndian(data);
        byte form = (byte)(species >> 11);
        species &= 0x3FF;
        byte min = data[2];
        byte max = data[3];
        return new EncounterSlot8b(this, species, form, min, max);
    }

    public override bool IsMatchLocation(int location)
    {
        if (base.IsMatchLocation(location))
            return true;
        return CanCrossoverTo(location);
    }

    private bool CanCrossoverTo(int location)
    {
        if (Type is SlotType.Surf)
        {
            return Location switch
            {
                486 => location is 167, // Route 223 -> Pokémon League
                167 => location is 486, // Pokémon League -> Route 223
                420 => location is 489, // Route 229 -> Route 230
                489 => location is 420, // Route 230 -> Route 229

                // All other crossover surf locations are identical slot lists.
                _ => false,
            };
        }

        // No crossover
        return false;
    }

    public override IEnumerable<EncounterSlot> GetMatchingSlots(PKM pk, EvoCriteria[] chain)
    {
        foreach (var slot in Slots)
        {
            foreach (var evo in chain)
            {
                if (slot.Species != evo.Species)
                    continue;

                if (!slot.IsLevelWithinRange(pk.Met_Level))
                    break;

                if (slot.Form != evo.Form && slot.Species is not (int)Species.Burmy)
                    break;

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

                yield return slot;
                break;
            }
        }
    }
    private static bool IsInaccessibleHoneySlotLocation(EncounterSlot8b 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 TID/SID 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.TID, pk.SID);
        return LocationID_HoneyTree[trees.Tree1] != location
               && LocationID_HoneyTree[trees.Tree2] != location
               && LocationID_HoneyTree[trees.Tree3] != location
               && LocationID_HoneyTree[trees.Tree4] != location;
    }

    private static readonly ushort[] LocationID_HoneyTree =
    {
        359, // 00 Route 205 Floaroma
        361, // 01 Route 205 Eterna
        362, // 02 Route 206
        364, // 03 Route 207
        365, // 04 Route 208
        367, // 05 Route 209
        373, // 06 Route 210 Solaceon
        375, // 07 Route 210 Celestic
        378, // 08 Route 211
        379, // 09 Route 212 Hearthome
        383, // 10 Route 212 Pastoria
        385, // 11 Route 213
        392, // 12 Route 214
        394, // 13 Route 215
        400, // 14 Route 218
        404, // 15 Route 221
        407, // 16 Route 222
        197, // 17 Valley Windworks
        199, // 18 Eterna Forest
        201, // 19 Fuego Ironworks
        253, // 20 Floaroma Meadow
    };
}