using System;
using System.Collections.Generic;

namespace PKHeX.Core;

/// <inheritdoc cref="EncounterArea" />
/// <summary>
/// <see cref="GameVersion.GSC"/> encounter area
/// </summary>
public sealed record EncounterArea2 : EncounterArea
{
    private static readonly byte[] BCC_SlotRates = { 20, 20, 10, 10, 05, 05, 10, 10, 05, 05 };
    private static readonly byte[] RatesGrass = { 30, 30, 20, 10, 5, 4, 1 };
    private static readonly byte[] RatesSurf = { 60, 30, 10 };

    internal readonly EncounterTime Time;
    public readonly byte Rate;
    public readonly IReadOnlyList<byte> Rates;
    public readonly EncounterSlot2[] Slots;

    protected override IReadOnlyList<EncounterSlot> Raw => Slots;

    public static EncounterArea2[] GetAreas(BinLinkerAccessor input, GameVersion game)
    {
        var result = new EncounterArea2[input.Length];
        for (int i = 0; i < result.Length; i++)
            result[i] = new EncounterArea2(input[i], game);
        return result;
    }

    private EncounterArea2(ReadOnlySpan<byte> data, GameVersion game) : base(game)
    {
        Location = data[0];
        Time = (EncounterTime)data[1];
        var type = (Type = (SlotType)data[2]) & (SlotType)0xF;
        Rate = data[3];

        var next = data[4..];
        if (type is > SlotType.Surf and not SlotType.BugContest) // Not Grass/Surf
        {
            const int size = 5;
            int count = next.Length / size;
            Rates = next[..count].ToArray();
            Slots = ReadSlots(next[count..], count);
        }
        else
        {
            const int size = 4;
            int count = next.Length / size;
            Rates = type switch
            {
                SlotType.BugContest => BCC_SlotRates,
                SlotType.Grass => RatesGrass,
                _ => RatesSurf,
            };
            Slots = ReadSlots(next, count);
        }
    }

    private EncounterSlot2[] ReadSlots(ReadOnlySpan<byte> data, int count)
    {
        const int size = 4;
        var slots = new EncounterSlot2[count];
        for (int i = 0; i < slots.Length; i++)
        {
            var entry = data.Slice(i * size, size);
            byte max = entry[3];
            byte min = entry[2];
            byte slotNum = entry[1];
            byte species = entry[0];
            slots[i] = new EncounterSlot2(this, species, min, max, slotNum);
        }
        return slots;
    }

    public override IEnumerable<EncounterSlot> GetMatchingSlots(PKM pk, EvoCriteria[] chain)
    {
        if (pk is not ICaughtData2 {CaughtData: not 0} pk2)
            return GetSlotsFuzzy(chain);

        if (pk2.Met_Location != Location)
            return Array.Empty<EncounterSlot>();
        return GetSlotsSpecificLevelTime(chain, pk2.Met_TimeOfDay, pk2.Met_Level);
    }

    private IEnumerable<EncounterSlot2> GetSlotsSpecificLevelTime(EvoCriteria[] chain, int time, int lvl)
    {
        foreach (var slot in Slots)
        {
            foreach (var evo in chain)
            {
                if (slot.Species != evo.Species)
                    continue;

                if (slot.Form != evo.Form)
                {
                    if (slot.Species != (int)Species.Unown || evo.Form >= 26) // Don't yield !? forms
                        break;
                }

                if (!slot.IsLevelWithinRange(lvl))
                    break;

                if (!Time.Contains(time))
                    break;

                yield return slot;
                break;
            }
        }
    }

    private IEnumerable<EncounterSlot2> GetSlotsFuzzy(EvoCriteria[] chain)
    {
        foreach (var slot in Slots)
        {
            foreach (var evo in chain)
            {
                if (slot.Species != evo.Species)
                    continue;

                if (slot.Form != evo.Form)
                {
                    if (slot.Species != (int) Species.Unown || evo.Form >= 26) // Don't yield !? forms
                        break;
                }
                if (slot.LevelMin > evo.LevelMax)
                    break;

                yield return slot;
                break;
            }
        }
    }
}