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 = data[0] | (data[1] << 8);
            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 pkm, EvoCriteria[] chain)
        {
            foreach (var slot in Slots)
            {
                foreach (var evo in chain)
                {
                    if (slot.Species != evo.Species)
                        continue;

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

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

                    if (Type is SlotType.HoneyTree && IsInaccessibleHoneySlotLocation(slot, pkm))
                        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
        };
    }
}