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

namespace PKHeX.Core
{
    /// <inheritdoc cref="EncounterArea" />
    /// <summary>
    /// <see cref="GameVersion.Gen3"/> encounter area
    /// </summary>
    public sealed record EncounterArea3 : EncounterArea
    {
        public readonly int Rate;
        public readonly EncounterSlot3[] Slots;

        protected override IReadOnlyList<EncounterSlot> Raw => Slots;

        public static EncounterArea3[] GetAreas(BinLinkerAccessor input, GameVersion game)
        {
            var result = new EncounterArea3[input.Length];
            for (int i = 0; i < result.Length; i++)
                result[i] = new EncounterArea3(input[i], game);
            return result;
        }

        public static EncounterArea3[] GetAreasSwarm(BinLinkerAccessor input, GameVersion game)
        {
            var result = new EncounterArea3[input.Length];
            for (int i = 0; i < result.Length; i++)
                result[i] = new EncounterArea3(input[i], game, SlotType.Swarm | SlotType.Grass);
            return result;
        }

        private EncounterArea3(ReadOnlySpan<byte> data, GameVersion game) : base(game)
        {
            Location = data[0] | (data[1] << 8);
            Type = (SlotType)data[2];
            Rate = data[3];

            Slots = ReadRegularSlots(data);
        }

        private EncounterArea3(ReadOnlySpan<byte> data, GameVersion game, SlotType type) : base(game)
        {
            Location = data[0] | (data[1] << 8);
            Type = type;
            Rate = data[3];

            Slots = ReadSwarmSlots(data);
        }

        private EncounterSlot3[] ReadRegularSlots(ReadOnlySpan<byte> data)
        {
            const int size = 10;
            int count = (data.Length - 4) / size;
            var slots = new EncounterSlot3[count];
            for (int i = 0; i < slots.Length; i++)
            {
                int offset = 4 + (size * i);
                var entry = data.Slice(offset, size);
                slots[i] = ReadRegularSlot(entry);
            }

            return slots;
        }

        private EncounterSlot3 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 EncounterSlot3(this, species, form, min, max, slotNum, mpi, mpc, sti, stc);
        }

        private EncounterSlot3[] ReadSwarmSlots(ReadOnlySpan<byte> data)
        {
            const int size = 14;
            int count = (data.Length - 4) / size;
            var slots = new EncounterSlot3[count];
            for (int i = 0; i < slots.Length; i++)
            {
                int offset = 4 + (size * i);
                var entry = data.Slice(offset, size);
                slots[i] = ReadSwarmSlot(entry);
            }

            return slots;
        }

        private EncounterSlot3Swarm ReadSwarmSlot(ReadOnlySpan<byte> entry)
        {
            ushort species = ReadUInt16LittleEndian(entry);
            // form always 0
            byte slotNum = entry[3];
            byte min = entry[4];
            byte max = entry[5];

            int[] moves =
            {
                ReadUInt16LittleEndian(entry[06..]),
                ReadUInt16LittleEndian(entry[08..]),
                ReadUInt16LittleEndian(entry[10..]),
                ReadUInt16LittleEndian(entry[12..]),
            };

            return new EncounterSlot3Swarm(this, species, min, max, slotNum, moves);
        }

        public override IEnumerable<EncounterSlot> GetMatchingSlots(PKM pkm, IReadOnlyList<EvoCriteria> chain)
        {
            if (pkm.Format != 3) // Met Location and Met Level are changed on PK3->PK4
                return GetSlotsFuzzy(chain);
            if (pkm.Met_Location != Location)
                return Array.Empty<EncounterSlot>();
            return GetSlotsMatching(chain, pkm.Met_Level);
        }

        private IEnumerable<EncounterSlot3> GetSlotsMatching(IReadOnlyList<EvoCriteria> chain, int lvl)
        {
            foreach (var slot in Slots)
            {
                foreach (var evo in chain)
                {
                    if (slot.Species != evo.Species)
                        continue;

                    if (slot.Form != evo.Form)
                        break;
                    if (!slot.IsLevelWithinRange(lvl))
                        break;

                    yield return slot;
                    break;
                }
            }
        }

        private IEnumerable<EncounterSlot3> GetSlotsFuzzy(IReadOnlyList<EvoCriteria> chain)
        {
            foreach (var slot in Slots)
            {
                foreach (var evo in chain)
                {
                    if (slot.Species != evo.Species)
                        continue;

                    if (slot.Form != evo.Form)
                        break;
                    if (slot.LevelMin > evo.Level)
                        break;

                    yield return slot;
                    break;
                }
            }
        }
    }
}