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

namespace PKHeX.Core
{
    /// <inheritdoc cref="EncounterArea" />
    /// <summary>
    /// <see cref="GameVersion.GO"/> encounter area for direct-to-HOME transfers.
    /// </summary>
    public sealed record EncounterArea8g : EncounterArea, ISpeciesForm
    {
        /// <summary> Species for the area </summary>
        /// <remarks> Due to how the encounter data is packaged by PKHeX, each species-form is grouped together. </remarks>
        public int Species { get; }
        /// <summary> Form of the Species </summary>
        public int Form { get; }
        public readonly EncounterSlot8GO[] Slots;

        protected override IReadOnlyList<EncounterSlot> Raw => Slots;

        private EncounterArea8g(ushort species, byte form, EncounterSlot8GO[] slots) : base(GameVersion.GO)
        {
            Species = species;
            Form = form;
            Location = Locations.GO8;
            Slots = slots;
        }

        internal static EncounterArea8g[] GetArea(BinLinkerAccessor data)
        {
            var areas = new EncounterArea8g[data.Length];
            for (int i = 0; i < areas.Length; i++)
                areas[i] = GetArea(data[i]);
            return areas;
        }

        private const int meta = 4;
        private const int entrySize = (2 * sizeof(int)) + 2;

        private static EncounterArea8g GetArea(ReadOnlySpan<byte> data)
        {
            var species = ReadUInt16LittleEndian(data);
            var form = data[2];
            var import = (PogoImportFormat)data[3];

            data = data[meta..];
            var result = new EncounterSlot8GO[data.Length / entrySize];
            var area = new EncounterArea8g(species, form, result);
            for (int i = 0; i < result.Length; i++)
            {
                var offset = i * entrySize;
                var entry = data.Slice(offset, entrySize);
                result[i] = ReadSlot(entry, area, species, form, import);
            }

            return area;
        }

        private static EncounterSlot8GO ReadSlot(ReadOnlySpan<byte> entry, EncounterArea8g area, ushort species, byte form, PogoImportFormat format)
        {
            int start = ReadInt32LittleEndian(entry);
            int end = ReadInt32LittleEndian(entry[4..]);
            var sg = entry[8];
            var shiny = (Shiny)(sg & 0x3F);
            var gender = (Gender)(sg >> 6);
            var type = (PogoType)entry[9];
            return new EncounterSlot8GO(area, species, form, start, end, shiny, gender, type, format);
        }

        public override IEnumerable<EncounterSlot> GetMatchingSlots(PKM pkm, EvoCriteria[] chain)
        {
            if (pkm.TSV == 0) // HOME doesn't assign TSV=0 to accounts.
                yield break;

            // Find the first chain that has slots defined.
            // Since it is possible to evolve before transferring, we only need the highest evolution species possible.
            // PoGoEncTool has already extrapolated the evolutions to separate encounters!
            var sf = FindCriteriaToIterate(pkm, chain);
            if (sf == default)
                yield break;

            var species = pkm.Species;
            var ball = (Ball)pkm.Ball;
            var met = Math.Max(sf.LevelMin, pkm.Met_Level);
            EncounterSlot8GO? deferredIV = null;

            foreach (var slot in Slots)
            {
                if (!slot.IsLevelWithinRange(met))
                    continue;
                if (!slot.IsBallValid(ball, species))
                    continue;
                if (!slot.Shiny.IsValid(pkm))
                    continue;
                if (slot.Gender != Gender.Random && (int)slot.Gender != pkm.Gender)
                    continue;

                if (!slot.GetIVsValid(pkm))
                {
                    deferredIV ??= slot;
                    continue;
                }

                yield return slot;
            }

            if (deferredIV != null)
                yield return deferredIV;
        }

        private EvoCriteria FindCriteriaToIterate(PKM pkm, EvoCriteria[] chain)
        {
            foreach (var evo in chain)
            {
                if (evo.Species != Species)
                    continue;

                if (evo.Form == Form)
                    return evo;

                // Check for form mismatches
                if (FormInfo.IsFormChangeable(Species, Form, evo.Form, pkm.Format))
                    return evo;
                if (Species == (int)Core.Species.Burmy)
                    return evo;
                break;
            }
            return default;
        }
    }
}