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

namespace PKHeX.Core
{
    /// <summary>
    /// Generation 5 Evolution Branch Entries
    /// </summary>
    public static class EvolutionSet5
    {
        private static EvolutionMethod GetMethod(ReadOnlySpan<byte> data)
        {
            var method = data[0]; // other byte unnecessary
            int arg = ReadUInt16LittleEndian(data[2..]);
            int species = ReadUInt16LittleEndian(data[4..]);

            if (method == 0)
                throw new ArgumentOutOfRangeException(nameof(method));

            var lvl = EvolutionSet6.EvosWithArg.Contains(method) ? 0 : arg;
            return new EvolutionMethod(method, species, argument: arg, level: (byte)lvl);
        }

        private const int bpe = 6; // bytes per evolution entry
        private const int entries = 7; // amount of entries per species
        private const int size = entries * bpe; // bytes per species entry

        public static IReadOnlyList<EvolutionMethod[]> GetArray(ReadOnlySpan<byte> data)
        {
            var evos = new EvolutionMethod[data.Length / size][];
            for (int i = 0; i < evos.Length; i++)
            {
                int offset = i * size;
                var rawEntries = data.Slice(offset, size);
                var count = ScanCountEvolutions(rawEntries);
                if (count == 0)
                {
                    evos[i] = Array.Empty<EvolutionMethod>();
                    continue;
                }

                var set = new EvolutionMethod[count];
                for (int j = 0; j < set.Length; j++)
                    set[j] = GetMethod(rawEntries.Slice(j * bpe, bpe));
                evos[i] = set;
            }
            return evos;
        }

        private static int ScanCountEvolutions(ReadOnlySpan<byte> data)
        {
            for (int count = 0; count < entries; count++)
            {
                var methodOffset = count * bpe;
                var method = data[methodOffset];
                if (method == 0)
                    return count;
            }
            return entries;
        }
    }
}