PKHeX/PKHeX.Core/Legality/Evolutions/EvolutionSets/EvolutionSet4.cs

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

namespace PKHeX.Core;

/// <summary>
/// Generation 4 Evolution Branch Entries
/// </summary>
public static class EvolutionSet4
{
    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));

        // To have the same structure as gen 6
        // Gen 4 Method 6 is Gen 6 Method 7, G4 7 = G6 8, and so on
        if (method > 6)
            method++;

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

    public static IReadOnlyList<EvolutionMethod[]> GetArray(ReadOnlySpan<byte> data)
    {
        const int bpe = 6; // bytes per evolution entry
        const int entries = 7; // amount of entries per species
        const int size = (entries * bpe) + 2; // bytes per species entry, + 2 alignment bytes

        var evos = new EvolutionMethod[data.Length / size][];
        for (int i = 0; i < evos.Length; i++)
        {
            int offset = i * size;
            int count = 0;
            for (; count < entries; count++)
            {
                var methodOffset = offset + (count * bpe);
                var method = data[methodOffset];
                if (method == 0)
                    break;
            }
            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(data.Slice(offset + (j * bpe), bpe));
            evos[i] = set;
        }
        return evos;
    }
}