PKHeX/PKHeX.Core/Saves/Substructures/Gen3/Gen3MysteryData.cs

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

namespace PKHeX.Core;

public abstract class Gen3MysteryData
{
    public readonly byte[] Data;

    protected Gen3MysteryData(byte[] data) => Data = data;

    public uint Checksum
    {
        get => ReadUInt32LittleEndian(Data.AsSpan(0));
        set => WriteUInt32LittleEndian(Data.AsSpan(0), value);
    }

    public bool IsChecksumValid() => Checksum == GetChecksum(Data);
    public void FixChecksum() => Checksum = GetChecksum(Data);

    private static uint GetChecksum(ReadOnlySpan<byte> data)
    {
        uint sum = 0;
        for (var i = 4; i < data.Length; i++)
        {
            var b = data[i];
            sum += b;
        }

        return sum;
    }
}