PKHeX/PKHeX.Core/Saves/Blocks/BlockInfoRSBOX.cs

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

namespace PKHeX.Core;

/// <summary>
/// Gen3 <see cref="GameVersion.RSBOX"/> Block Info
/// </summary>
public sealed class BlockInfoRSBOX : BlockInfo
{
    public readonly uint SaveCount;
    private const int ChecksumRegionSize = 0x1FF8;

    public BlockInfoRSBOX(ReadOnlySpan<byte> data, int offset)
    {
        Offset = offset;
        Length = 4 + ChecksumRegionSize;

        // Values stored in Big Endian format
        ID = ReadUInt32BigEndian(data[(Offset + 4)..]);
        SaveCount = ReadUInt32BigEndian(data[(Offset + 8)..]);
    }

    protected override bool ChecksumValid(ReadOnlySpan<byte> data)
    {
        var chk = GetChecksum(data);
        var old = ReadUInt32BigEndian(data[Offset..]);
        return chk == old;
    }

    protected override void SetChecksum(Span<byte> data)
    {
        var chk = GetChecksum(data);
        var span = data[Offset..];
        WriteUInt32BigEndian(span, chk);
    }

    private uint GetChecksum(ReadOnlySpan<byte> data)
    {
        var span = data.Slice(Offset + 4, ChecksumRegionSize);
        return Checksums.CheckSum16BigInvert(span);
    }
}