PKHeX/PKHeX.Core/Saves/Blocks/BlockInfo3DS.cs

using System;

namespace PKHeX.Core
{
    /// <summary>
    /// Gen6+ Block Info (inside BEEF chunk)
    /// </summary>
    public abstract class BlockInfo3DS : BlockInfo
    {
        private readonly int BlockInfoOffset;

        // ==chunk def== @ BlockInfoOffset
        // u64 timestamp1
        // u64 timestamp2
        // u8[4] BEEF magic
        // n*{blockInfo}, where n varies per sav type

        // ==block info def==
        // u32 length
        // u16 id
        // u16 checksum

        // when stored, each block size is rounded up to nearest 0x200, and the next chunk is immediately after

        protected BlockInfo3DS(int bo, uint id, int ofs, int len)
        {
            BlockInfoOffset = bo;
            ID = id;
            Offset = ofs;
            Length = len;
        }

        private int ChecksumOffset => BlockInfoOffset + 0x14 + ((int)ID * 8) + 6;
        protected abstract ushort GetChecksum(byte[] data);

        protected override bool ChecksumValid(byte[] data)
        {
            ushort chk = GetChecksum(data);
            var old = BitConverter.ToUInt16(data, ChecksumOffset);
            return chk == old;
        }

        protected override void SetChecksum(byte[] data)
        {
            ushort chk = GetChecksum(data);
            BitConverter.GetBytes(chk).CopyTo(data, ChecksumOffset);
        }
    }

    public sealed class BlockInfo6 : BlockInfo3DS
    {
        public BlockInfo6(int bo, uint id, int ofs, int len) : base(bo, id, ofs, len) { }
        protected override ushort GetChecksum(byte[] data) => Checksums.CRC16_CCITT(new ReadOnlySpan<byte>(data, Offset, Length));
    }

    public sealed class BlockInfo7 : BlockInfo3DS
    {
        public BlockInfo7(int bo, uint id, int ofs, int len) : base(bo, id, ofs, len) { }
        protected override ushort GetChecksum(byte[] data) => Checksums.CRC16Invert(new ReadOnlySpan<byte>(data, Offset, Length));
    }

    public sealed class BlockInfo7b : BlockInfo3DS
    {
        public BlockInfo7b(int bo, uint id, int ofs, int len) : base(bo, id, ofs, len) { }
        protected override ushort GetChecksum(byte[] data) => Checksums.CRC16NoInvert(new ReadOnlySpan<byte>(data, Offset, Length));
    }
}