PKHeX/PKHeX.Core/Saves/Blocks/BlockInfoNDS.cs

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

namespace PKHeX.Core
{
    /// <summary>
    /// Gen5 Block Info
    /// </summary>
    public sealed class BlockInfoNDS : BlockInfo
    {
        private readonly int ChecksumOffset;
        private readonly int ChecksumMirror;

        public BlockInfoNDS(int offset, int length, int chkOffset, int chkMirror)
        {
            Offset = offset;
            Length = length;
            ChecksumOffset = chkOffset;
            ChecksumMirror = chkMirror;
        }

        private ushort GetChecksum(Span<byte> data) => Checksums.CRC16_CCITT(data.Slice(Offset, Length));

        protected override bool ChecksumValid(Span<byte> data)
        {
            ushort chk = GetChecksum(data);
            if (chk != ReadUInt16LittleEndian(data[ChecksumOffset..]))
                return false;
            if (chk != ReadUInt16LittleEndian(data[ChecksumMirror..]))
                return false;
            return true;
        }

        protected override void SetChecksum(Span<byte> data)
        {
            ushort chk = GetChecksum(data);
            WriteUInt16LittleEndian(data[ChecksumOffset..], chk);
            WriteUInt16LittleEndian(data[ChecksumMirror..], chk);
        }
    }
}