Hacker Checksum

crc32_hacker.cs

public unsafe static uint ChecksumHacker(ReadOnlySpan<byte> arr)
{
    if (arr.Length == 0) return 0;

    fixed (byte* ptr = arr)
    {
        uint sum0 = 0, sum1 = 0, sum2 = 0, sum3 = 0;
        uint z = 0;

        ulong tmp0 = 0, tmp1 = 0;
        int limit2 = 64;

        int limit = arr.Length - 32;
        while (z < limit)
        {
            ulong l1 = *(ulong*)(ptr + z);
            ulong l2 = *(ulong*)(ptr + z + 8);
            ulong l3 = *(ulong*)(ptr + z + 16);
            ulong l4 = *(ulong*)(ptr + z + 24);

            tmp0 += (l1 & 0x00ff00ff00ff00ff) + (l2 & 0x00ff00ff00ff00ff)
                + (l3 & 0x00ff00ff00ff00ff) + (l4 & 0x00ff00ff00ff00ff);
                
            tmp1 += ((l1 & 0xff00ff00ff00ff00) >> 8) + ((l2 & 0xff00ff00ff00ff00) >> 8)
                + ((l3 & 0xff00ff00ff00ff00) >> 8) + ((l4 & 0xff00ff00ff00ff00) >> 8);

            limit2--;
            z += 32;
            if (limit2 == 0)
            {
                sum0 += (uint)(tmp0 & 0xffff) + (uint)((tmp0 & 0x0000ffff00000000) >> 32);
                sum1 += (uint)(tmp1 & 0xffff) + (uint)((tmp1 & 0x0000ffff00000000) >> 32);
                sum2 += (uint)((tmp0 & 0xffff0000) >> 16) + (uint)(tmp0 >> 48);
                sum3 += (uint)((tmp1 & 0xffff0000) >> 16) + (uint)(tmp1 >> 48);

                tmp0 = 0; tmp1 = 0; limit2 = 64;
            }
        }

        if (limit2 != 64)
        {
            sum0 += (uint)(tmp0 & 0xffff) + (uint)((tmp0 & 0x0000ffff00000000) >> 32);
            sum1 += (uint)(tmp1 & 0xffff) + (uint)((tmp1 & 0x0000ffff00000000) >> 32);
            sum2 += (uint)((tmp0 & 0xffff0000) >> 16) + (uint)(tmp0 >> 48);
            sum3 += (uint)((tmp1 & 0xffff0000) >> 16) + (uint)(tmp1 >> 48);
        }

        ...
        
    }
}