Expert BigEndian Checksum

crc32_bigendian.cs

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

    fixed (byte* ptr = arr)
    {
        uint sum = 0;
        int z = 0;

        var limit = arr.Length - 4;
        while (z < limit)
        {
            sum += *(uint*)(ptr + z);
            z += 4;
        }

        int rem = (arr.Length - z);
        switch (rem & 3)
        {
            case 3:
                sum += (uint)(ptr[z + 2]) << 8;
                sum += (uint)(ptr[z + 1]) << 16;
                sum += (uint)(ptr[z + 0]) << 24;
                break;
            case 2:
                sum += (uint)(ptr[z + 1]) << 16;
                sum += (uint)(ptr[z + 0]) << 24;
                break;
            case 1:
                sum += (uint)(ptr[z + 0]) << 24;
                break;
        }

        return sum;
    }
}