Simple Big-endian Checksum

crc32.cs

// Compute a 32-bit big-endian checksum on the N-byte buffer.  If the
// buffer is not a multiple of 4 bytes length, compute the sum that would
// have occurred if the buffer was padded with zeros to the next multiple
// of four bytes.
public static uint Checksum(ReadOnlySpan<byte> arr)
{
    if (arr.Length == 0) return 0;

    uint sum0 = 0, sum1 = 0, sum2 = 0, sum = 0, N = (uint)arr.Length;
    int z = 0;

    while (N >= 4)
    {
        sum0 += arr[z + 0];
        sum1 += arr[z + 1];
        sum2 += arr[z + 2];
        sum += arr[z + 3];
        z += 4;
        N -= 4;
    }

    sum += (sum2 << 8) + (sum1 << 16) + (sum0 << 24);
    switch (N & 3)
    {
        case 3:
            sum += (uint)(arr[z + 2]) << 8;
            sum += (uint)(arr[z + 1]) << 16;
            sum += (uint)(arr[z + 0]) << 24;
            break;
        case 2:
            sum += (uint)(arr[z + 1]) << 16;
            sum += (uint)(arr[z + 0]) << 24;
            break;
        case 1:
            sum += (uint)(arr[z + 0]) << 24;
            break;
    }
        
    return sum;
}