Question

I’ve been trying to parse them for a couple days, and I can’t quite grok it. Here they are:

int left = S->buflen >> 3;
int fill = 64 - left;
if(left && (((datalen >> 3) & 0x3F) >= (unsigned)fill)){
   some code here
}

If it helps, this is in the reference implementation of the SHA-3 candidate BLAKE256.

Answer 1

An alternative without bitops might help clear up the meaning:

int left = S->buflen / 8;
int fill = 64 - left;
if (left != 0) {
    int tmp = datalen / 8;
    if ((tmp % 64) >= (unsigned)fill) {
        /* Some code here */
    }
}

This of course assumes that buflen and datalen are nonnegative, since the right shift operator has platform-dependent behavior for negative numbers.