1. /* Function to reverse bits. Algorithm from:
    2.  * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */
    3. static unsigned long rev(unsigned long v) {
    4.     unsigned long s = 8 * sizeof(v);  // bit size; must be power of 2
    5.     unsigned long mask = ~0;
    6.     while ((s >>= 1) > 0) {
    7.         mask ^= (mask << s);
    8.         v = ((v >> s) & mask) | ((v << s) & ~mask);
    9.     }
    10.     return v;
    11. }