find the index of the highest bit set of a 32-bit number without loops obviously

Question

Here\'s a tough one(atleast i had a hard time :P):

find the index of the highest bit set of a 32-bit number without using any loops.

Answer 1

Paislee's solution is actually pretty easy to make tail-recursive, though, it's a much slower solution than the suggested floor(log2(n));

int firstset_tr(int bits, int final_dec) {

     // pass in 0 for final_dec on first call, or use a helper function

     if (bits & 0x80000000) {
      return 31-final_dec;
     } else {
      return firstset_tr( ((bits << 1) | 1), final_dec+1 );
     }
}

This function also works for other bit sizes, just change the check, e.g.

if (bits & 0x80) {   // for 8-bit
  return 7-final_dec;
}