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

this can be done as a binary search, reducing complexity of O(N) (for an N-bit word) to O(log(N)). A possible implementation is:

int highest_bit_index(uint32_t value)
{ 
  if(value == 0) return 0;
  int depth = 0;
  int exponent = 16;

  while(exponent > 0)
  {
    int shifted = value >> (exponent);
    if(shifted > 0)
    {
      depth += exponent;
      if(shifted == 1) return depth + 1;
      value >>= exponent;
    }
    exponent /= 2;
  }

  return depth + 1;
}

the input is a 32 bit unsigned integer. it has a loop that can be converted into 5 levels of if-statements , therefore resulting in 32 or so if-statements. you could also use recursion to get rid of the loop, or the absolutely evil "goto" ;)