algorithm - 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

With recursion:

int firstset(int bits) {        
     return (bits & 0x80000000) ? 31 : firstset((bits << 1) | 1) - 1;
}

• Assumes [31,..,0] indexing
• Returns -1 if no bits set
• | 1 prevents stack overflow by capping the number of shifts until a 1 is reached (32)
• Not tail recursive :)