Answer 4
The following C expression computes the logarithm of a 16-bit number in O(log n) steps, assuming exactly one bit is set in the number:

(n & 0xAAAA) ? 1 : 0

| (n & 0xCCCC) ? 2 : 0

| (n & 0xF0F0) ? 4 : 0

| (n & 0xFF00) ? 8 : 0

Contributor: David Tweed

Published July 2003