How to check if a number is a power of 2

Question

Today I needed a simple algorithm for checking if a number is a power of 2.

The algorithm needs to be:

1. Simple
2. Correct for any ulong

Answer 1

Some sites that document and explain this and other bit twiddling hacks are:

• http://graphics.stanford.edu/~seander/bithacks.html
  (http://graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2)
• http://bits.stephan-brumme.com/
  (http://bits.stephan-brumme.com/isPowerOfTwo.html)

And the grandaddy of them, the book "Hacker's Delight" by Henry Warren, Jr.:

• http://www.hackersdelight.org/

As Sean Anderson's page explains, the expression ((x & (x - 1)) == 0) incorrectly indicates that 0 is a power of 2. He suggests to use:

(!(x & (x - 1)) && x)

to correct that problem.