Constant-time exponent of 2 detector

notes
In your common FFT program, you want to make sure that the frame you're working with has a
size that is a power of 2.  This tells you in just a few operations.  Granted, you won't
be using this algorithm inside a loop, so the savings aren't that great, but every little
hack helps  ;)
code
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// Quit if size isn't a power of 2
if ((-size ^ size) & size) return;

// If size is an unsigned int, the above might not compile.
// You'd want to use this instead:
if (((~size + 1) ^ size) & size) return;

Comments

I think I prefer:

if (! (size & (size - 1))) return;

I'm not positive this is fewer instructions than the above, but I think it's easier to see why it works (n and n-1 will share bits unless n is a power of two), and it doesn't require two's-complement.

 - Tom 7