Finding if a number is a power of 2
Subtract 1 from the number, then and it with the original number. If the result is zero, it was a power of two.
if (((n-1) & n) == 0) {
// power of two!
}
(sorry, my PHP is rusty...)
If it's a power of 2? Well, one way is to convert it to binary, and verify the presence of only 1 1
...:
$bin = decbin($number);
if (preg_match('/^0*10*$/', $bin)) {
//Even Power Of 2
}
One way is to use bitwise AND. If a number $x
is a power of two (e.g., 8=1000), it will have no bits in common with its predecessor (7=0111). So you can write:
($x & ($x - 1)) == 0
Note: This will give a false positive for $x == 0.