Well, the answer is pretty simple. If you have an int value:
int log2(int value) {
return Integer.SIZE - Integer.numberOfLeadingZeros(value);
}
The same exists for Long…
[Edit]
If shaving milliseconds is an issue here, Integer.numberOfLeadingZeros(int) is reasonably efficient, but still does 15 operations… Expanding a reasonable amount of memory (300 bytes, static) you could shave that to between 1 and 8 operations, depending on the range of your integers.