Find out number of bits needed to represent a positive integer in binary?

Question

This is probably pretty basic, but to save me an hour or so of grief can anyone tell me how you can work out the number of bits required to represent a given positive intege

Answer 1

If you're trying to avoid a loop and you care about speed, you can use a method like this:

int value = ...;
int count = 0;
if( value < 0 ) { value = 0; count = 32; }
if( value >= 0x7FFF ) { value >>= 16; count += 16; }
if( value >= 0x7F ) { value >>= 8; count += 8; }
if( value >= 0x7 ) { value >>= 4; count += 4; }
if( value >= 0x3 ) { value >>= 2; count += 2; }
if( value >= 0x1 ) { value >>= 1; count += 1; }

Java doesn't have unsigned integers, so that first if( value < 0 ) is a little questionable. Negative numbers always set the most significant bit, so arguably require the full word to to represent them. Adapt that behavior if you care.

Incidentally, to handle a 64-bit integer, replace the if( value < 0 ) line with these two:

if( value < 0 ) { value = 0; count = 64; }
if( value >= 0x7FFFFFFF ) { value >>= 32; count += 32; }