Comments on: Binary Fun – Bits in Bed with Actionscript

By: Andy Stricker

Andy Stricker — Tue, 20 Apr 2010 20:40:19 +0000

countBits is called population count or hamming weight in computer science. There is a good chapter about the optimization available for population count in the O’Reilly book “Beautiful Code” in chapter Chapter 10: “The Quest for an Accelerated Population Count” (http://oreilly.com/catalog/9780596510046). I’ll like to see how those improvements apply for high level languages.

Following an optimized function for 32 bit unsigned int population count in Javascript (derived from http://en.wikipedia.org/wiki/Hamming_weight):

function pop(x) {
   x &= 0xFFFFFFFF;
   x -= (x >> 1) & 0x55555555;
   x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
   x = (x + (x >> 4)) & 0x0f0f0f0f;
   return (x * 0x01010101) >> 24;
}

By: betabong

betabong — Thu, 01 Oct 2009 07:41:29 +0000

Love that!! And it’s about 40% faster too :) Allow me one tiny little correction though:

function is_1_bit(value:uint): Boolean
{
	return value != 0 && ( value == 0x80000000 || (value & -value) == value );
}

By: Jackson Dunstan

Jackson Dunstan — Thu, 01 Oct 2009 01:19:53 +0000

Your function to check if an int has only a single one value is the same as a function that checks if an int is a power of two. You can do so without the loop for extra speed like this:

function is_1_bit(value:uint): Boolean
{
	return value == 0x80000000 || (value & -value) == value;
}

The formula won’t work for (1<<31), hence the need for the explicit check.

By: betabong

betabong — Tue, 22 Sep 2009 16:52:23 +0000

XOR is deadly easy :-)

function xor( v1:uint , v2:uint ) : uint {
	return v1 ^ v2;
}

By: peko

peko — Tue, 22 Sep 2009 14:50:49 +0000

What about binary xor?
AS3 don`t have an default operator :(