Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!ut-emx!nwagner
From: nwagner@ut-emx.uucp (Neal R. Wagner)
Newsgroups: comp.lang.c
Subject: count of bits set in a long
Keywords: bits set, count of, long
Message-ID: <37545@ut-emx.uucp>
Date: 24 Sep 90 22:37:26 GMT
Organization: The University of Texas at Austin; Austin, Texas
Lines: 31


  I need a fast method to count the number of bits that are set in a 32-bit
integer on a Sun 3/80 running 4.0.3 SunOS.  The brute-force method follows.
Is there a faster way in C?  How close in speed to a hand-coded assembly
routine would a fast method in C be (after the latter is run through the
optimizer)?
  Thanks in advance for your help.  Send e-mail and I will summarize for the
net.

===============================================================================
#define	BIT_SETSIZE	32
typedef	unsigned long	bit_set;
#define BIT_ISSET(bit,bitset)	(((bitset) & (1<<(bit))) >> (bit))
int bit_count();

main()
{
	bit_set i;
	for (i=0; i<=16; i++)
		printf("%d has %d bits on\n", i, bit_count(i));
}

intint bit_count(i)
bit_set i;
{
	int j, k;
	k = 0;
	for (j=0; j<BIT_SETSIZE; j++) k = k + BIT_ISSET(j, i);
	return k;
}
===============================================================================