From:	DavMac AT iname DOT com (Davin McCall)
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Re[4]: Bit counting?
Date:	Mon, 23 Aug 1999 09:13:25 GMT
Organization:	Monash Uni
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On Sat, 21 Aug 1999 09:43:53 +0700, Batchex <thedark1 AT Phreaker DOT net>
wrote:

>I've tried this approach, but it will only gives the correct bit count
>if value is as high as 15. Higher than 15 (16 and higher) will give
>incorrect bit count. I've tried 8 bit values and go as far as 16 bit
>sequences.

Look, the algorithm is correct; you only really need to read my
description carefully to see how it works. Nevertheless, I've gone to
the trouble of actually implementing it (albeit in a very inefficient
manner), and the source code follows. It works perfectly for all
32-bit or less numbers.

--- (begin) ----

/* bit count program */

#include <stdio.h>
#include <stdlib.h>

unsigned long binstrtoval(const char *binstr)
{
  unsigned long val = 0;

  while( *binstr != '\0' ) {
    val <<= 1;
    if( *binstr == '1' ) val |= 1;
    binstr++;
  } /* end-while */

  return val;
}

main()
{
  char number[20];
  unsigned long val, tempval;
  unsigned long masked1, masked2, bmask1, bmask2;

  printf( "Enter a binary number: " ); fflush(stdout);
  fgets(number, 20, stdin);

  val = binstrtoval(number); printf( "decimal value: %d\n", val );

  bmask1 = binstrtoval("01010101010101010101010101010101");
  bmask2 = bmask1 << 1;
  masked1 = val & bmask1;
  masked2 = (val & bmask2) >> 1;
  tempval = masked1 + masked2;

  bmask1 = binstrtoval("00110011001100110011001100110011");
  bmask2 = bmask1 << 2;
  masked1 = tempval & bmask1;
  masked2 = (tempval & bmask2) >> 2;
  tempval = masked1 + masked2;

  bmask1 = binstrtoval("00001111000011110000111100001111");
  bmask2 = bmask1 << 4;
  masked1 = tempval & bmask1;
  masked2 = (tempval & bmask2) >> 4;
  tempval = masked1 + masked2;

  bmask1 = binstrtoval("00000000111111110000000011111111");
  bmask2 = bmask1 << 8;
  masked1 = tempval & bmask1;
  masked2 = (tempval & bmask2) >> 8;
  tempval = masked1 + masked2;

  bmask1 = binstrtoval("00000000000000001111111111111111");
  bmask2 = bmask1 << 16;
  masked1 = tempval & bmask1;
  masked2 = (tempval & bmask2) >> 16;
  tempval = masked1 + masked2;

  printf( "\nNumber of bits set: %d\n", tempval );
}

__________________________________________________________
       *** davmac - sharkin'!! davmac AT iname DOT com ***
my programming page: http://yoyo.cc.monash.edu.au/~davmac/

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