lib/libc/string/popcount.c

   1 /*      $NetBSD: mi_vector_hash.c,v 1.1 2009/07/20 17:03:37 joerg Exp $ */
   2 /*-
   3  * Copyright (c) 2009 The NetBSD Foundation, Inc.
   4  * All rights reserved.
   5  *
   6  * This code is derived from software contributed to The NetBSD Foundation
   7  * by Joerg Sonnenberger.
   8  *
   9  * Redistribution and use in source and binary forms, with or without
  10  * modification, are permitted provided that the following conditions
  11  * are met:
  12  *
  13  * 1. Redistributions of source code must retain the above copyright
  14  *    notice, this list of conditions and the following disclaimer.
  15  * 2. Redistributions in binary form must reproduce the above copyright
  16  *    notice, this list of conditions and the following disclaimer in
  17  *    the documentation and/or other materials provided with the
  18  *    distribution.
  19  *
  20  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  21  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  22  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  23  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  24  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  25  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
  26  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  27  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
  28  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  29  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  30  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  */
  33
  34 #include <sys/cdefs.h>
  35 __RCSID("$NetBSD: mi_vector_hash.c,v 1.1 2009/07/20 17:03:37 joerg Exp $");
  36
  37 #include <limits.h>
  38 #include <strings.h>
  39
  40 #if UINT_MAX > 0xffffffffUL
  41 #error "Unsupported architecture"
  42 #endif
  43
  44 /*
  45  * This a hybrid algorithm for bit counting between parallel counting and
  46  * using multiplication.  The idea is to sum up the bits in each Byte, so
  47  * that the final accumulation can be done with a single multiplication.
  48  * If the platform has a slow multiplication instruction, it can be replaced
  49  * by the commented out version below.
  50  */
  51
  52 unsigned int
  53 popcount(unsigned int v)
  54 {
  55         unsigned int c;
  56
  57         v = v - ((v >> 1) & 0x55555555U);
  58         v = (v & 0x33333333U) + ((v >> 2) & 0x33333333U);
  59         v = ((v + (v >> 4)) & 0x0f0f0f0fU;
  60         c = (v * 0x01010101U) >> 24;
  61         /*
  62          * v = (v >> 16) + v;
  63          * v = (v >> 8) + v;
  64          * c = v & 255;
  65          */
  66
  67         return c;
  68 }