lib/libc/stdlib/bsearch.c

   1 /*      $NetBSD: bsearch.c,v 1.14 2010/11/27 18:33:54 christos Exp $    */
   2
   3 /*
   4  * Copyright (c) 1990, 1993
   5  *      The Regents of the University of California.  All rights reserved.
   6  *
   7  * Redistribution and use in source and binary forms, with or without
   8  * modification, are permitted provided that the following conditions
   9  * are met:
  10  * 1. Redistributions of source code must retain the above copyright
  11  *    notice, this list of conditions and the following disclaimer.
  12  * 2. Redistributions in binary form must reproduce the above copyright
  13  *    notice, this list of conditions and the following disclaimer in the
  14  *    documentation and/or other materials provided with the distribution.
  15  * 3. Neither the name of the University nor the names of its contributors
  16  *    may be used to endorse or promote products derived from this software
  17  *    without specific prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  21  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  22  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  23  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  24  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  25  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  26  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  27  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  28  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  29  * SUCH DAMAGE.
  30  */
  31
  32 #include <sys/cdefs.h>
  33 #if defined(LIBC_SCCS) && !defined(lint)
  34 #if 0
  35 static char sccsid[] = "@(#)bsearch.c   8.1 (Berkeley) 6/4/93";
  36 #else
  37 __RCSID("$NetBSD: bsearch.c,v 1.14 2010/11/27 18:33:54 christos Exp $");
  38 #endif
  39 #endif /* LIBC_SCCS and not lint */
  40
  41 #include <assert.h>
  42 #include <errno.h>
  43 #include <stdlib.h>
  44
  45 /*
  46  * Perform a binary search.
  47  *
  48  * The code below is a bit sneaky.  After a comparison fails, we
  49  * divide the work in half by moving either left or right. If lim
  50  * is odd, moving left simply involves halving lim: e.g., when lim
  51  * is 5 we look at item 2, so we change lim to 2 so that we will
  52  * look at items 0 & 1.  If lim is even, the same applies.  If lim
  53  * is odd, moving right again involes halving lim, this time moving
  54  * the base up one item past p: e.g., when lim is 5 we change base
  55  * to item 3 and make lim 2 so that we will look at items 3 and 4.
  56  * If lim is even, however, we have to shrink it by one before
  57  * halving: e.g., when lim is 4, we still looked at item 2, so we
  58  * have to make lim 3, then halve, obtaining 1, so that we will only
  59  * look at item 3.
  60  */
  61 void *
  62 bsearch(const void *key, const void *base0, size_t nmemb, size_t size,
  63     int (*compar)(const void *, const void *))
  64 {
  65         const char *base = base0;
  66         size_t lim;
  67         int cmp;
  68         const void *p;
  69
  70         _DIAGASSERT(key != NULL);
  71         _DIAGASSERT(base0 != NULL);
  72         _DIAGASSERT(compar != NULL);
  73
  74         for (lim = nmemb; lim != 0; lim >>= 1) {
  75                 p = base + (lim >> 1) * size;
  76                 cmp = (*compar)(key, p);
  77                 if (cmp == 0)
  78                         return __UNCONST(p);
  79                 if (cmp > 0) {  /* key > p: move right */
  80                         base = (const char *)p + size;
  81                         lim--;
  82                 }               /* else move left */
  83         }
  84         return (NULL);
  85 }