lib/libc/stdlib/bsearch.c

   1 /*
   2  * Copyright (c) 1990, 1993
   3  *      The Regents of the University of California.  All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  * 3. All advertising materials mentioning features or use of this software
  14  *    must display the following acknowledgement:
  15  *      This product includes software developed by the University of
  16  *      California, Berkeley and its contributors.
  17  * 4. Neither the name of the University nor the names of its contributors
  18  *    may be used to endorse or promote products derived from this software
  19  *    without specific prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  *
  33  * @(#)bsearch.c        8.1 (Berkeley) 6/4/93
  34  * $DragonFly: src/lib/libc/stdlib/bsearch.c,v 1.5 2005/11/20 12:37:48 swildner Exp $
  35  */
  36
  37 #include <stddef.h>
  38 #include <stdlib.h>
  39
  40 /*
  41  * Perform a binary search.
  42  *
  43  * The code below is a bit sneaky.  After a comparison fails, we
  44  * divide the work in half by moving either left or right. If lim
  45  * is odd, moving left simply involves halving lim: e.g., when lim
  46  * is 5 we look at item 2, so we change lim to 2 so that we will
  47  * look at items 0 & 1.  If lim is even, the same applies.  If lim
  48  * is odd, moving right again involes halving lim, this time moving
  49  * the base up one item past p: e.g., when lim is 5 we change base
  50  * to item 3 and make lim 2 so that we will look at items 3 and 4.
  51  * If lim is even, however, we have to shrink it by one before
  52  * halving: e.g., when lim is 4, we still looked at item 2, so we
  53  * have to make lim 3, then halve, obtaining 1, so that we will only
  54  * look at item 3.
  55  */
  56 void *
  57 bsearch(const void *key, const void *base0, size_t nmemb, size_t size,
  58         int (*compar)(const void *, const void *))
  59 {
  60         const char *base = base0;
  61         size_t lim;
  62         int cmp;
  63         const void *p;
  64
  65         for (lim = nmemb; lim != 0; lim >>= 1) {
  66                 p = base + (lim >> 1) * size;
  67                 cmp = (*compar)(key, p);
  68                 if (cmp == 0)
  69                         return ((void *)p);
  70                 if (cmp > 0) {  /* key > p: move right */
  71                         base = (char *)p + size;
  72                         lim--;
  73                 }               /* else move left */
  74         }
  75         return (NULL);
  76 }