sys/libkern/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  * 4. Neither the name of the University nor the names of its contributors
  14  *    may be used to endorse or promote products derived from this software
  15  *    without specific prior written permission.
  16  *
  17  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  27  * SUCH DAMAGE.
  28  *
  29  * @(#)bsearch.c        8.1 (Berkeley) 6/4/93
  30  * $FreeBSD: src/sys/libkern/bsearch.c,v 1.8 2005/01/07 00:24:32 imp Exp $
  31  * $DragonFly: src/sys/libkern/bsearch.c,v 1.1 2007/11/18 17:53:02 pavalos Exp $
  32  */
  33
  34 #include <sys/param.h>
  35 #include <sys/libkern.h>
  36
  37 /*
  38  * Perform a binary search.
  39  *
  40  * The code below is a bit sneaky.  After a comparison fails, we
  41  * divide the work in half by moving either left or right. If lim
  42  * is odd, moving left simply involves halving lim: e.g., when lim
  43  * is 5 we look at item 2, so we change lim to 2 so that we will
  44  * look at items 0 & 1.  If lim is even, the same applies.  If lim
  45  * is odd, moving right again involes halving lim, this time moving
  46  * the base up one item past p: e.g., when lim is 5 we change base
  47  * to item 3 and make lim 2 so that we will look at items 3 and 4.
  48  * If lim is even, however, we have to shrink it by one before
  49  * halving: e.g., when lim is 4, we still looked at item 2, so we
  50  * have to make lim 3, then halve, obtaining 1, so that we will only
  51  * look at item 3.
  52  */
  53 void *
  54 bsearch(const void *key, const void *base0, size_t nmemb, size_t size,
  55         int (*compar)(const void *, const void *))
  56 {
  57         const char *base = base0;
  58         size_t lim;
  59         int cmp;
  60         const void *p;
  61
  62         for (lim = nmemb; lim != 0; lim >>= 1) {
  63                 p = base + (lim >> 1) * size;
  64                 cmp = (*compar)(key, p);
  65                 if (cmp == 0)
  66                         return ((void *)(uintptr_t)p);
  67                 if (cmp > 0) {  /* key > p: move right */
  68                         base = (const char *)p + size;
  69                         lim--;
  70                 }               /* else move left */
  71         }
  72         return (NULL);
  73 }