bsearch.c revision 92905 
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
34#if defined(LIBC_SCCS) && !defined(lint)
35static char sccsid[] = "@(#)bsearch.c	8.1 (Berkeley) 6/4/93";
36#endif /* LIBC_SCCS and not lint */
37#include <sys/cdefs.h>
38__FBSDID("$FreeBSD: head/lib/libc/stdlib/bsearch.c 92905 2002-03-21 22:49:10Z obrien $");
39
40#include <stddef.h>
41#include <stdlib.h>
42
43/*
44 * Perform a binary search.
45 *
46 * The code below is a bit sneaky.  After a comparison fails, we
47 * divide the work in half by moving either left or right. If lim
48 * is odd, moving left simply involves halving lim: e.g., when lim
49 * is 5 we look at item 2, so we change lim to 2 so that we will
50 * look at items 0 & 1.  If lim is even, the same applies.  If lim
51 * is odd, moving right again involes halving lim, this time moving
52 * the base up one item past p: e.g., when lim is 5 we change base
53 * to item 3 and make lim 2 so that we will look at items 3 and 4.
54 * If lim is even, however, we have to shrink it by one before
55 * halving: e.g., when lim is 4, we still looked at item 2, so we
56 * have to make lim 3, then halve, obtaining 1, so that we will only
57 * look at item 3.
58 */
59void *
60bsearch(key, base0, nmemb, size, compar)
61	const void *key;
62	const void *base0;
63	size_t nmemb;
64	size_t size;
65	int (*compar)(const void *, const void *);
66{
67	const char *base = base0;
68	size_t lim;
69	int cmp;
70	const void *p;
71
72	for (lim = nmemb; lim != 0; lim >>= 1) {
73		p = base + (lim >> 1) * size;
74		cmp = (*compar)(key, p);
75		if (cmp == 0)
76			return ((void *)p);
77		if (cmp > 0) {	/* key > p: move right */
78			base = (char *)p + size;
79			lim--;
80		}		/* else move left */
81	}
82	return (NULL);
83}
84