bsearch.c revision 1573 
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
38#include <stddef.h>
39#include <stdlib.h>
40
41/*
42 * Perform a binary search.
43 *
44 * The code below is a bit sneaky.  After a comparison fails, we
45 * divide the work in half by moving either left or right. If lim
46 * is odd, moving left simply involves halving lim: e.g., when lim
47 * is 5 we look at item 2, so we change lim to 2 so that we will
48 * look at items 0 & 1.  If lim is even, the same applies.  If lim
49 * is odd, moving right again involes halving lim, this time moving
50 * the base up one item past p: e.g., when lim is 5 we change base
51 * to item 3 and make lim 2 so that we will look at items 3 and 4.
52 * If lim is even, however, we have to shrink it by one before
53 * halving: e.g., when lim is 4, we still looked at item 2, so we
54 * have to make lim 3, then halve, obtaining 1, so that we will only
55 * look at item 3.
56 */
57void *
58bsearch(key, base0, nmemb, size, compar)
59	register const void *key;
60	const void *base0;
61	size_t nmemb;
62	register size_t size;
63	register int (*compar) __P((const void *, const void *));
64{
65	register const char *base = base0;
66	register size_t lim;
67	register int cmp;
68	register const void *p;
69
70	for (lim = nmemb; lim != 0; lim >>= 1) {
71		p = base + (lim >> 1) * size;
72		cmp = (*compar)(key, p);
73		if (cmp == 0)
74			return ((void *)p);
75		if (cmp > 0) {	/* key > p: move right */
76			base = (char *)p + size;
77			lim--;
78		}		/* else move left */
79	}
80	return (NULL);
81}
82