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bsearch.c revision 1.5 
      1 /*	$NetBSD: bsearch.c,v 1.5 1995/12/28 08:52:10 thorpej Exp $	*/
      2 
      3 /*
      4  * Copyright (c) 1990 Regents of the University of California.
      5  * 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. All advertising materials mentioning features or use of this software
     16  *    must display the following acknowledgement:
     17  *	This product includes software developed by the University of
     18  *	California, Berkeley and its contributors.
     19  * 4. Neither the name of the University nor the names of its contributors
     20  *    may be used to endorse or promote products derived from this software
     21  *    without specific prior written permission.
     22  *
     23  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     24  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     25  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     26  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     27  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     28  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     29  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     30  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     31  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     32  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     33  * SUCH DAMAGE.
     34  */
     35 
     36 #if defined(LIBC_SCCS) && !defined(lint)
     37 #if 0
     38 static char *sccsid = "from: @(#)bsearch.c	5.4 (Berkeley) 2/23/91";
     39 #else
     40 static char *rcsid = "$NetBSD: bsearch.c,v 1.5 1995/12/28 08:52:10 thorpej Exp $";
     41 #endif
     42 #endif /* LIBC_SCCS and not lint */
     43 
     44 #include <stdlib.h>
     45 
     46 /*
     47  * Perform a binary search.
     48  *
     49  * The code below is a bit sneaky.  After a comparison fails, we
     50  * divide the work in half by moving either left or right. If lim
     51  * is odd, moving left simply involves halving lim: e.g., when lim
     52  * is 5 we look at item 2, so we change lim to 2 so that we will
     53  * look at items 0 & 1.  If lim is even, the same applies.  If lim
     54  * is odd, moving right again involes halving lim, this time moving
     55  * the base up one item past p: e.g., when lim is 5 we change base
     56  * to item 3 and make lim 2 so that we will look at items 3 and 4.
     57  * If lim is even, however, we have to shrink it by one before
     58  * halving: e.g., when lim is 4, we still looked at item 2, so we
     59  * have to make lim 3, then halve, obtaining 1, so that we will only
     60  * look at item 3.
     61  */
     62 void *
     63 bsearch(key, base0, nmemb, size, compar)
     64 	register const void *key;
     65 	const void *base0;
     66 	size_t nmemb;
     67 	register size_t size;
     68 	register int (*compar) __P((const void *, const void *));
     69 {
     70 	register const char *base = base0;
     71 	register int lim, cmp;
     72 	register const void *p;
     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 ((void *)p);
     79 		if (cmp > 0) {	/* key > p: move right */
     80 			base = (char *)p + size;
     81 			lim--;
     82 		} /* else move left */
     83 	}
     84 	return (NULL);
     85 }
     86