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bsearch.c revision 1.1 
      1 /*
      2  * Copyright (c) 1990 Regents of the University of California.
      3  * 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)
     35 static char sccsid[] = "@(#)bsearch.c	5.4 (Berkeley) 2/23/91";
     36 #endif /* LIBC_SCCS and not lint */
     37 
     38 #include <stddef.h>		/* size_t */
     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  */
     57 void *
     58 bsearch(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 int lim, cmp;
     67 	register const void *p;
     68 
     69 	for (lim = nmemb; lim != 0; lim >>= 1) {
     70 		p = base + (lim >> 1) * size;
     71 		cmp = (*compar)(key, p);
     72 		if (cmp == 0)
     73 			return ((void *)p);
     74 		if (cmp > 0) {	/* key > p: move right */
     75 			base = (char *)p + size;
     76 			lim--;
     77 		} /* else move left */
     78 	}
     79 	return (NULL);
     80 }
     81