bsearch.c (3156B)
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 #include <stddef.h> 35 #include <stdlib.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(key, base0, nmemb, size, compar) 55 const void *key; 56 const void *base0; 57 size_t nmemb; 58 size_t size; 59 int (*compar)(const void *, const void *); 60 { 61 const char *base = base0; 62 size_t lim; 63 int cmp; 64 const void *p; 65 66 for (lim = nmemb; lim != 0; lim >>= 1) { 67 p = base + (lim >> 1) * size; 68 cmp = (*compar)(key, p); 69 if (cmp == 0) 70 return ((void *)p); 71 if (cmp > 0) { /* key > p: move right */ 72 base = (char *)p + size; 73 lim--; 74 } /* else move left */ 75 } 76 return (NULL); 77 }