gonzui

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