src/port/bsearch_arg.c

   1 /*
   2  * bsearch_arg.c: bsearch variant with a user-supplied pointer
   3  *
   4  * Copyright (c) 2021, PostgreSQL Global Development Group
   5  * Copyright (c) 1990 Regents of the University of California.
   6  * All rights reserved.
   7  *
   8  * Redistribution and use in source and binary forms, with or without
   9  * modification, are permitted provided that the following conditions
  10  * are met:
  11  * 1. Redistributions of source code must retain the above copyright
  12  *    notice, this list of conditions and the following disclaimer.
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice, this list of conditions and the following disclaimer in the
  15  *    documentation and/or other materials provided with the distribution.
  16  * 3. [rescinded 22 July 1999]
  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  * src/port/bsearch_arg.c
  34  */
  35
  36 #include "c.h"
  37
  38 /*
  39  * Perform a binary search.
  40  *
  41  * The code below is a bit sneaky.  After a comparison fails, we
  42  * divide the work in half by moving either left or right. If lim
  43  * is odd, moving left simply involves halving lim: e.g., when lim
  44  * is 5 we look at item 2, so we change lim to 2 so that we will
  45  * look at items 0 & 1.  If lim is even, the same applies.  If lim
  46  * is odd, moving right again involves halving lim, this time moving
  47  * the base up one item past p: e.g., when lim is 5 we change base
  48  * to item 3 and make lim 2 so that we will look at items 3 and 4.
  49  * If lim is even, however, we have to shrink it by one before
  50  * halving: e.g., when lim is 4, we still looked at item 2, so we
  51  * have to make lim 3, then halve, obtaining 1, so that we will only
  52  * look at item 3.
  53  */
  54 void *
  55 bsearch_arg(const void *key, const void *base0,
  56                         size_t nmemb, size_t size,
  57                         int (*compar) (const void *, const void *, void *),
  58                         void *arg)
  59 {
  60         const char *base = (const char *) base0;
  61         int                     lim,
  62                                 cmp;
  63         const void *p;
  64
  65         for (lim = nmemb; lim != 0; lim >>= 1)
  66         {
  67                 p = base + (lim >> 1) * size;
  68                 cmp = (*compar) (key, p, arg);
  69                 if (cmp == 0)
  70                         return (void *) p;
  71                 if (cmp > 0)
  72                 {                                               /* key > p: move right */
  73                         base = (const char *) p + size;
  74                         lim--;
  75                 }                                               /* else move left */
  76         }
  77         return (NULL);
  78 }