StdLib/LibC/StdLib/Bsearch.c

   1 /** @file
   2   Binary search utility function.
   3
   4   This utility makes use of a comparison function to search arrays of
   5   unspecified type. Where an argument declared as size_t nmemb specifies the
   6   length of the array for a function, nmemb can have the value zero on a call
   7   to that function; the comparison function is not called, a search finds no
   8   matching element. Pointer arguments on such a call shall still have valid
   9   values.
  10
  11   The implementation shall ensure that the second argument of the comparison
  12   function is a pointer to an element of the array. The first argument shall
  13   equal key.
  14
  15   The comparison function shall not alter the contents of the array. The
  16   implementation may reorder elements of the array between calls to the
  17   comparison function, but shall not alter the contents of any individual
  18   element.
  19
  20   When the same objects (consisting of size bytes, irrespective of their
  21   current positions in the array) are passed more than once to the comparison
  22   function, the results shall be consistent with one another. That is, the same
  23   object shall always compare the same way with the key.
  24
  25   A sequence point occurs immediately before and immediately after each call to
  26   the comparison function, and also between any call to the comparison function
  27   and any movement of the objects passed as arguments to that call.
  28
  29   Copyright (c) 2010, Intel Corporation. All rights reserved.<BR>
  30
  31  * Copyright (c) 1990, 1993
  32  *  The Regents of the University of California.  All rights reserved.
  33  *
  34  * Redistribution and use in source and binary forms, with or without
  35  * modification, are permitted provided that the following conditions
  36  * are met:
  37  * 1. Redistributions of source code must retain the above copyright
  38  *    notice, this list of conditions and the following disclaimer.
  39  * 2. Redistributions in binary form must reproduce the above copyright
  40  *    notice, this list of conditions and the following disclaimer in the
  41  *    documentation and/or other materials provided with the distribution.
  42  * 4. Neither the name of the University nor the names of its contributors
  43  *    may be used to endorse or promote products derived from this software
  44  *    without specific prior written permission.
  45  *
  46  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  47  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  48  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  49  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  50  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  51  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  52  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  53  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  54  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  55  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  56  * SUCH DAMAGE.
  57
  58   ("$FreeBSD: src/lib/libc/stdlib/bsearch.c,v 1.4.10.1.2.1 2009/10/25 01:10:29 kensmith Exp $");
  59 **/
  60 #include  <LibConfig.h>
  61 #include  <sys/EfiCdefs.h>
  62 #include  <stdlib.h>
  63
  64 /*
  65  * Perform a binary search.
  66  *
  67  * The code below is a bit sneaky.  After a comparison fails, we
  68  * divide the work in half by moving either left or right. If lim
  69  * is odd, moving left simply involves halving lim: e.g., when lim
  70  * is 5 we look at item 2, so we change lim to 2 so that we will
  71  * look at items 0 & 1.  If lim is even, the same applies.  If lim
  72  * is odd, moving right again involes halving lim, this time moving
  73  * the base up one item past p: e.g., when lim is 5 we change base
  74  * to item 3 and make lim 2 so that we will look at items 3 and 4.
  75  * If lim is even, however, we have to shrink it by one before
  76  * halving: e.g., when lim is 4, we still looked at item 2, so we
  77  * have to make lim 3, then halve, obtaining 1, so that we will only
  78  * look at item 3.
  79  */
  80 void *
  81 bsearch(
  82   const void *key,
  83   const void *base0,
  84   size_t nmemb,
  85   size_t size,
  86   int (*compar)(const void *, const void *)
  87   )
  88 {
  89   const char *base = base0;
  90   size_t      lim;
  91   int         cmp;
  92   const void *p;
  93
  94   for (lim = nmemb; lim != 0; lim >>= 1) {
  95     p = base + (lim >> 1) * size;
  96     cmp = (*compar)(key, p);
  97     if (cmp == 0)
  98       return ((void *)p);
  99     if (cmp > 0) {  /* key > p: move right */
 100       base = (char *)p + size;
 101       lim--;
 102     }   /* else move left */
 103   }
 104   return (NULL);
 105 }