bsearch_arg.c

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/*
 * bsearch_arg.c: bsearch variant with a user-supplied pointer
 *
 * Copyright (c) 2021-2024, PostgreSQL Global Development Group
 * Copyright (c) 1990 Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. [rescinded 22 July 1999]
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * src/port/bsearch_arg.c
 */
 
#include "c.h"
 
/*
 * Perform a binary search.
 *
 * The code below is a bit sneaky.  After a comparison fails, we
 * divide the work in half by moving either left or right. If lim
 * is odd, moving left simply involves halving lim: e.g., when lim
 * is 5 we look at item 2, so we change lim to 2 so that we will
 * look at items 0 & 1.  If lim is even, the same applies.  If lim
 * is odd, moving right again involves halving lim, this time moving
 * the base up one item past p: e.g., when lim is 5 we change base
 * to item 3 and make lim 2 so that we will look at items 3 and 4.
 * If lim is even, however, we have to shrink it by one before
 * halving: e.g., when lim is 4, we still looked at item 2, so we
 * have to make lim 3, then halve, obtaining 1, so that we will only
 * look at item 3.
 */
void *
bsearch_arg(const void *key, const void *base0,
            size_t nmemb, size_t size,
            int (*compar) (const void *, const void *, void *),
            void *arg)
{
    const char *base = (const char *) base0;
    size_t      lim;
    int         cmp;
    const void *p;
 
    for (lim = nmemb; lim != 0; lim >>= 1)
    {
        p = base + (lim >> 1) * size;
        cmp = (*compar) (key, p, arg);
        if (cmp == 0)
            return (void *) p;
        if (cmp > 0)
        {                       /* key > p: move right */
            base = (const char *) p + size;
            lim--;
        }                       /* else move left */
    }
    return (NULL);
}