usr/src/lib/libc/stdlib/bsearch.c

/*
 * Copyright (c) 1990 Regents of the University of California.
 * All rights reserved.
 *
 * %sccs.include.redist.c%
 */

#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)bsearch.c   5.2 (Berkeley) %G%";
#endif /* LIBC_SCCS and not lint */

#include <stddef.h>             /* size_t */

/*
 * 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 involes 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(key, base0, nmemb, size, compar)
        register void *key;
        void *base0;
        size_t nmemb;
        register size_t size;
        register int (*compar)();
{
        register char *base = base0;
        register int lim, cmp;
        register void *p;

        for (lim = nmemb; lim != 0; lim >>= 1) {
                p = base + (lim >> 1) * size;
                cmp = (*compar)(key, p);
                if (cmp == 0)
                        return (p);
                if (cmp > 0) {  /* key > p: move right */
                        base = (char *)p + size;
                        lim--;
                } /* else move left */
        }
        return (NULL);
}