qsort2.c

/*-
 * Copyright (c) 1980, 1983 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted
 * provided that: (1) source distributions retain this entire copyright
 * notice and comment, and (2) distributions including binaries display
 * the following acknowledgement:  ``This product includes software
 * developed by the University of California, Berkeley and its contributors''
 * in the documentation or other materials provided with the distribution
 * and in all advertising materials mentioning features or use of this
 * software. 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 ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

#if defined(LIBC_SCCS) && !defined(lint)
static char sccsid[] = "@(#)qsort.c	5.7 (Berkeley) 5/17/90";
#endif /* LIBC_SCCS and not lint */

#include <stdlib.h>

/*
 * qsort.c:
 * Our own version of the system qsort routine which is faster by an average
 * of 25%, with lows and highs of 10% and 50%.
 * The THRESHold below is the insertion sort threshold, and has been adjusted
 * for records of size 48 bytes.
 * The MTHREShold is where we stop finding a better median.
 */

#define		THRESH		4		/* threshold for insertion */
#define		MTHRESH		6		/* threshold for median */

static  int		(*qcmp)();		/* the comparison routine */
static  int		qsz;			/* size of each record */
static  int		thresh;			/* THRESHold in chars */
static  int		mthresh;		/* MTHRESHold in chars */

/*
 * qsort:
 * First, set up some global parameters for qst to share.  Then, quicksort
 * with qst(), and then a cleanup insertion sort ourselves.  Sound simple?
 * It's not...
 */

typedef int (*compFn)();

static void qst(char* base, char* max);

void qsort2(char* base, int n, int size, compFn compar)
{
    char c, *i, *j, *lo, *hi;
    char *min, *max;

    if (n <= 1)
        return;
    qsz = size;
    qcmp = compar;
    thresh = qsz * THRESH;
    mthresh = qsz * MTHRESH;
    max = base + n * qsz;
    if (n >= THRESH) {
        qst(base, max);
        hi = base + thresh;
    } else {
        hi = max;
    }
    /*
     * First put smallest element, which must be in the first THRESH, in
     * the first position as a sentinel.  This is done just by searching
     * the first THRESH elements (or the first n if n < THRESH), finding
     * the min, and swapping it into the first position.
     */
    for (j = lo = base; (lo += qsz) < hi; )
        if (qcmp(j, lo) > 0)
            j = lo;
    if (j != base) {
        /* swap j into place */
        for (i = base, hi = base + qsz; i < hi; ) {
            c = *j;
            *j++ = *i;
            *i++ = c;
        }
    }
    /*
     * With our sentinel in place, we now run the following hyper-fast
     * insertion sort.  For each remaining element, min, from [1] to [n-1],
     * set hi to the index of the element AFTER which this one goes.
     * Then, do the standard insertion sort shift on a character at a time
     * basis for each element in the frob.
     */
    for (min = base; (hi = min += qsz) < max; ) {
        while (qcmp(hi -= qsz, min) > 0)
            /* void */;
        if ((hi += qsz) != min) {
            for (lo = min + qsz; --lo >= min; ) {
                c = *lo;
                for (i = j = lo; (j -= qsz) >= hi; i = j)
                    *i = *j;
                *i = c;
            }
        }
    }
}

/*
 * qst:
 * Do a quicksort
 * First, find the median element, and put that one in the first place as the
 * discriminator.  (This "median" is just the median of the first, last and
 * middle elements).  (Using this median instead of the first element is a big
 * win).  Then, the usual partitioning/swapping, followed by moving the
 * discriminator into the right place.  Then, figure out the sizes of the two
 * partions, do the smaller one recursively and the larger one via a repeat of
 * this code.  Stopping when there are less than THRESH elements in a partition
 * and cleaning up with an insertion sort (in our caller) is a huge win.
 * All data swaps are done in-line, which is space-losing but time-saving.
 * (And there are only three places where this is done).
 */

static void qst(char* base, char* max)
{
    char c, *i, *j, *jj;
    int ii;
    char *mid, *tmp;
    int lo, hi;

    /*
     * At the top here, lo is the number of characters of elements in the
     * current partition.  (Which should be max - base).
     * Find the median of the first, last, and middle element and make
     * that the middle element.  Set j to largest of first and middle.
     * If max is larger than that guy, then it's that guy, else compare
     * max with loser of first and take larger.  Things are set up to
     * prefer the middle, then the first in case of ties.
     */
    lo = max - base;		/* number of elements as chars */
    do	{
        mid = i = base + qsz * ((lo / qsz) >> 1);
        if (lo >= mthresh) {
            j = (qcmp((jj = base), i) > 0 ? jj : i);
            if (qcmp(j, (tmp = max - qsz)) > 0) {
                /* switch to first loser */
                j = (j == jj ? i : jj);
                if (qcmp(j, tmp) < 0)
                    j = tmp;
            }
            if (j != i) {
                ii = qsz;
                do	{
                    c = *i;
                    *i++ = *j;
                    *j++ = c;
                } while (--ii);
            }
        }
        /*
         * Semi-standard quicksort partitioning/swapping
         */
        for (i = base, j = max - qsz; ; ) {
            while (i < mid && qcmp(i, mid) <= 0)
                i += qsz;
            while (j > mid) {
                if (qcmp(mid, j) <= 0) {
                    j -= qsz;
                    continue;
                }
                tmp = i + qsz;	/* value of i after swap */
                if (i == mid) {
                    /* j <-> mid, new mid is j */
                    mid = jj = j;
                } else {
                    /* i <-> j */
                    jj = j;
                    j -= qsz;
                }
                goto swap;
            }
            if (i == mid) {
                break;
            } else {
                /* i <-> mid, new mid is i */
                jj = mid;
                tmp = mid = i;	/* value of i after swap */
                j -= qsz;
            }
        swap:
            ii = qsz;
            do	{
                c = *i;
                *i++ = *jj;
                *jj++ = c;
            } while (--ii);
            i = tmp;
        }
        /*
         * Look at sizes of the two partitions, do the smaller
         * one first by recursion, then do the larger one by
         * making sure lo is its size, base and max are update
         * correctly, and branching back.  But only repeat
         * (recursively or by branching) if the partition is
         * of at least size THRESH.
         */
        i = (j = mid) + qsz;
        if ((lo = j - base) <= (hi = max - i)) {
            if (lo >= thresh)
                qst(base, j);
            base = i;
            lo = hi;
        } else {
            if (hi >= thresh)
                qst(i, max);
            max = j;
        }
    } while (lo >= thresh);
}