src/util/qsort.c

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#include "util.h"

#ifndef lint
static char rcsid[] UNUSED = "$Id: qsort.c,v 1.5 2002/08/25 05:30:13 fabio Exp $";
#endif

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

static  int             (*qcmp)(const void *, const void *);
                                                /* the comparison routine */
static  int             qsz;                    /* size of each record */
static  int             thresh;                 /* THRESHold in chars */
static  int             mthresh;                /* MTHRESHold in chars */
static  void            qst ARGS((char *base, char *max));

/*---------------------------------------------------------------------------*/
/* Definition of exported functions                                          */
/*---------------------------------------------------------------------------*/

#undef min
#undef max

void
qsort(
  void *vbase,
  size_t n,
  size_t size,
  int (*compar)(const void *, const void *))
{
        register char c, *i, *j, *lo, *hi;
        char *min, *max, *base;

        if (n <= 1)
                return;
        base = (char *) vbase;
        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;
                        }
                }
        }
}

/*---------------------------------------------------------------------------*/
/* Definition of static functions                                            */
/*---------------------------------------------------------------------------*/

static void
qst(char *base, char *max)
{
        register char c, *i, *j, *jj;
        register 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);
}

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