qsort.c

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/*
 * Copyright (c) 2015 Cisco and/or its affiliates.
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * Imported into CLIB by Eliot Dresselhaus from:
 *
 *  This file is part of
 *      MakeIndex - A formatter and format independent index processor
 *
 *  This file is public domain software donated by
 *  Nelson Beebe (beebe@science.utah.edu).
 *
 *  modifications copyright (c) 2003 Cisco Systems, Inc.
 */

#include <vppinfra/clib.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 */

typedef struct {
  word qsz;                     /* size of each record */
  word thresh;                  /* THRESHold in chars */
  word mthresh;                 /* MTHRESHold in chars */
  int (*qcmp) (const void *, const void *); /* the comparison routine */
} qst_t;

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

/*
 * qqsort: 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...
 */

void
qsort (void * base, uword n, uword size,
       int (*compar) (const void *, const void *))
{
  char *i;
  char *j;
  char *lo;
  char *hi;
  char *min;
  char c;
  char *max;
  qst_t _q, * q = &_q;

  if (n <= 1)
    return;

  q->qsz = size;
  q->qcmp = compar;
  q->thresh = q->qsz * THRESH;
  q->mthresh = q->qsz * MTHRESH;
  max = base + n * q->qsz;
  if (n >= THRESH) {
    qst(q, base, max);
    hi = base + q->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 += q->qsz) < hi;) {
    if ((*compar) (j, lo) > 0)
      j = lo;
  }
  if (j != base) {                     /* swap j into place */
    for (i = base, hi = base + q->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 += q->qsz) < max;) {
    while ((*q->qcmp) (hi -= q->qsz, min) > 0);
    if ((hi += q->qsz) != min) {
      for (lo = min + q->qsz; --lo >= min;) {
        c = *lo;
        for (i = j = lo; (j -= q->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(qst_t *q, char *base, char *max)
{
  char *i;
  char *j;
  char *jj;
  char *mid;
  int ii;
  char c;
  char   *tmp;
  int     lo;
  int     hi;
  int qsz = q->qsz;

  lo = (int)(max - base);               /* number of elements as chars */
  do {
    /*
     * 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.
     */
    mid = i = base + qsz * ((unsigned) (lo / qsz) >> 1);
    if (lo >= q->mthresh) {
      j = ((*q->qcmp) ((jj = base), i) > 0 ? jj : i);
      if ((*q->qcmp) (j, (tmp = max - qsz)) > 0) {
        /* switch to first loser */
        j = (j == jj ? i : jj);
        if ((*q->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 && (*q->qcmp) (i, mid) <= 0)
        i += qsz;
      while (j > mid) {
        if ((*q->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 = (int)(j - base)) <= (hi = (int)(max - i))) {
      if (lo >= q->thresh)
        qst(q, base, j);
      base = i;
      lo = hi;
    } else {
      if (hi >= q->thresh)
        qst(q, i, max);
      max = j;
    }
  } while (lo >= q->thresh);
}