qsort.c

早期freebsd实现

/*
 * 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		qsz;			/* size of each record */
static  int		(*qcmp)();		/* the comparison routine */

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...
 */
qsort (base, n, size, compar)
     char *base;
     int n;
     int size;
     int (*compar)();
{
  register char *i, *j, *lo, *hi, *min;
  register int c;
  char *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);
      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).
 */

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

  lo = 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 * ((lo/qsz) >> 1);
      if (lo >= mthresh)
	{
	  j = ((*qcmp) ((jj = base), i) > 0 ? jj : i);
	  if ((*qcmp) (j, (tmp = max - qsz)) > 0)
	    {
	      j = (j == jj ? i : jj);	/* switch to first loser */
	      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);
}