glib/gqsort.c

/* GLIB - Library of useful routines for C programming
 * Copyright (C) 1991, 1992, 1996, 1997 Free Software Foundation, Inc.
 * Copyright (C) 2000 Eazel, Inc.
 * Copyright (C) 1995-1997  Peter Mattis, Spencer Kimball and Josh MacDonald
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

/*
 * This file was originally part of the GNU C Library, and was modified to allow
 * user data to be passed in to the sorting function.
 *
 * Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
 * Modified by Maciej Stachowiak (mjs@eazel.com)
 *
 * Modified by the GLib Team and others 1997-2000.  See the AUTHORS
 * file for a list of people on the GLib Team.  See the ChangeLog
 * files for a list of changes.  These files are distributed with
 * GLib at ftp://ftp.gtk.org/pub/gtk/.  */

#include <string.h>

#include "glib.h"

/* Byte-wise swap two items of size SIZE. */
#define SWAP(a, b, size)						      \
  do									      \
    {									      \
      register size_t __size = (size);					      \
      register char *__a = (a), *__b = (b);				      \
      do								      \
	{								      \
	  char __tmp = *__a;						      \
	  *__a++ = *__b;						      \
	  *__b++ = __tmp;						      \
	} while (--__size > 0);						      \
    } while (0)

/* Discontinue quicksort algorithm when partition gets below this size.
   This particular magic number was chosen to work best on a Sun 4/260. */
#define MAX_THRESH 4

/* Stack node declarations used to store unfulfilled partition obligations. */
typedef struct
{
  char *lo;
  char *hi;
}
stack_node;

/* The next 4 #defines implement a very fast in-line stack abstraction. */
#define STACK_SIZE	(8 * sizeof(unsigned long int))
#define PUSH(low, high)	((void) ((top->lo = (low)), (top->hi = (high)), ++top))
#define	POP(low, high)	((void) (--top, (low = top->lo), (high = top->hi)))
#define	STACK_NOT_EMPTY	(stack < top)


/* Order size using quicksort.  This implementation incorporates
 * four optimizations discussed in Sedgewick:
 *
 * 1. Non-recursive, using an explicit stack of pointer that store the next
 *    array partition to sort.  To save time, this maximum amount of space
 *    required to store an array of MAX_INT is allocated on the stack.  Assuming
 *    a 32-bit integer, this needs only 32 * sizeof(stack_node) == 136 bits.
 *    Pretty cheap, actually.
 *
 * 2. Chose the pivot element using a median-of-three decision tree.  This
 *    reduces the probability of selecting a bad pivot value and eliminates
 *    certain * extraneous comparisons.
 *
 * 3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving insertion
 *    sort to order the MAX_THRESH items within each partition.  This is a big
 *    win, since insertion sort is faster for small, mostly sorted array
 *    segments.
 *
 * 4. The larger of the two sub-partitions is always pushed onto the stack
 *    first, with the algorithm then concentrating on the smaller partition.
 *    This *guarantees* no more than log (n) stack size is needed (actually O(1)
 *    in this case)!
 */

void
g_qsort_with_data (gconstpointer    pbase,
		   gint             total_elems,
		   size_t           size,
		   GCompareFuncData compare_func,
		   gpointer         user_data)
{
  register char *base_ptr = (char *) pbase;

  /* Allocating SIZE bytes for a pivot buffer facilitates a better
   * algorithm below since we can do comparisons directly on the pivot.
   */
  char *pivot_buffer = (char *) alloca (size);
  const size_t max_thresh = MAX_THRESH * size;

  g_return_if_fail (total_elems > 0);
  g_return_if_fail (pbase != NULL);
  g_return_if_fail (compare_func != NULL);

  if (total_elems > MAX_THRESH)
    {
      char *lo = base_ptr;
      char *hi = &lo[size * (total_elems - 1)];
      /* Largest size needed for 32-bit int!!! */
      stack_node stack[STACK_SIZE];
      stack_node *top = stack + 1;

      while (STACK_NOT_EMPTY)
	{
	  char *left_ptr;
	  char *right_ptr;

	  char *pivot = pivot_buffer;

	  /* Select median value from among LO, MID, and HI. Rearrange
	   * LO and HI so the three values are sorted. This lowers the
	   * probability of picking a pathological pivot value and
	   * skips a comparison for both the LEFT_PTR and RIGHT_PTR. */

	  char *mid = lo + size * ((hi - lo) / size >> 1);

	  if ((*compare_func) ((void *) mid, (void *) lo, user_data) < 0)
	    SWAP (mid, lo, size);
	  if ((*compare_func) ((void *) hi, (void *) mid, user_data) < 0)
	    SWAP (mid, hi, size);
	  else
	    goto jump_over;
	  if ((*compare_func) ((void *) mid, (void *) lo, user_data) < 0)
	    SWAP (mid, lo, size);
	jump_over:;
	  memcpy (pivot, mid, size);
	  pivot = pivot_buffer;

	  left_ptr = lo + size;
	  right_ptr = hi - size;

	  /* Here's the famous ``collapse the walls'' section of quicksort.
	   * Gotta like those tight inner loops!  They are the main reason
	   * that this algorithm runs much faster than others. */
	  do
	    {
	      while ((*compare_func)
		     ((void *) left_ptr, (void *) pivot,
		      user_data) < 0)
		left_ptr += size;

	      while ((*compare_func)
		     ((void *) pivot, (void *) right_ptr,
		      user_data) < 0)
		right_ptr -= size;

	      if (left_ptr < right_ptr)
		{
		  SWAP (left_ptr, right_ptr, size);
		  left_ptr += size;
		  right_ptr -= size;
		}
	      else if (left_ptr == right_ptr)
		{
		  left_ptr += size;
		  right_ptr -= size;
		  break;
		}
	    }
	  while (left_ptr <= right_ptr);

	  /* Set up pointers for next iteration.  First determine whether
	   * left and right partitions are below the threshold size.  If so,
	   * ignore one or both.  Otherwise, push the larger partition's
	   * bounds on the stack and continue sorting the smaller one. */

	  if ((size_t) (right_ptr - lo) <= max_thresh)
	    {
	      if ((size_t) (hi - left_ptr) <= max_thresh)
		/* Ignore both small partitions. */
		POP (lo, hi);
	      else
		/* Ignore small left partition. */
		lo = left_ptr;
	    }
	  else if ((size_t) (hi - left_ptr) <= max_thresh)
				/* Ignore small right partition. */
	    hi = right_ptr;
	  else if ((right_ptr - lo) > (hi - left_ptr))
	    {
				/* Push larger left partition indices. */
	      PUSH (lo, right_ptr);
	      lo = left_ptr;

	    }
	  else
	    {
				/* Push larger right partition indices. */
	      PUSH (left_ptr, hi);
	      hi = right_ptr;
	    }
	}
    }

  /* Once the BASE_PTR array is partially sorted by quicksort the rest
   * is completely sorted using insertion sort, since this is efficient
   * for partitions below MAX_THRESH size. BASE_PTR points to the beginning
   * of the array to sort, and END_PTR points at the very last element in
   * the array (*not* one beyond it!). */

  {
    char *const end_ptr = &base_ptr[size * (total_elems - 1)];
    char *tmp_ptr = base_ptr;
    char *thresh = MIN (end_ptr, base_ptr + max_thresh);
    register char *run_ptr;

    /* Find smallest element in first threshold and place it at the
     * array's beginning.  This is the smallest array element,
     * and the operation speeds up insertion sort's inner loop. */

    for (run_ptr = tmp_ptr + size; run_ptr <= thresh;
	 run_ptr +=
	   size) if ((*compare_func) ((void *) run_ptr, (void *) tmp_ptr,
				      user_data) < 0)
	     tmp_ptr = run_ptr;

    if (tmp_ptr != base_ptr)
      SWAP (tmp_ptr, base_ptr, size);

    /* Insertion sort, running from left-hand-side up to right-hand-side.  */

    run_ptr = base_ptr + size;
    while ((run_ptr += size) <= end_ptr)
      {
	tmp_ptr = run_ptr - size;
	while ((*compare_func)
	       ((void *) run_ptr, (void *) tmp_ptr,
		user_data) < 0)
	  tmp_ptr -= size;

	tmp_ptr += size;
	if (tmp_ptr != run_ptr)
	  {
	    char *trav;

	    trav = run_ptr + size;
	    while (--trav >= run_ptr)
	      {
		char c = *trav;
		char *hi, *lo;

		for (hi = lo = trav;
		     (lo -= size) >= tmp_ptr; hi = lo)
		  *hi = *lo;
		*hi = c;
	      }
	  }
      }
  }
}