queue.c

/*
    Generic queue utilities.

    Copyright (C) 2002 Robert Lipe, robertlipe@usa.net

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program 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 General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111 USA

 */

#include "queue.h"
#include "stddef.h"

void
enqueue(queue *new_el, queue *old)
{
	new_el->next = old->next;
	new_el->prev = old;
	old->next->prev = new_el;
	old->next = new_el;
}

queue *
dequeue(queue *element)
{
	queue *prev = element->prev;
	queue *next = element->next;

	next->prev = prev;
	prev->next = next;
	
	QUEUE_INIT(element);
	return element;
}

/*
 * The following sorting code was derived from linked-list mergesort
 * sample code by Simon Tatham, code obtained from:
 *  http://www.chiark.greenend.org.uk/~sgtatham/algorithms/listsort.html
 * Modified for use with gpsbabel's queues by Paul Fox, October 2006.
 *
 * Original description and copyright messages follow...
 */

/*
 * Demonstration code for sorting a linked list.
 * 
 * The algorithm used is Mergesort, because that works really well
 * on linked lists, without requiring the O(N) extra space it needs
 * when you do it on arrays.
 * 
 * ... 
 */

/*
 * This file is copyright 2001 Simon Tatham.
 * 
 * Permission is hereby granted, free of charge, to any person
 * obtaining a copy of this software and associated documentation
 * files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use,
 * copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following
 * conditions:
 * 
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT.  IN NO EVENT SHALL SIMON TATHAM BE LIABLE FOR
 * ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF
 * CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */


void
sortqueue (queue *qh, int (*cmp)(const queue *, const queue *))
{

	queue *p, *q, *e, *tail, *oldhead, *list;
	int insize, nmerges, psize, qsize, i;

	/*
	 * Special case: if `list' is empty, we're done.
	 */
	if (QUEUE_EMPTY(qh))
		return;

	/*
	 * The algorithm doesn't really want the extra list head
	 * element.	 So remove the list head for now.  Put it back later.
	 */

	list = QUEUE_FIRST(qh);
	dequeue(qh);

	insize = 1;

	while (1) {
		p = list;
		oldhead = list;	 /* only used for circular linkage */
		list = NULL;
		tail = NULL;

		nmerges = 0;  /* count number of merges we do in this pass */

		while (p) {
			nmerges++;	/* there exists a merge to be done */
			/* step `insize' places along from p */
			q = p;
			psize = 0;
			for (i = 0; i < insize; i++) {
				psize++;
				q = (q->next == oldhead ? NULL : q->next);
				if (!q) break;
			}

			/* if q hasn't fallen off end, we have
			 * two lists to merge */
			qsize = insize;

			/* now we have two lists; merge them */
			while (psize > 0 || (qsize > 0 && q)) {

				/* decide whether next element of
				 * merge comes from p or q
				 */
				if (psize == 0) {
					/* p is empty; e must come from q. */
					e = q; q = q->next; qsize--;
					if (q == oldhead) q = NULL;
				} else if (qsize == 0 || !q) {
					/* q is empty; e must come from p. */
					e = p; p = p->next; psize--;
					if (p == oldhead) p = NULL;
				} else if (cmp(p,q) <= 0) {
					/* First element of p is
					 * lower (or same); e must
					 * come from p.
					 */
					e = p; p = p->next; psize--;
					if (p == oldhead) p = NULL;
				} else {
					/* First element of q is
					 * lower; e must come from
					 * q.
					 */
					e = q; q = q->next; qsize--;
					if (q == oldhead) q = NULL;
				}

				/* add the next element to the merged list */
				if (tail) {
					tail->next = e;
				} else {
					list = e;
				}

				/* Maintain reverse pointers in a
				 * doubly linked list.  */
				e->prev = tail;

				tail = e;
			}

			/* now p has stepped `insize' places
			 * along, and q has too */
			p = q;
		}
		tail->next = list;
		list->prev = tail;

		/* If we have done only one merge, we're finished.
		 * Allow for nmerges==0, the empty list case.
		 */
		if (nmerges <= 1) {

			/* Put the list head back at the start of the list */
			ENQUEUE_TAIL(list, qh);
			return;

		}

		/* Otherwise repeat, merging lists twice the size */
		insize *= 2;
	}
}