#
/* q.c */

/* EMACS_MODES: c !fill */

/* General-purpose queue manipulation routines.  Contains the following
 * routines:
 *	q_deq		dequeue and return first element from queue
 *	q_del		delete element from queue
 * All other general-purpose queue manipulation routines are macros
 * defined in "q.h".
 */

#include	"q.h"

#define	FALSE		0
#define	TRUE		1


q_elt q_deq (q)

/* Dequeue and return the first element of the specified queue.  Returns
 * a pointer to the first element if any, or NULL if the queue is empty.
 *
 * Arguments:
 */

register queue	*q;
{
	register q_elt	temp;		/* temp for result */
	
	if ((temp = q->q_head) == NULL)	/* queue empty? */
		return (NULL);		/* yes, show none */
	q->q_head = temp->qe_next;	/* else unlink */
	temp->qe_next = NULL;		/* avoid dangling pointers */
	if (q->q_head == NULL)		/* queue empty? */
		q->q_tail = NULL;	/* yes, update tail pointer too */
	q->q_len--;			/* update queue length */
	return (temp);
}


q_del (q, elt)

/* Delete the specified element from the queue.  This requires scanning
 * the queue from the top to find and remove the element, so it takes
 * O(queue length) time to execute.  Note that this routine must not
 * run at interrupt level.
 * Returns TRUE if the element is successfully deleted, or FALSE if
 * the element is not found in the queue.
 */

register queue	*q;			/* the queue */
register q_elt	elt;			/* element to delete */
{
	register q_elt	*tmp;		/* temp for chaining */
	
	for (tmp = &q->q_head; *tmp != elt;
	     tmp = (q_elt *)(((q_elt)tmp)->qe_next))
		if (*tmp == NULL)	/* find ptr. in queue to elt. */
			return (FALSE); /* if not in queue, punt */
			
	*tmp = (*tmp)->qe_next;		/* else link it out of the queue */

	if (q->q_tail == elt) {		/* at end of queue? */
		if (tmp == &q->q_head)	/* yes; if first elt, zero out tail */
			q->q_tail = NULL;
		else			/* otherwise tail is previous elt */
			q->q_tail = (q_elt)tmp;
	}
	elt->qe_next = NULL;		/* avoid dangling pointers */
	q->q_len--;			/* update element count */
	return (TRUE);
}