/*
    -- l_queue.c -- Tony Givargis
    Linked list Queue implementation for medium to large memory models.
*/

#include <malloc.h>
#include "l_queue.h"

/*---------------------------------------------------------------------------*/

/*
    EL_QueueCreate

    Creates an empty queue and returns an EL_Queue* ADT to caller. This ADT
    is passed to subsequent queue functions as the identifier (handle) of
    this particular queue. Returns EL_Queue* on success, NULL otherwise.
    --> O(1)
*/
EL_Queue* EL_QueueCreate(void) {

    EL_Queue* q = malloc(sizeof(EL_Queue));

    if( q != NULL ) {

        q->head = NULL;
        q->tail = NULL;
        q->size = 0;
    }
    return q;
}

/*---------------------------------------------------------------------------*/

/*
    EL_QueueDestroy

    Destroys queue (q).
    --> O(n)
*/
void EL_QueueDestroy(EL_Queue* q) {

    while( q->head != NULL ) {

        q->tail = q->head;
        q->head = q->head->link;
        free(q->tail);
    }
    free(q);
}

/*---------------------------------------------------------------------------*/

/*
    EL_QueuePush

    Pushes data (element) onto the queue (q). Retruns data element on
    success, NULL otherwise.
    --> O(1)
*/
void* EL_QueuePush(EL_Queue* q, void* element) {

    EL_QueueNode* n = malloc(sizeof(EL_QueueNode));

    if( n != NULL ) {

        n->element = element;
        n->link = NULL;
        q->size++;

        if( q->tail != NULL ) {		/* queue is not empty */

            q->tail->link = n;
        }
        if( q->head == NULL ) {		/* queue is empty */

            q->head = n;
	}
	q->tail = n;
    }
    return n;
}

/*---------------------------------------------------------------------------*/

/*
    EL_QueuePop

    Pops data element from the queue (q). Returns data element on success,
    NULL otherwise.
    --> O(1)
*/
void* EL_QueuePop(EL_Queue* q) {

    void* element = NULL;
    EL_QueueNode* n = q->head;

    if( n != NULL ) {

	q->size--;
	q->head = n->link;
	element = n->element;
	if( q->head == NULL ) {

	    q->tail = NULL;
	}
	free(n);
    }
    return element;
}