#include "dqueue.h"
  #include "dlist.h"
  #include <stdlib.h>

  dqueue
  dq_create(void)
  {
    dqueue q = (dqueue)malloc(sizeof(struct dqueued));
    q -> head = dcons(NULL, NULL, NULL);
    q -> tail = dcons(NULL, NULL, NULL);
    q -> head -> next = q -> tail;
    q -> tail -> prev = q -> head;
    return q;
  }

  int
  dq_empty(dqueue q)
  {
    return q -> head -> next == q -> tail;
  }

void dq_destory(dqueue q)
{
	while(!dq_empty(q))
		dq_deq_back(q);
}

  void
  dq_enq_front(dqueue q, void *element)
  {
    create_and_link(element, q -> head, q -> head -> next);
  }

  void
  dq_enq_back(dqueue q, void *element)
  {
    create_and_link(element, q -> tail -> prev, q -> tail);
  }

  void *
  dq_deq_front(dqueue q)
  {
    //assert(!empty(q));
    return unlink_and_free(q -> head -> next);
  }

  void *
  dq_deq_back(dqueue q)
  {
    //assert(!empty(q));
    return unlink_and_free(q -> tail -> prev);
  }