/** An instance of this class represents a queue capable of holding items of
**  type T (the generic type parameter).
**  The implementation is based on storing the queue items in a linked
**  structure of objects arising from the Link1 class.
**
**  Author: R. McCloskey
**  Date: March 2012
*/
public class QueueViaLink1<T> implements Queue<T> {

   /*  i n s t a n c e    v a r i a b l e s  */

   private Link1<T> front;  // reference to Link1 object holding front item 
   private Link1<T> rear;   // reference to Link1 object holding rear item 
   private int numItems;    // # of items on the queue


   /*  c o n s t r u c t o r s  */

   public QueueViaLink1() 
   { 
      front = null;  rear = null;  numItems = 0;
   }


   /*  o b s e r v e r s  */

   
   public int sizeOf() { return numItems; }

   public boolean isEmpty() { return sizeOf() == 0; }

   public T frontOf() 
   { 
      return front.item;   
      // alternative: return front.getItem();
      // alternative: return item(0)
    }

   public T item(int k)
   {
      Link1<T> p = front;           // have p begin at the front
      for (int i=0; i!=k; i++) {    // follow next references k times
         p = p.next;     // alternative: p = p.getNext();
      }
      // return the item in the node to which p points
      return p.item;   // alternative: return p.getItem();
   }


   public String toString()
   {
      StringBuilder s = new StringBuilder();
      Link1<T> p = front;
      while (p != null) {
         s.append(p.item.toString() + ",");
         p = p.next;
      }
      return s.substring(0,Math.max(0, s.length()-1));
   }
   // Alternative O(N^2)-time loop for above method
   //   final int N = sizeOf();
   //   for (int i=0; i != N; i++) {
   //      s.append(item(i).toString() + ",");
   //   }


   /*  m u t a t o r s  */

   public void enqueue(T elem)  
   { 
      Link1<T> newRear = new Link1<T>(elem, null);
      if (isEmpty()) { 
         front = newRear;
      }
      else { 
         rear.next = newRear;     // alternative: rear.setNext(newRear);
      }
      rear = newRear;
      numItems = numItems + 1;
   }

   public void dequeue()
   { 
      front = front.next;  // or front = front.getNext();
      numItems = numItems - 1;
      if (front == null) { rear = null; }  // not necessary
   }

}