//  Bare-bones, array-based implementation of a fixed-capacity queue
//  in the style of Sedgwick & Wayne fixed-capacity stack (p. 135).
//
//  front is the index of the first element in the queue.
//  back is the index of the first free element after the end of the queue.
//  (front == back) is defined to be an empty queue.
//  
public class FixedCapacityQueue<Item>
{
    private Item [] a;
    private int capacity;
    private int front;
    private int back;

    public FixedCapacityQueue(int cap) {   
        a = (Item[]) new Object[cap];  
	front = back = 0;                 // not strictly necessary:  java initializes instance variables to zero
        capacity = cap;
    }

    public boolean isEmpty()  {  return front == back;  }
    
    public int size()  {
        if (back >= front) return back - front;
	else return capacity - (back - front);   // wraparound case
    }

    public void enqueue(Item item) {
        a[back] = item;
	back = (back + 1) % capacity;    // handles wraparound
    }

    public Item dequeue() {
        Item d = a[front];
	a[front] = null;                   // Avoid loitering
	front = (front + 1) % capacity;    // handles wraparound
	return d;
    }
}