ResizingArrayQueue.java

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package Fundamentals;

/**
 *
 * @author stefan0
 */
/*************************************************************************
 *  Compilation:  javac ResizingArrayQueue.java
 *  Execution:    java ResizingArrayQueue < input.txt
 *  Data files:   http://algs4.cs.princeton.edu/13stacks/tobe.txt  
 *  
 *  Queue implementation with a resizing array.
 *
 *  % java ResizingArrayQueue < tobe.txt 
 *  to be or not to be (2 left on queue)
 *
 *************************************************************************/

import java.util.Iterator;
import java.util.NoSuchElementException;

public class ResizingArrayQueue<Item> implements Iterable<Item> {
    private Item[] q;            // queue elements
    private int N = 0;           // number of elements on queue
    private int first = 0;       // index of first element of queue
    private int last  = 0;       // index of next available slot

    // cast needed since no generic array creation in Java
    public ResizingArrayQueue() {
        q = (Item[]) new Object[2];
    }

    public boolean isEmpty() { return N == 0;    }
    public int size()        { return N;         }

    // resize the underlying array
    private void resize(int max) {
        assert max >= N;
        Item[] temp = (Item[]) new Object[max];
        for (int i = 0; i < N; i++) {
            temp[i] = q[(first + i) % q.length];
        }
        q = temp;
        first = 0;
        last  = N;
    }


    public void enqueue(Item item) {
        // double size of array if necessary and recopy to front of array
        if (N == q.length) resize(2*q.length);   // double size of array if necessary
        q[last++] = item;                        // add item
        if (last == q.length) last = 0;          // wrap-around
        N++;
    }

    // remove the least recently added item 
    public Item dequeue() {
        if (isEmpty()) throw new RuntimeException("Queue underflow");
        Item item = q[first];
        q[first] = null;                            // to avoid loitering
        N--;
        first++;
        if (first == q.length) first = 0;           // wrap-around
        // shrink size of array if necessary
        if (N > 0 && N == q.length/4) resize(q.length/2); 
        return item;
    }

    public Iterator<Item> iterator() { return new ArrayIterator(); }

    // an iterator, doesn't implement remove() since it's optional
    private class ArrayIterator implements Iterator<Item> {
        private int i = 0;
        public boolean hasNext()  { return i < N;                               }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = q[(i + first) % q.length];
            i++;
            return item;
        }
    }

   /**
     * A test client.
     */
    public static void main(String[] args) {
        ResizingArrayQueue<String> q = new ResizingArrayQueue<String>();
        /*
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            if (!item.equals("-")) q.enqueue(item);
            else if (!q.isEmpty()) StdOut.print(q.dequeue() + " ");
        }
        StdOut.println("(" + q.size() + " left on queue)");
        */
    }

}