// CSE 373, Winter 2013, Marty Stepp
// This class defines an implementation of the stack ADT using an array
// as the internal data structure.

import java.util.Arrays;
import java.util.EmptyStackException;

public class ArrayStack<E> implements Stack<E> {
	private E[] elements;
	private int size;
	
	// Constructs a new empty stack.
	public ArrayStack() {
		elements = (E[]) (new Object[5]);
		size = 0;
	}
	
	// Adds the given value to the top of this stack.
	public void push(E value) {
		if (size == elements.length) {
			// copy into an array twice as large
			elements = Arrays.copyOf(elements, 2 * size);
		}
		elements[size] = value;
		size++;
	}

	// Removes and returns the top element of this stack.
	// Throws an EmptyStackException if the stack is empty.
	public E pop() {
		if (size == 0) {
			throw new EmptyStackException();
		}
		
		E top = elements[size - 1];
		elements[size - 1] = null;
		size--;
		return top;
	}

	// Returns the top element of this stack without removing it.
	// Throws an EmptyStackException if the stack is empty.
	public E peek() {
		if (size == 0) {
			throw new EmptyStackException();
		}
		return elements[size - 1];
	}

	// Returns true if the stack does not contain any elements.
	public boolean isEmpty() {
		return (size == 0);
	}

	// Returns the number of elements contained in this stack.
	public int size() {
		return size;
	}
}