/*
 *  Copyright 2001-2004 The Apache Software Foundation
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.commons.collections;

import java.util.ArrayList;
import java.util.EmptyStackException;

/**
 * An implementation of the {@link java.util.Stack} API that is based on an
 * ArrayList instead of a Vector, so it is not
 * synchronized to protect against multi-threaded access.  The implementation
 * is therefore operates faster in environments where you do not need to
 * worry about multiple thread contention.
 * 

 * The removal order of an ArrayStack is based on insertion 
 * order: The most recently added element is removed first.  The iteration
 * order is not the same as the removal order.  The iterator returns
 * elements from the bottom up, whereas the {@link #remove()} method removes
 * them from the top down.
 * 

 * Unlike Stack, ArrayStack accepts null entries.
 *
 * @see java.util.Stack
 * @since Commons Collections 1.0
 * @version $Revision: 1.17 $ $Date: 2004/02/18 01:15:42 $
 * 
 * @author Craig R. McClanahan
 * @author Paul Jack
 * @author Stephen Colebourne
 */
public class ArrayStack extends ArrayList implements Buffer {

    /** Ensure serialization compatibility */    
    private static final long serialVersionUID = 2130079159931574599L;

    /**
     * Constructs a new empty ArrayStack. The initial size
     * is controlled by ArrayList and is currently 10.
     */
    public ArrayStack() {
        super();
    }

    /**
     * Constructs a new empty ArrayStack with an initial size.
     * 
     * @param initialSize  the initial size to use
     * @throws IllegalArgumentException  if the specified initial size
     *  is negative
     */
    public ArrayStack(int initialSize) {
        super(initialSize);
    }

    /**
     * Return true if this stack is currently empty.
     * 

     * This method exists for compatibility with java.util.Stack.
     * New users of this class should use isEmpty instead.
     * 
     * @return true if the stack is currently empty
     */
    public boolean empty() {
        return isEmpty();
    }

    /**
     * Returns the top item off of this stack without removing it.
     *
     * @return the top item on the stack
     * @throws EmptyStackException  if the stack is empty
     */
    public Object peek() throws EmptyStackException {
        int n = size();
        if (n <= 0) {
            throw new EmptyStackException();
        } else {
            return get(n - 1);
        }
    }

    /**
     * Returns the n'th item down (zero-relative) from the top of this
     * stack without removing it.
     *
     * @param n  the number of items down to go
     * @return the n'th item on the stack, zero relative
     * @throws EmptyStackException  if there are not enough items on the
     *  stack to satisfy this request
     */
    public Object peek(int n) throws EmptyStackException {
        int m = (size() - n) - 1;
        if (m < 0) {
            throw new EmptyStackException();
        } else {
            return get(m);
        }
    }

    /**
     * Pops the top item off of this stack and return it.
     *
     * @return the top item on the stack
     * @throws EmptyStackException  if the stack is empty
     */
    public Object pop() throws EmptyStackException {
        int n = size();
        if (n <= 0) {
            throw new EmptyStackException();
        } else {
            return remove(n - 1);
        }
    }

    /**
     * Pushes a new item onto the top of this stack. The pushed item is also
     * returned. This is equivalent to calling add.
     *
     * @param item  the item to be added
     * @return the item just pushed
     */
    public Object push(Object item) {
        add(item);
        return item;
    }

    /**
     * Returns the one-based position of the distance from the top that the
     * specified object exists on this stack, where the top-most element is
     * considered to be at distance 1.  If the object is not
     * present on the stack, return -1 instead.  The
     * equals() method is used to compare to the items
     * in this stack.
     *
     * @param object  the object to be searched for
     * @return the 1-based depth into the stack of the object, or -1 if not found
     */
    public int search(Object object) {
        int i = size() - 1;        // Current index
        int n = 1;                 // Current distance
        while (i >= 0) {
            Object current = get(i);
            if ((object == null && current == null) ||
                (object != null && object.equals(current))) {
                return n;
            }
            i--;
            n++;
        }
        return -1;
    }

    /**
     * Returns the element on the top of the stack.
     *
     * @return the element on the top of the stack
     * @throws BufferUnderflowException  if the stack is empty
     */
    public Object get() {
        int size = size();
        if (size == 0) {
            throw new BufferUnderflowException();
        }
        return get(size - 1);
    }

    /**
     * Removes the element on the top of the stack.
     *
     * @return the removed element 
     * @throws BufferUnderflowException  if the stack is empty
     */
    public Object remove() {
        int size = size();
        if (size == 0) {
            throw new BufferUnderflowException();
        }
        return remove(size - 1);
    }

}