/**
 * $RCSfile: LinkedList.java,v $
 * $Revision: 1.2 $
 * $Date: 2000/12/21 17:47:20 $
 *
 * Copyright (C) 2000 CoolServlets.com. All rights reserved.
 *
 * ===================================================================
 * The Apache Software License, Version 1.1
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:
 *       "This product includes software developed by
 *        CoolServlets.com (http://www.Yasna.com)."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Jive" and "CoolServlets.com" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For written permission, please
 *    contact webmaster@Yasna.com.
 *
 * 5. Products derived from this software may not be called "Jive",
 *    nor may "Jive" appear in their name, without prior written
 *    permission of CoolServlets.com.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL COOLSERVLETS.COM OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of CoolServlets.com. For more information
 * on CoolServlets.com, please see <http://www.Yasna.com>.
 */

package com.Yasna.util;

import java.util.*;

/**
 * Simple LinkedList implementation. The main feature is that list nodes
 * are public, which allows very fast delete operations (when one has a
 * reference to the node that is to be deleted).<p>
 *
 * The linked list implementation was specifically written for the CoolServlets
 * cache system. While it can be used as a general purpose linked list, for
 * most applications, it is more suitable to use the linked list that is part
 * of the Java Collections package.
 */
public class LinkedList {

    /**
     * The root of the list keeps a reference to both the first and last
     * elements of the list.
     */
    private LinkedListNode head = new LinkedListNode("head", null, null);

    /**
     * Creates a new linked list.
     */
    public LinkedList() {
        head.next = head.previous = head;
    }

    /**
     * Returns the first linked list node in the list.
     *
     * @return the first element of the list.
     */
    public LinkedListNode getFirst() {
        LinkedListNode node = head.next;
        if (node == head) {
            return null;
        }
        return node;
    }

    /**
     * Returns the last linked list node in the list.
     *
     * @return the last element of the list.
     */
    public LinkedListNode getLast() {
        LinkedListNode node = head.previous;
        if (node == head) {
            return null;
        }
        return node;
    }

    /**
     * Adds a node to the beginning of the list.
     *
     * @param node the node to add to the beginning of the list.
     */
    public LinkedListNode addFirst(LinkedListNode node) {
        node.next = head.next;
        node.previous = head;
        node.previous.next = node;
        node.next.previous = node;
        return node;
    }

    /**
     * Adds an object to the beginning of the list by automatically creating a
     * a new node and adding it to the beginning of the list.
     *
     * @param object the object to add to the beginning of the list.
     * @return the node created to wrap the object.
     */
    public LinkedListNode addFirst(Object object) {
        LinkedListNode node = new LinkedListNode(object, head.next, head);
        node.previous.next = node;
        node.next.previous = node;
        return node;
    }

    /**
     * Adds an object to the end of the list by automatically creating a
     * a new node and adding it to the end of the list.
     *
     * @param object the object to add to the end of the list.
     * @return the node created to wrap the object.
     */
    public LinkedListNode addLast(Object object) {
        LinkedListNode node = new LinkedListNode(object, head, head.previous);
        node.previous.next = node;
        node.next.previous = node;
        return node;
    }

    /**
     * Erases all elements in the list and re-initializes it.
     */
    public void clear() {
        //Remove all references in the list.
        LinkedListNode node = getLast();
        while (node != null) {
            node.remove();
            node = getLast();
        }

        //Re-initialize.
        head.next = head.previous = head;
    }

    /**
     * Returns a String representation of the linked list with a comma
     * delimited list of all the elements in the list.
     *
     * @return a String representation of the LinkedList.
     */
    public String toString() {
        LinkedListNode node = head.next;
        StringBuffer buf = new StringBuffer();
        while (node != head) {
            buf.append(node.toString()).append(", ");
            node = node.next;
        }
        return buf.toString();
    }
}