content: /**
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You 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.activemq.util;
  
  /**
   * Provides a base class for you to extend when you want object to maintain a
   * doubly linked list to other objects without using a collection class.
   *
   * @author chirino
   */
  public class LinkedNode {
  
      protected LinkedNode next = this;
      protected LinkedNode prev = this;
      protected boolean tail = true;
  
      public LinkedNode getHeadNode() {
          if (isHeadNode()) {
              return this;
          }
          if (isTailNode()) {
              return next;
          }
          LinkedNode rc = prev;
          while (!rc.isHeadNode()) {
              rc = rc.prev;
          }
          return rc;
      }
  
      public LinkedNode getTailNode() {
          if (isTailNode()) {
              return this;
          }
          if (isHeadNode()) {
              return prev;
          }
          LinkedNode rc = next;
          while (!rc.isTailNode()) {
              rc = rc.next;
          }
          return rc;
      }
  
      public LinkedNode getNext() {
          return tail ? null : next;
      }
  
      public LinkedNode getPrevious() {
          return prev.tail ? null : prev;
      }
  
      public boolean isHeadNode() {
          return prev.isTailNode();
      }
  
      public boolean isTailNode() {
          return tail;
      }
  
      /**
       * @param rightHead the node to link after this node.
       * @return this
       */
      public LinkedNode linkAfter(LinkedNode rightHead) {
  
          if (rightHead == this) {
              throw new IllegalArgumentException("You cannot link to yourself");
          }
          if (!rightHead.isHeadNode()) {
              throw new IllegalArgumentException("You only insert nodes that are the first in a list");
          }
  
          LinkedNode rightTail = rightHead.prev;
  
          if (tail) {
              tail = false;
          } else {
              rightTail.tail = false;
          }
  
          rightHead.prev = this; // link the head of the right side.
          rightTail.next = next; // link the tail of the right side
          next.prev = rightTail; // link the head of the left side
          next = rightHead; // link the tail of the left side.
  
          return this;
      }
  
      /**
       * @param leftHead the node to link after this node.
       * @return this
       */
      public LinkedNode linkBefore(LinkedNode leftHead) {
  
          if (leftHead == this) {
              throw new IllegalArgumentException("You cannot link to yourself");
          }
          if (!leftHead.isHeadNode()) {
              throw new IllegalArgumentException("You only insert nodes that are the first in a list");
          }
  
          // The left side is no longer going to be a tail..
          LinkedNode leftTail = leftHead.prev;
          leftTail.tail = false;
  
          leftTail.next = this; // link the tail of the left side.
          leftHead.prev = prev; // link the head of the left side.
          prev.next = leftHead; // link the tail of the right side.
          prev = leftTail; // link the head of the right side.
  
          return leftHead;
      }
  
      /**
       * Removes this node out of the linked list it is chained in.
       */
      public void unlink() {
          // If we are allready unlinked...
          if (prev == this) {
              reset();
              return;
          }
  
          if (tail) {
              prev.tail = true;
          }
  
          // Update the peers links..
          next.prev = prev;
          prev.next = next;
  
          // Update our links..
          reset();
      }
  
      public void reset() {
          next = this;
          prev = this;
          tail = true;
      }
  }
  