code: /**
   * Copyright (c) 2004-2005, Regents of the University of California
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions
   * are met:
   *
   * Redistributions of source code must retain the above copyright notice,
   * this list of conditions and the following disclaimer.
   *
   * 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.
   *
   * Neither the name of the University of California, Los Angeles nor the
   * names of its contributors may be used to endorse or promote products
   * derived from this software without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   * "AS IS" AND ANY EXPRESS 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 THE COPYRIGHT
   * OWNER OR 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.
   */
  
  package avrora.sim.util;
  
  
  
  /**
   * The <code>TransactionalList</code> class implements a list of objects that has the special property that it
   * can be frozen temporarily and unfrozen. During the time that the list is "frozen", requests to add or remove
   * objects from the list are queued. Upon unfreezing, the queue of requests is processed in order, and then
   * the list is free to have objects added and removed again.
   *
   * @author Ben L. Titzer
   */
  public class TransactionalList {
      /**
       * The <code>Link</code> class is used internally to represent links for the list of objects and
       * updates.
       */
      public static class Link {
          public boolean addTransaction; // used for transactions; true if the transaction was an add, false if it was a remove
          public final Object object;
          public Link next;
  
          Link(Object o) {
              object = o;
          }
  
          Link(Object o, boolean a) {
              object = o;
              addTransaction = a;
          }
      }
  
      protected Link head;
      protected Link tail;
  
      protected Link transHead;
      protected Link transTail;
  
      protected int nesting;
  
      /**
       * The <code>add()</code> method allows another probe to be inserted into the multicast set. It will be
       * inserted at the end of the list of current probes and will therefore fire after any probes already in
       * the multicast set.
       *
       * @param b the probe to insert
       */
      public void add(Object b) {
          if (nesting > 0) {
              addTransaction(b, true);
              return;
          }
  
          if (head == null) {
              head = tail = new Link(b);
          } else {
              tail.next = new Link(b);
              tail = tail.next;
          }
      }
  
      /**
       * The <code>remove</code> method removes a probe from the multicast set. The order of the remaining
       * probes is not changed. The comparison used is reference equality, not the <code>.equals()</code>
       * method.
       *
       * @param o the probe to remove
       */
      public void remove(Object o) {
          if (nesting > 0) {
              addTransaction(o, false);
              return;
          }
  
          Link prev = null;
          Link pos = head;
          while (pos != null) {
              Link next = pos.next;
  
              // matched?
              if (pos.object == o) {
                  // remove the head ?
                  if (prev == null) head = pos.next;
                  // somewhere in the middle (or at end)
                  else prev.next = pos.next;
                  // remove the tail item ?
                  if (pos == tail) tail = prev;
  
              } else {
                  // no match; continue
                  prev = pos;
              }
              pos = next;
          }
      }
  
      private void addTransaction(Object o, boolean isAdd) {
          if (transHead == null) {
              transHead = transTail = new Link(o, isAdd);
          } else {
              transTail.next = new Link(o, isAdd);
              transTail = transTail.next;
          }
      }
  
      /**
       * The <code>isEmpty()</code> method tests whether this list is empty. If this list is currently in
       * a transaction (frozen), this method does not consider pending updates to the list.
       *
       * @return true if there are no objects in this list; false otherwise
       */
      public boolean isEmpty() {
          return head == null;
      }
  
      /**
       * The <code>beginTransaction()</code> method freezes this list so that further requests for updates
       * (such as to add or remove objects) are queued until the transaction is completed, which is triggered
       * by a called to <code>endTransaction()</code>. When this method is called, all pending requests to
       * add or remove objects are processed in order.
       */
      public void beginTransaction() {
          nesting++;
      }
  
      /**
       * The <code>endTransaction()</code> method unlocks this list from the transaction and will process
       * any queued adds or removes in order from the time the <code>beginTransaction()</code> method was called.
       */
      public void endTransaction() {
          nesting--;
          if ( nesting == 0 ) {
              Link thead = transHead;
              transHead = null;
              transTail = null;
              for (Link pos = thead; pos != null; pos = pos.next) {
                  if ( pos.addTransaction ) add(pos.object);
                  else remove(pos.object);
              }
          }
      }
  
      public Link getHead() {
          return head;
      }
  
  }
  