code: package avrora.sim.clock;
  
  /**
   * 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.
   */
  
  import avrora.sim.Simulator;
  import cck.util.Util;
  
  /**
   * The <code>SystemClock</code> class represents a wrapper around the system clock that
   * measures actual wall clock time passed in simulation. This implementation encapsulates
   * the <code>System.currentTimeMillis()</code> method, and thus provides a precision
   * of approximately 1000 cycles per second.
   *
   * @author Ben L. Titzer
   */
  public class SystemClock extends Clock {
  
      private static final SystemClock instance = new SystemClock();
  
      private SystemClock() {
          super("system", 1000);
      }
  
      /**
       * The <code>get()</code> method retrieves the singleton instance of the system clock.
       * @return an instance of the <code>SystemClock</code> class
       */
      public static SystemClock get() {
          return instance;
      }
  
      /**
       * The <code>getCount()</code> method returns the number of clock cycles (ticks) that have elapsed for
       * this clock.
       *
       * @return the number of elapsed time ticks in clock cycles
       */
      public long getCount() {
          return System.currentTimeMillis();
      }
  
      /**
       * The <code>insertEvent()</code> method inserts an event into the event queue of the clock with the
       * specified delay in clock cycles. The event will then be executed at the future time specified.
       *
       * @param e      the event to be inserted
       * @param cycles the number of cycles in the future at which to fire
       */
      public void insertEvent(Simulator.Event e, long cycles) {
          throw Util.unimplemented();
      }
  
      /**
       * The <code>removeEvent()</code> method removes an event from the event queue of the clock. The
       * comparison used is reference equality, not <code>.equals()</code>.
       *
       * @param e the event to remove
       */
      public void removeEvent(Simulator.Event e) {
          throw Util.unimplemented();
      }
  
      /**
       * The <code>getFirstEventDelta()</code> method returns the number of clock cycles until
       * the first event in the event queue will fire. This method will return -1 if there are no
       * events in the queue.
       * @return the delta in clock cycles of the first event in the queue; -1 if there are no
       * events in the queue
       */
      public long getFirstEventDelta() {
          throw Util.unimplemented();
      }
  }
  