package nl.tudelft.simulation.language.concurrent;
   
   import java.util.LinkedHashMap;
   import java.util.Map;
   
   /**
    * Monitor class. In the Java programming language there is a lock associated with every object. The language does not provide a
    * way to perform separate lock and unlock operations; instead, they are implicitly performed by high-level constructs that
    * always arrange to pair such operations correctly. This Monitor class, however, provides separate monitorenter and monitorexit
   * instructions that implement the lock and unlock operations. The class is final for now, as it is not the idea that the class
   * should be extended. It has only static methods.
   * <p>
   * Copyright (c) 2002-2023 Delft University of Technology, Jaffalaan 5, 2628 BX Delft, the Netherlands. All rights reserved. See
   * for project information <a href="https://simulation.tudelft.nl/" target="_blank"> https://simulation.tudelft.nl</a>. The DSOL
   * project is distributed under a three-clause BSD-style license, which can be found at
   * <a href="https://https://simulation.tudelft.nl/dsol/docs/latest/license.html" target="_blank">
   * https://https://simulation.tudelft.nl/dsol/docs/latest/license.html</a>.
   * </p>
   * @author <a href="mailto:phmjacobs@hotmail.com">Peter H.M. Jacobs</a>
   * @author <a href="http://tudelft.nl/averbraeck">Alexander Verbraeck</a>
   */
  public final class Monitor
  {
      /** the locks held. */
      private static Map<Object, MonitorThread> locks = new LinkedHashMap<>();
  
      /**
       * constructs a new Monitor.
       */
      private Monitor()
      {
          // unreachable code
      }
  
      /**
       * locks an object for the current thread.
       * @param object Object; the object to lock
       */
      public static void lock(final Object object)
      {
          Monitor.lock(object, Thread.currentThread());
      }
  
      /**
       * locks an object for the given requestor.
       * @param object Object; the object to lock.
       * @param requestor Thread; the requesting thread.
       */
      public static void lock(final Object object, final Thread requestor)
      {
          synchronized (Monitor.locks)
          {
              if (Monitor.get(object) == null)
              {
                  Monitor.locks.put(object, new MonitorThread(requestor, object));
              }
              else
              {
                  MonitorThread thread = Monitor.get(object);
                  if (thread.getOwner().equals(requestor))
                  {
                      thread.increaseCounter();
                  }
                  else
                  {
                      synchronized (object)
                      {
                          // We wait until we gained access to the monitor
                          Monitor.locks.put(object, new MonitorThread(requestor, object));
                      }
                  }
              }
          }
      }
  
      /**
       * unlocks an object locked by the current Thread.
       * @param object Object; the object to unlock
       */
      public static void unlock(final Object object)
      {
          Monitor.unlock(object, Thread.currentThread());
      }
  
      /**
       * unlocks an object locked by owner.
       * @param object Object; the object to unlock.
       * @param owner Thread; the owning thread.
       */
      public static void unlock(final Object object, final Thread owner)
      {
          synchronized (Monitor.locks)
          {
              MonitorThread thread = Monitor.get(object);
              if (thread == null)
              {
                  throw new IllegalMonitorStateException("object(" + object + ") is not locked");
              }
              if (!thread.getOwner().equals(owner))
             {
                 throw new IllegalMonitorStateException(owner + " cannot" + " unlock object owned by " + thread.getOwner());
             }
             thread.decreaseCounter();
             if (thread.getCounter() == 0)
             {
                 thread.interrupt();
                 Monitor.locks.remove(object);
             }
         }
     }
 
     /**
      * returns the MonitorThread for a specific key.
      * @param key Object; the key to resolve
      * @return the MonitorThread
      */
     private static MonitorThread get(final Object key)
     {
         return locks.get(key);
     }
 
     /**
      * A MonitorThread is used to lock an object.
      */
     private static class MonitorThread extends Thread
     {
         /** the monitor to use. */
         private Object object = null;
 
         /** the owning thread. */
         private Thread owner = null;
 
         /** the counter. */
         private int counter = 0;
 
         /**
          * constructs a new MonitorThread.
          * @param owner Thread; the owning thread
          * @param object Object; the object
          */
          MonitorThread(final Thread owner, final Object object)
         {
             super("MonitorThread on " + object.getClass());
             this.setDaemon(true);
             this.owner = owner;
             synchronized (object)
             {
                 this.object = object;
                 increaseCounter();
                 this.start();
             }
             synchronized (owner)
             {
                 try
                 {
                     this.owner.wait();
                 }
                 catch (InterruptedException exception)
                 {
                     exception = null;
                     /*
                      * This interrupted exception is thrown because this monitor thread has started and interrupted its
                      * constructor. We now know object is locked and may therefore return.
                      */
                 }
             }
         }
 
         /**
          * @return Returns the counter.
          */
         public synchronized int getCounter()
         {
             return this.counter;
         }
 
         /**
          * decreases the counter with one.
          */
         public synchronized void decreaseCounter()
         {
             this.counter = Math.max(0, this.counter - 1);
         }
 
         /**
          * increases the counter of this thread with one.
          */
         public synchronized void increaseCounter()
         {
             this.counter++;
         }
 
         /**
          * @return Returns the owning thread.
          */
         public Thread getOwner()
         {
             return this.owner;
         }
 
         /** {@inheritDoc} */
         @Override
         public void run()
         {
             try
             {
                 // We lock the object
                 synchronized (this.object)
                 {
                     // Since we have locked the object, we can now return
                     // the constructor
                     this.owner.interrupt();
 
                     // We join
                     this.join();
                 }
             }
             catch (Exception exception)
             {
                 // This is OK.. We use this construction in the
                 // MonitorTest.unlock to release a lock
                 exception = null;
             }
         }
     }
 }