Iterator Tricks

//JAVA CODE
import java.util.concurrent.*;
import java.lang.ref.*;
import groovy.lang.Closure;
import java.util.*;

public class Generator<T> implements Iterator<T>, Iterable<T>{
   Semaphore availSemaphore=new Semaphore(0);
   Semaphore emptySemaphore=new Semaphore(1);

   //the thread can push one value at at time into pushedValue
   T pushedValue=null;

      //pull value moves it from pushedValue to pulledValue
   //until it is released by next()
   T pulledValue=null;
   boolean hasPulledValue=false;

   Thread internalThread;

   Generator(Closure closure){
      internalThread=new GeneratorThread<T>(this,closure);
      internalThread.setDaemon(true);
      internalThread.start();
   }

   private void pullValue(){
      availSemaphore.acquireUninterruptibly();
      pulledValue=pushedValue;
      pushedValue=null;
      hasPulledValue=true;
      emptySemaphore.release();
   }

   public boolean hasNext(){
      if (!hasPulledValue)
	 pullValue();
      return emptySemaphore.availablePermits() != 2;
   }

   public T next(){
      if (!hasNext())
	 throw new NoSuchElementException("Closure has no more values");
      T retval=pulledValue;
      hasPulledValue=false;
      return retval;
   }

   public void remove(){
      throw new UnsupportedOperationException(
	 "Remove is not supported on generators");
   }

   public Iterator<T> iterator(){
      return this;
   }

   public void finalize(){
      internalThread.interrupt();
   }

   static class GeneratorThread<T> extends Thread{
      WeakReference<Generator<T>> generatorRef;
      Closure closure;
      public GeneratorThread(Generator<T> generator, Closure cl){
	 generatorRef=new WeakReference<Generator<T>>(generator);
	 closure=cl;
      }

      public void run(){
	 closure.call(new SaveClosure<T>(this));
	 Generator generator=generatorRef.get();

	 //NOTE: when the closure completes, pullValue() will block forever
	 //waiting for more available data. This release() allows it to
	 //get in one last time, and read a variable indicating that the
	 //thread has died and isn't producing any more data. one final
	 //pullValue() run will have emptySemaphore==1 and
	 //availSemaphore==1, and it will make emptySemaphore==2 thus
	 //indicating that the thread has died
	 if (generator!=null){
	    generator.availSemaphore.release();
	 }
	 //NOTE: if the generator has been garbage collected, we don't care
	 //about letting the generator pull a termination condition.
      }
   }

   static class SaveClosure<T> extends Closure{
      WeakReference<Generator<T>> generatorRef;
      Semaphore emptySemaphore;
      Semaphore availSemaphore;
      public SaveClosure(GeneratorThread<T> gt){
	 super(gt,null);
	 generatorRef=gt.generatorRef;
	 Generator<T> generator=generatorRef.get();
	 if (generator!=null){
	    emptySemaphore=generator.emptySemaphore;
	    availSemaphore=generator.availSemaphore;
	 }else{
	    throw new GeneratorDisposedException();
	 }
      }

      public void doCall(T value){
	 try{
	    emptySemaphore.acquire();
	 }catch(InterruptedException e){
	    throw new GeneratorDisposedException();
	 }
	 Generator<T> generator=generatorRef.get();
	 if (generator!=null){
	    generator.pushedValue=value;
	 }else{
	    throw new GeneratorDisposedException();
	 }
	 availSemaphore.release();
      }
   }

   /**
    * A GeneratorDisposedException is used to terminate the thread
    * that was generating values, once the Generator has been garbage
    * collected.
    */
   static public class GeneratorDisposedException extends RuntimeException{
   }
}