javaxt.utils.Generator Maven / Gradle / Ivy

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package javaxt.utils;

import java.util.Iterator;
import java.util.NoSuchElementException;

//******************************************************************************
//**  Generator
//******************************************************************************
/**
 * A custom iterator that yields its values one at a time. This is a great
 * alternative to arrays or lists when dealing with large data. By yielding one
 * entry at a time, this iterator can help avoid out of memory exceptions.
 *
 * Subclasses must define a method called {@link #run()} and may call
 * {@link yield(T)} to return values one at a time. Example:
 * 
 *  * Generator<String> generator = new Generator<String>() {
 * 
 * 	@Override
 * 	public void run() {
 * 
 * 		BufferedReader br = file.getBufferedReader("UTF-8");
 * 
 * 		String row;
 * 		while ((row = br.readLine()) != null) {
 * 			yield(row);
 * 		}
 * 
 * 		br.close();
 * 	}
 * };
 * 
 *
 * @author Adrian Kuhn <akuhn(at)iam.unibe.ch>
 *
 ******************************************************************************/

public abstract class Generator implements Iterable {

	public abstract void run();

	@Override
	public Iterator iterator() {
		return new Iter();
	}

	private static final Object DONE = new Object();
	private static final Object EMPTY = new Object();
	private Object drop = EMPTY;
	private Thread th = null;

	private synchronized Object take() {
		while (drop == EMPTY) {
			try {
				wait();
			} catch (InterruptedException ex) {
				Thread.currentThread().interrupt();
			}
		}
		Object temp = drop;
		if (drop != DONE)
			drop = EMPTY;
		notifyAll();
		return temp;
	}

	private synchronized void put(Object value) {
		if (drop == DONE)
			throw new IllegalStateException();
		if (drop != EMPTY)
			throw new IllegalStateException();
		drop = value;
		notifyAll();
		while (drop != EMPTY) {
			try {
				wait();
			} catch (InterruptedException ex) {
				Thread.currentThread().interrupt();
			}
		}
	}

	protected void yield(T value) {
		put(value);
	}

	public synchronized void done() {
		if (drop == DONE)
			throw new IllegalStateException();
		if (drop != EMPTY)
			throw new IllegalStateException();
		drop = DONE;
		notifyAll();
	}

	private class Iter implements Iterator, Runnable {

		private Object next = EMPTY;

		public Iter() {
			if (th != null)
				throw new IllegalStateException("Can not run coroutine twice");
			th = new Thread(this);
			th.setDaemon(true);
			th.start();
		}

		@Override
		public void run() {
			Generator.this.run();
			done();
		}

		@Override
		public boolean hasNext() {
			if (next == EMPTY)
				next = take();
			return next != DONE;
		}

		@Override
		@SuppressWarnings("unchecked")
		public T next() {
			if (next == EMPTY)
				next = take();
			if (next == DONE)
				throw new NoSuchElementException();
			Object temp = next;
			next = EMPTY;
			return (T) temp;
		}

		@Override
		public void remove() {
			throw new UnsupportedOperationException();

		}

		@SuppressWarnings("deprecation")
		@Override
		protected void finalize() throws Throwable {
			th.stop(); // let's commit suicide
		}

	}

}