package geniusweb.protocol;

import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * Object containing function that will be called exactly once, on demand or at
 * given deadline.
 *
 */
public class WillBeCalled {
	private static volatile ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(
			2);
	private final Runnable runnable;
	private volatile AtomicBoolean done = new AtomicBoolean(false);

	/**
	 * @param r       the runnable that will be run once, either when the user
	 *                calls {@link #complete(Void)} or after delayMs, whichever
	 *                comes first.
	 * @param delayMs the delay after this will be called anywa (the 'deadline'
	 *                in milliseconds. If null, no deadline call will be made
	 *                and the user must call {@link #complete(Void)} himself.
	 */
	public WillBeCalled(Runnable r, Long delayMs) {
		this.runnable = r;
		if (delayMs != null) {
			executor.schedule(() -> complete(), delayMs, TimeUnit.MILLISECONDS);
		}
	}

	/**
	 * Tells the runnable to be executed. Only the first time the function is
	 * run, subsequent calls do nothing. Failures in runnable are NOT caught
	 * here, as they are probably bugs
	 * 
	 * We must avoid synchronized here because users may use synchronized blocks
	 * already.
	 */
	public void complete() {
		if (!done.compareAndSet(false, true))
			return;
		runnable.run();
	}
}