sclass RestartableCountdown implements AutoCloseable {
  java.util.Timer timer;
  long targetTime; // in sys time

  ifndef RestartableCountdown_noStats
  long /*firings,*/ totalSleepTime; // stats
  endifndef
  
  synchronized void setTargetTime(long targetTime, Runnable action) {
    if (targetTime <= 0)
      stop();
    else if (targetTime != this.targetTime) {
      start(targetTime-sysNow(), action);
      this.targetTime = targetTime;
    }
  }
  
  // stops the countdown and restarts it
  synchronized void start(long delayMS, O action) {
    stop();
    if (delayMS <= 0)
      thread { callF(action); }
    else {
      ifndef RestartableCountdown_noStats
        totalSleepTime += delayMS;
      endifndef
      timer = doLater_daemon(delayMS, action);
      targetTime = sysNow()+delayMS;
    }
  }
  
  void start(double delaySeconds, O action) {
    start(toMS(delaySeconds), action);
  }
  
  synchronized void stop() {
    cancelTimer(timer);
    timer = null;
    targetTime = 0;
  }
  
  public void close() { stop(); }
}