// only executes one action at once. keeps track of
// optimal timestamps though, so actions may pile up
sclass FlexibleRateTimer implements AutoCloseable {
  double hertz;
  transient Runnable action;
  transient double timestamp;
  transient DoLater doLater;
  transient new Flag started;
  transient volatile bool disposed;
  transient long delay; // how much are we currently delayed
  transient L<Runnable> onFrequencyChanged;
  
  static double maxHertz = 1000;
  
  *() {}
  *(double *hertz) {}
  *(double *hertz, Runnable *action) {}
  
  void start {
    if (!started.raise()) ret;
    _kaboom();
  }
  
  synchronized void _kaboom {
    if (disposed) ret;
    dispose doLater;
    if (timestamp == 0) timestamp = sysNow();
    timestamp += 1000/getFrequency();
    long _timestamp = lround(timestamp);
    doLater = new DoLater(_timestamp, r {
      delay = sysNow()-_timestamp;
      pcallF(action);
      _kaboom();
    });
    doLater.enable();
  }
  
  void cancel { close(); }
  
  public void close {
    set disposed;
    dispose doLater;
  }
  
  void setRunnableAndStart(Runnable action) {
    this.action = action;
    start();
  }
  
  // takes affect _after_ next trigger
  synchronized void setFrequency(double hertz) {
    hertz = clamp(hertz, 0, maxHertz);
    if (hertz == this.hertz) ret;
    this.hertz = hertz;
    pcallFAll(onFrequencyChanged);
  }
  
  // reschedule next action
  synchronized void setFrequencyImmediately(double hertz) {
    hertz = clamp(hertz, 0, maxHertz);
    if (hertz == this.hertz) ret;
    if (!started.isUp()) ret;
    if (!doLater.cancel()) ret;
    timestamp -= 1000/getFrequency();
    this.hertz = hertz;
    pcallFAll(onFrequencyChanged);
    _kaboom();
  }
  
  synchronized double getFrequency() { ret hertz; }
  
  long currentDelay() { ret delay; }
  
  synchronized void onFrequencyChanged(Runnable r) {
    onFrequencyChanged = addOrCreate(onFrequencyChanged,  r);
  }
}