// `fn` will be called with context `context` if supplied (optional)
function TimerControl(fn, context) {

  var epoch = 0;
  var running = false;

  // initialize, effectively
  fn.call(context || this, epoch);

  this.play = function() {
    if(running) return;
    running = true;
    d3.timer(function(t) {
      fn.call(context || this, epoch + t);
      if (!running) {
        epoch += t;
        return true;
      }
      return false;
    })
  }

  this.pause = function() {
    running = false;
  }

  this.reset = function() {
    epoch = 0;
    fn.call(context || this, epoch);
  }

}