var Damper = function(fullValue, restoreCountdownMS) {
  this._fullValue = fullValue; // 'Constant'
  this._restoreCountdownMS = restoreCountdownMS; // 'Constant'

  // Mutable state.
  this._countdownMS = 0; // Always at full strength initially.
  this._lastTimeMS = Date.now();

  this._isPaused = false;
};

Damper.prototype.update = function() {
  var deltaMS = Date.now() - this._lastTimeMS;
  if (!this._isPaused) {
    this._countdownMS -= deltaMS;
    this._countdownMS = Math.max(0, this._countdownMS);
  }

  this._lastTimeMS = Date.now();
};

function lerp(v0, v1, t) {
    return v0*(1-t)+v1*t
}

Damper.prototype.pause = function() {
  this._isPaused = true;
}
Damper.prototype.resume = function() {
  this._isPaused = false;
}

Damper.prototype.dampen = function() {
  this._countdownMS = this._restoreCountdownMS; // Reset the countdown 'clock'.
};

Damper.prototype.getValue = function() {
  if (this._countdownMS > 0) {
    var t = this._countdownMS / this._restoreCountdownMS;
    return lerp(this._fullValue, 0, t);
  } else {
    return this._fullValue;
  }
};

/*
States:

- stopped (default)
- 



          Date.now() // Milliseconds




      function damper(fullValue, restoreDuration) {
        this._fullValue = fullValue;
        this._restoreDuration = restoreDuration;
        this._clock = new THREE.Clock(false);
        this._clock.stop();
        this._resumeTime = this._clock.elapsedTime;
      };
      damper.prototype.pause = function() {
        this._clock.stop();
//        this._isPaused = true;
      };
      damper.prototype.resume = function() {
        this._clock.start();
//        this._isPaused = false;
        this._resumeTime = this._clock.elapsedTime;
      };
      damper.prototype.getValue = function() {
        var currentTime = this._clock.elapsedTime;
        var = (currentTime - this._resumeTime) / this._restoreDuration;
        if (this._isPaused) {
          return 0;
        } else if (currentTime - this._restoreDuration > this._resumeTime) {
          // Still restoring.
          return lerp(0, this._fullValue, )
        } else {
        }
        // body...
      };
      damper.prototype.update = function() {
        clock.getDelta();
      };
*/