float  lerp(float a, float b, float f)
{
    return a + f * (b - a);
}

class Envelope {
  public:
    float attack, decay, sustain, release, start;
    int period;

  // attack, decay and release are in millis
  // sustain is an amplitude (float of 0-1)

  Envelope (float a, float d, float s, float r) {
    start = millis();

    attack = a;
    decay = d;
    sustain = s;
    release = r;
  }

  // You have to specify how long the period is (the time from
  // noteOn to noteOff) in millis
  void trigger (int p) {
    start = millis();
    period = p;
  }

  float value () {
    int t = millis() - start;

    if (t < attack) {
      return lerp (0, 1, 1.0 / attack * t);
    } else if (t < attack + decay) {
      return lerp (1, sustain, 1.0 / decay * (t - attack));
    } else if (t < period - release) {
      return sustain;
    } else if (t < period) {
      return lerp (sustain, 0, 1.0 / release * (t - period + release ));
    } else {
      return 0;
    }
  }
};