// This looks at a number of periods of a given frequency starting at a certain time in the audio

// and returns an intensity value.

// No phase adjustment here, so you have to call this twice to get meaningful (complex) results.

srecord noeq SumOfVibrations(IAudioSample sample, int channel, double start, double freq, int periods) {

  double period, end;

  double rawSum() {

    period = sample.sampleRate()/freq;

    double sum = 0, t = start;

    for p to periods: {

      // Subtract an expected trough from an expected neighboring peak and add to overall sum.

      // Nota bene: Trough and peak have the same area (=length), so this is basically a Haar-like feature!

      // By the use of which we automatically get around nasty complications like DC offsets in the input data.

      sum += sample.sampleSum(channel, t, t+period/2)

           - sample.sampleSum(channel, t+period/2, t+period);

      t += period;

    }

    end = t;

    ret sum;

  }

  // alternate calculation adjusted for duration

  double sumDividedByDuration() {

    ret rawSum()/(end-start);

  }

  // Not divided by duration - this seems like the best frequency detector at this point.

  // As in a proper FFT/DCT, we return a complex value to represent phase.

  // Call abs() to get the desired intensity value.

  Complex complexSum() {

    SumOfVibrations sum2 = new(sample, channel, start+period/4, freq, periods);

    ret Complex(rawSum(), sum2.rawSum());

  }

  double amplitude() { ret abs(complexSum()); }

}