/// <summary>
/// Calculate filtering gain so that frequency response is normalized onto [0, 1] range.
/// </summary>
/// <param name="filter"></param>
/// <param name="fftSize"></param>
/// <returns>Gain for filtering operations</returns>
public static float EstimateGain(this IOnlineFilter filter, int fftSize = 512)
{
var unit = DiscreteSignal.Unit(fftSize);
// get impulse response
var response = unit.Samples.Select(s => filter.Process(s)).ToArray();
// get frequency response
var spectrum = new float[fftSize / 2 + 1];
var fft = new RealFft(fftSize);
fft.MagnitudeSpectrum(response, spectrum);
return(1 / spectrum.Max(s => Math.Abs(s)));
}