static Matrixf melfilter()

in StreamingASR/StreamingASR/app/src/main/cpp/librosa/librosa.h [101:143]

• 37 lines of code
• 4 McCabe index (conditional complexity)

static Matrixf melfilter(int sr, int n_fft, int n_mels, int fmin, int fmax){
  int n_f = n_fft/2+1;
  Vectorf fft_freqs = (Vectorf::LinSpaced(n_f, 0.f, static_cast<float>(n_f-1))*sr)/n_fft;

  float f_min = 0.f;
  float f_sp = 200.f/3.f;
  float min_log_hz = 1000.f;
  float min_log_mel = (min_log_hz-f_min)/f_sp;
  float logstep = logf(6.4f)/27.f;

  auto hz_to_mel = [=](int hz, bool htk = false) -> float {
    if (htk){
      return 2595.0f*log10f(1.0f+hz/700.0f);
    }
    float mel = (hz-f_min)/f_sp;
    if (hz >= min_log_hz){
      mel = min_log_mel+logf(hz/min_log_hz)/logstep;
    }
    return mel;
  };
  auto mel_to_hz = [=](Vectorf &mels, bool htk = false) -> Vectorf {
    if (htk){
      return 700.0f*(Vectorf::Constant(n_mels+2, 10.f).array().pow(mels.array()/2595.0f)-1.0f);
    }
    return (mels.array()>min_log_mel).select(((mels.array()-min_log_mel)*logstep).exp()*min_log_hz, (mels*f_sp).array()+f_min);
  };

  float min_mel = hz_to_mel(fmin);
  float max_mel = hz_to_mel(fmax);
  Vectorf mels = Vectorf::LinSpaced(n_mels+2, min_mel, max_mel);
  Vectorf mel_f = mel_to_hz(mels);
  Vectorf fdiff = mel_f.segment(1, mel_f.size() - 1) - mel_f.segment(0, mel_f.size() - 1);
  Matrixf ramps = mel_f.replicate(n_f, 1).transpose().array() - fft_freqs.replicate(n_mels + 2, 1).array();

  Matrixf lower = -ramps.topRows(n_mels).array()/fdiff.segment(0, n_mels).transpose().replicate(1, n_f).array();
  Matrixf upper = ramps.bottomRows(n_mels).array()/fdiff.segment(1, n_mels).transpose().replicate(1, n_f).array();
  Matrixf weights = (lower.array()<upper.array()).select(lower, upper).cwiseMax(0);

  auto enorm = (2.0/(mel_f.segment(2, n_mels)-mel_f.segment(0, n_mels)).array()).transpose().replicate(1, n_f);
  weights = weights.array()*enorm;

  return weights;
}