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static float* calculateAutocorrelationUsingBruteForce(float *m_samples) {
    float *autocorr = (float *)malloc(sizeof(float)*N);
    for (int tau = 0; tau < N; tau++) {
        float sum = 0;
        for (int t = 0; t < N - tau; t++) sum += m_samples[t] * m_samples[t + tau];
        autocorr[tau] = sum;
    }
    return autocorr;
}

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static float* calculateAutocorrelationUsingFFT(float *m_samples) {
    // calculate autocorrelation according to the Wiener–Khinchin theorem
    // prepare calculation
    FFTSetup m_fftSetup = vDSP_create_fftsetup(LOG_2N, kFFTRadix2);
    DSPSplitComplex m_splitComplex;
    m_splitComplex.realp = (float *)malloc(sizeof(float)*N/2);
    m_splitComplex.imagp = (float *)malloc(sizeof(float)*N/2);
    vDSP_ctoz((DSPComplex *)m_samples, 2, &m_splitComplex, 1, N/2);
    // perform FFT
    vDSP_fft_zrip(m_fftSetup, &m_splitComplex, 1, LOG_2N, kFFTDirection_Forward);
    // calculate the absolute squares respectively the squared magnitudes
    vDSP_zvmags(&m_splitComplex, 1, m_splitComplex.realp, 1, N/2);
    memset(m_splitComplex.imagp, 0, sizeof(float)*N/2);
    // perform IFFT
    vDSP_fft_zrip(m_fftSetup, &m_splitComplex, 1, LOG_2N, kFFTDirection_Inverse);
    // store autocorrelation result vector
    vDSP_ztoc(&m_splitComplex, 1, (DSPComplex *)m_samples, 2, N/2);
    return m_samples;
}