/**
 * complexData is overlapped 50%.
 * @param complexData
 * @return
 */
private Double[] autoCorrelation(final Complex[] complexData) {
    Complex[] toFFT = doubleAndPad(complexData);

    Double[] autoCorrelationAbsolute = new Double[toFFT.length];

    FFT.fftForward(toFFT);

    for(int j = 0; j < toFFT.length; j++) {
        //Same as Complex.mult(toFFT[j], toFFT[j].conjugate()) but simpler
        double square = toFFT[j].absoluteSquare();
        toFFT[j] = new Complex(square);
    }

    //Effective inverse FFT
    //forward FFT == inverse FFT for real numbers.
    FFT.fftForward(toFFT);

    for(int i = 0; i < toFFT.length; i++) {
        autoCorrelationAbsolute[i] = toFFT[i].absolute();
    }
    return autoCorrelationAbsolute;
}

/**
 * Returns a copy of data with the size doubled and the
 * second half set to zero.
 * @param data
 * @return
 */
private Complex[] doubleAndPad(Complex[] data) {
    Complex[] doubledData = Arrays.copyOf(data, data.length * 2);
    //Set the second half to zero
    for(int i = data.length; i < doubledData.length; i++) {
        doubledData[i] = new Complex(0);
    }
    return doubledData;
}

/**
 * Computes the frequency based off of the padded FFT in autoCorrelation
 * @param bin
 * @param data
 * @return
 */
public static double computeFrequency(int bin, AudioData data) {
    return bin * data.getFormat().getSampleRate() / data.getFftLength();
}