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package jm.audio.math;

/**
 * Abstract Class representing FFT's of real, single precision data.
 * Concrete classes are typically named RealFloatFFT_method, implement the
 * FFT using some particular method.
 * 
 * The physical layout of the mathematical data d[i] in the array data is as follows:
 * 
 * d[i] = data[i0 + stride*i]
 * 
 * The FFT (D[i]) of real data (d[i]) is complex, but restricted by symmetry:
 *  * D[n-i] = conj(D[i])
 * 
 * It turns out that there are still n `independent' values, so the transformation
 * can still be carried out in-place.
 * However, each Real FFT method tends to leave the real and imaginary parts
 * distributed in the data array in its own unique arrangment.
 * 
 * You must consult the documentation for the specific classes implementing
 * RealFloatFFT for the details.
 * Note, however, that each class's backtransform and inverse methods understand
 * thier own unique ordering of the transformed result and can invert it correctly.
 *
 * @author Bruce R. Miller [email protected]
 * @author Contribution of the National Institute of Standards and Technology,
 * @author not subject to copyright.
 */

public abstract class RealFloatFFT {

    int n;

    /**
     * Create an FFT for transforming n points of real, single precision data.
     */
    public RealFloatFFT(int n) {
        if (n <= 0)
            throw new IllegalArgumentException("The transform length must be >=0 : " + n);
        this.n = n;
    }

    protected void checkData(float data[], int i0, int stride) {
        if (i0 < 0)
            throw new IllegalArgumentException("The offset must be >=0 : " + i0);
        if (stride < 1)
            throw new IllegalArgumentException("The stride must be >=1 : " + stride);
        if (i0 + stride * (n - 1) + 1 > data.length)
            throw new IllegalArgumentException("The data array is too small for " + n + ":" +
                    "i0=" + i0 + " stride=" + stride +
                    " data.length=" + data.length);
    }

    /**
     * Compute the Fast Fourier Transform of data leaving the result in data.
     */
    public void transform(float data[]) {
        transform(data, 0, 1);
    }

    /**
     * Compute the Fast Fourier Transform of data leaving the result in data.
     */
    public abstract void transform(float data[], int i0, int stride);

    /**
     * Return data in wraparound order.
     *
     * @see wraparound format
     */
    public float[] toWraparoundOrder(float data[]) {
        return toWraparoundOrder(data, 0, 1);
    }

    /**
     * Return data in wraparound order.
     * i0 and stride are used to traverse data; the new array is in
     * packed (i0=0, stride=1) format.
     *
     * @see wraparound format
     */
    public abstract float[] toWraparoundOrder(float data[], int i0, int stride);

    /**
     * Compute the (unnomalized) inverse FFT of data, leaving it in place.
     */
    public void backtransform(float data[]) {
        backtransform(data, 0, 1);
    }

    /**
     * Compute the (unnomalized) inverse FFT of data, leaving it in place.
     */
    public abstract void backtransform(float data[], int i0, int stride);

    /**
     * Return the normalization factor.
     * Multiply the elements of the backtransform'ed data to get the normalized inverse.
     */
    public float normalization() {
        return 1.0f / ((float) n);
    }

    /**
     * Compute the (nomalized) inverse FFT of data, leaving it in place.
     */
    public void inverse(float data[]) {
        inverse(data, 0, 1);
    }

    /**
     * Compute the (nomalized) inverse FFT of data, leaving it in place.
     */
    public void inverse(float data[], int i0, int stride) {
        backtransform(data, i0, stride);

  /* normalize inverse fft with 1/n */
        float norm = normalization();
        for (int i = 0; i < n; i++)
            data[i0 + stride * i] *= norm;
    }

}