// Polyphase decimation filter.
//
// Convert an oversampled audio stream to non-oversampled.  Uses a
// windowed sinc FIR filter w/ Blackman window to control aliasing.
// Christian Floisand's 'blog explains it very well.
//
// This version has a very simple main processing loop (the decimate
// method) which vectorizes easily.
//
// Refs:
//   https://christianfloisand.wordpress.com/2012/12/05/audio-resampling-part-1/
//   https://christianfloisand.wordpress.com/2013/01/28/audio-resampling-part-2/
//   http://www.dspguide.com/ch16.htm
//   http://en.wikipedia.org/wiki/Window_function#Blackman_windows

#ifndef INCLUDE_M17MODDECIMATOR_H
#define INCLUDE_M17MODDECIMATOR_H

#include <cstdint>

class M17ModDecimator {

public:
    M17ModDecimator();
    ~M17ModDecimator();

    void initialize(
        double   decimatedSampleRate,
        double   passFrequency,
        unsigned oversampleRatio
    );

    double oversampleRate()  const { return mOversampleRate; }
    int oversampleRatio() const { return mRatio; }

    void decimate(int16_t *in, int16_t *out, unsigned int outCount);
    // N.B., input must have (ratio * outCount) samples.

private:
    double mDecimatedSampleRate;
    double mOversampleRate;
    int mRatio;                 // oversample ratio
    float *mKernel;
    unsigned int mKernelSize;
    float *mShift;              // shift register
    unsigned int mCursor;
};

#endif // INCLUDE_M17MODDECIMATOR_H