IIRFilter.h - mozsearch

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// Copyright 2016 The Chromium Authors. All rights reserved.

// Use of this source code is governed by a BSD-style license that can be

// found in the LICENSE file.

#ifndef IIRFilter_h

#define IIRFilter_h

#include "nsTArray.h"

typedef nsTArray<double> AudioDoubleArray;

namespace blink {

class IIRFilter final {

 public:

  // The maximum IIR filter order.  This also limits the number of feedforward

  // coefficients.  The maximum number of coefficients is 20 according to the

  // spec.

  const static size_t kMaxOrder = 19;

  IIRFilter(const AudioDoubleArray* feedforwardCoef,

            const AudioDoubleArray* feedbackCoef);

  ~IIRFilter();

  void process(const float* sourceP, float* destP, size_t framesToProcess);

  void reset();

  void getFrequencyResponse(int nFrequencies, const float* frequency,

                            float* magResponse, float* phaseResponse);

  bool buffersAreZero();

 private:

  // Filter memory

//

  // For simplicity, we assume |m_xBuffer| and |m_yBuffer| have the same length,

  // and the length is a power of two.  Since the number of coefficients has a

  // fixed upper length, the size of xBuffer and yBuffer is fixed. |m_xBuffer|

  // holds the old input values and |m_yBuffer| holds the old output values

  // needed to compute the new output value.

//

  // m_yBuffer[m_bufferIndex] holds the most recent output value, say, y[n].

  // Then m_yBuffer[m_bufferIndex - k] is y[n - k].  Similarly for m_xBuffer.

//

  // To minimize roundoff, these arrays are double's instead of floats.

  AudioDoubleArray m_xBuffer;

  AudioDoubleArray m_yBuffer;

  // Index into the xBuffer and yBuffer arrays where the most current x and y

  // values should be stored.  xBuffer[bufferIndex] corresponds to x[n], the

  // current x input value and yBuffer[bufferIndex] is where y[n], the current

  // output value.

  int m_bufferIndex;

  // Coefficients of the IIR filter.

  const AudioDoubleArray* m_feedback;

  const AudioDoubleArray* m_feedforward;

};

}  // namespace blink

#endif  // IIRFilter_h