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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
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