Source/WebCore/platform/audio/IIRFilter.h

/*
 * Copyright 2016 The Chromium Authors. All rights reserved.
 * Copyright (C) 2020, Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 *    notice, this list of conditions and the following disclaimer.
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 *    notice, this list of conditions and the following disclaimer in the
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 *
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#pragma once

#include <wtf/Vector.h>

namespace WebCore {

class IIRFilter {
    WTF_MAKE_FAST_ALLOCATED;
public:
    static constexpr size_t maxOrder { 20 };
    IIRFilter(const Vector<double>& feedforward, const Vector<double>& feedback);

    void reset();

    void process(const float* source, float* destination, size_t framesToProcess);
    void getFrequencyResponse(unsigned length, const float* frequency, float* magResponse, float* phaseResponse);
    double tailTime(double sampleRate, bool isFilterStable);

    const Vector<double>& feedforward() const { return m_feedforward; }
    const Vector<double>& feedback() const { return m_feedback; }

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.
    Vector<double> m_xBuffer;
    Vector<double> 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.
    size_t m_bufferIndex { 0 };

    // Those Vectors are owned by the IIRProcess, which owns the IIRFilters via
    // the IIRDSPKernels. This is a memory optimization to avoid having copies
    // of these vectors in each IIRDSPKernel / IIRFilter.
    const Vector<double>& m_feedforward;
    const Vector<double>& m_feedback;
};

} // namespace WebCore