JUCE/modules/juce_dsp/processors/juce_Oversampling.cpp

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2017 - ROLI Ltd.

   JUCE is an open source library subject to commercial or open-source
   licensing.

   By using JUCE, you agree to the terms of both the JUCE 5 End-User License
   Agreement and JUCE 5 Privacy Policy (both updated and effective as of the
   27th April 2017).

   End User License Agreement: www.juce.com/juce-5-licence
   Privacy Policy: www.juce.com/juce-5-privacy-policy

   Or: You may also use this code under the terms of the GPL v3 (see
   www.gnu.org/licenses).

   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.

  ==============================================================================
*/

namespace juce
{
namespace dsp
{

/** Abstract class for the provided oversampling engines used internally in
    the Oversampling class.
*/
template <typename SampleType>
class OversamplingEngine
{
public:
    //===============================================================================
    OversamplingEngine (size_t newNumChannels, size_t newFactor)
    {
        numChannels = newNumChannels;
        factor = newFactor;
    }

    virtual ~OversamplingEngine() {}

    //===============================================================================
    virtual SampleType getLatencyInSamples() = 0;
    size_t getFactor() { return factor; }

    virtual void initProcessing (size_t maximumNumberOfSamplesBeforeOversampling)
    {
        buffer.setSize (static_cast<int> (numChannels), static_cast<int> (maximumNumberOfSamplesBeforeOversampling * factor), false, false, true);
    }

    virtual void reset()
    {
        buffer.clear();
    }

    dsp::AudioBlock<SampleType> getProcessedSamples (size_t numSamples)
    {
        return dsp::AudioBlock<SampleType> (buffer).getSubBlock (0, numSamples);
    }

    virtual void processSamplesUp (dsp::AudioBlock<SampleType> &inputBlock) = 0;
    virtual void processSamplesDown (dsp::AudioBlock<SampleType> &outputBlock) = 0;

protected:
    //===============================================================================
    AudioBuffer<SampleType> buffer;
    size_t factor;
    size_t numChannels;
};


//===============================================================================
/** Dummy oversampling engine class which simply copies and pastes the input
    signal, which could be equivalent to a "one time" oversampling processing.
*/
template <typename SampleType>
class OversamplingDummy : public OversamplingEngine<SampleType>
{
public:
    //===============================================================================
    OversamplingDummy (size_t numChannels) : OversamplingEngine<SampleType> (numChannels, 1) {}
    ~OversamplingDummy() {}

    //===============================================================================
    SampleType getLatencyInSamples() override
    {
        return 0.f;
    }

    void processSamplesUp (dsp::AudioBlock<SampleType> &inputBlock) override
    {
        jassert (inputBlock.getNumChannels() <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumChannels()));
        jassert (inputBlock.getNumSamples() * OversamplingEngine<SampleType>::factor <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumSamples()));

        for (size_t channel = 0; channel < inputBlock.getNumChannels(); channel++)
            OversamplingEngine<SampleType>::buffer.copyFrom (static_cast<int> (channel), 0,
                inputBlock.getChannelPointer (channel), static_cast<int> (inputBlock.getNumSamples()));
    }

    void processSamplesDown (dsp::AudioBlock<SampleType> &outputBlock) override
    {
        jassert (outputBlock.getNumChannels() <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumChannels()));
        jassert (outputBlock.getNumSamples() * OversamplingEngine<SampleType>::factor <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumSamples()));

        outputBlock.copy (OversamplingEngine<SampleType>::getProcessedSamples (outputBlock.getNumSamples()));
    }

private:
    //===============================================================================
    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (OversamplingDummy)
};

//===============================================================================
/** Oversampling engine class performing 2 times oversampling using the Filter
    Design FIR Equiripple method. The resulting filter is linear phase,
    symmetric, and has every two samples but the middle one equal to zero,
    leading to specific processing optimizations.
*/
template <typename SampleType>
class Oversampling2TimesEquirippleFIR : public OversamplingEngine<SampleType>
{
public:
    //===============================================================================
    Oversampling2TimesEquirippleFIR (size_t numChannels,
                                     SampleType normalizedTransitionWidthUp,
                                     SampleType stopbandAttenuationdBUp,
                                     SampleType normalizedTransitionWidthDown,
                                     SampleType stopbandAttenuationdBDown) : OversamplingEngine<SampleType> (numChannels, 2)
    {
        coefficientsUp = *dsp::FilterDesign<SampleType>::designFIRLowpassHalfBandEquirippleMethod (normalizedTransitionWidthUp, stopbandAttenuationdBUp);
        coefficientsDown = *dsp::FilterDesign<SampleType>::designFIRLowpassHalfBandEquirippleMethod (normalizedTransitionWidthDown, stopbandAttenuationdBDown);

        auto N = coefficientsUp.getFilterOrder() + 1;
        stateUp.setSize (static_cast<int> (numChannels), static_cast<int> (N));

        N = coefficientsDown.getFilterOrder() + 1;
        auto Ndiv2 = N / 2;
        auto Ndiv4 = Ndiv2 / 2;

        stateDown.setSize (static_cast<int> (numChannels), static_cast<int> (N));
        stateDown2.setSize (static_cast<int> (numChannels), static_cast<int> (Ndiv4 + 1));

        position.resize (static_cast<int> (numChannels));
    }

    ~Oversampling2TimesEquirippleFIR() {}

    //===============================================================================
    SampleType getLatencyInSamples() override
    {
        return static_cast<SampleType> (coefficientsUp.getFilterOrder() + coefficientsDown.getFilterOrder()) * 0.5f;
    }

    void reset() override
    {
        OversamplingEngine<SampleType>::reset();

        stateUp.clear();
        stateDown.clear();
        stateDown2.clear();

        position.fill (0);
    }

    void processSamplesUp (dsp::AudioBlock<SampleType> &inputBlock) override
    {
        jassert (inputBlock.getNumChannels() <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumChannels()));
        jassert (inputBlock.getNumSamples() * OversamplingEngine<SampleType>::factor <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumSamples()));

        // Initialization
        auto fir = coefficientsUp.getRawCoefficients();
        auto N = coefficientsUp.getFilterOrder() + 1;
        auto Ndiv2 = N / 2;
        auto numSamples = inputBlock.getNumSamples();

        // Processing
        for (size_t channel = 0; channel < inputBlock.getNumChannels(); channel++)
        {
            auto bufferSamples = OversamplingEngine<SampleType>::buffer.getWritePointer (static_cast<int> (channel));
            auto buf = stateUp.getWritePointer (static_cast<int> (channel));
            auto samples = inputBlock.getChannelPointer (channel);

            for (size_t i = 0; i < numSamples; i++)
            {
                // Input
                buf[N - 1] = 2 * samples[i];

                // Convolution
                auto out = static_cast<SampleType> (0.0);
                for (size_t k = 0; k < Ndiv2; k += 2)
                    out += (buf[k] + buf[N - k - 1]) * fir[k];

                // Outputs
                bufferSamples[i << 1] = out;
                bufferSamples[(i << 1) + 1] = buf[Ndiv2 + 1] * fir[Ndiv2];

                // Shift data
                for (size_t k = 0; k < N - 2; k += 2)
                    buf[k] = buf[k + 2];
            }
        }
    }

    void processSamplesDown (dsp::AudioBlock<SampleType> &outputBlock) override
    {
        jassert (outputBlock.getNumChannels() <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumChannels()));
        jassert (outputBlock.getNumSamples() * OversamplingEngine<SampleType>::factor <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumSamples()));

        // Initialization
        auto fir = coefficientsDown.getRawCoefficients();
        auto N = coefficientsDown.getFilterOrder() + 1;
        auto Ndiv2 = N / 2;
        auto Ndiv4 = Ndiv2 / 2;
        auto numSamples = outputBlock.getNumSamples();

        // Processing
        for (size_t channel = 0; channel < outputBlock.getNumChannels(); channel++)
        {
            auto bufferSamples = OversamplingEngine<SampleType>::buffer.getWritePointer (static_cast<int> (channel));
            auto buf = stateDown.getWritePointer (static_cast<int> (channel));
            auto buf2 = stateDown2.getWritePointer (static_cast<int> (channel));
            auto samples = outputBlock.getChannelPointer (channel);
            auto pos = position.getUnchecked (static_cast<int> (channel));

            for (size_t i = 0; i < numSamples; i++)
            {
                // Input
                buf[N - 1] = bufferSamples[i << 1];

                // Convolution
                auto out = static_cast<SampleType> (0.0);
                for (size_t k = 0; k < Ndiv2; k += 2)
                    out += (buf[k] + buf[N - k - 1]) * fir[k];

                // Output
                out += buf2[pos] * fir[Ndiv2];
                buf2[pos] = bufferSamples[(i << 1) + 1];

                samples[i] = out;

                // Shift data
                for (size_t k = 0; k < N - 2; k++)
                    buf[k] = buf[k + 2];

                // Circular buffer
                pos = (pos == 0 ? Ndiv4 : pos - 1);
            }

            position.setUnchecked (static_cast<int> (channel), pos);
        }

    }

private:
    //===============================================================================
    dsp::FIR::Coefficients<SampleType> coefficientsUp, coefficientsDown;
    AudioBuffer<SampleType> stateUp, stateDown, stateDown2;
    Array<size_t> position;

    //===============================================================================
    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (Oversampling2TimesEquirippleFIR)
};


//===============================================================================
/** Oversampling engine class performing 2 times oversampling using the Filter
    Design IIR Polyphase Allpass Cascaded method. The resulting filter is minimum
    phase, and provided with a method to get the exact resulting latency.
*/
template <typename SampleType>
class Oversampling2TimesPolyphaseIIR : public OversamplingEngine<SampleType>
{
public:
    //===============================================================================
    Oversampling2TimesPolyphaseIIR (size_t numChannels,
                                    SampleType normalizedTransitionWidthUp,
                                    SampleType stopbandAttenuationdBUp,
                                    SampleType normalizedTransitionWidthDown,
                                    SampleType stopbandAttenuationdBDown) : OversamplingEngine<SampleType> (2, numChannels)
    {
        auto structureUp = dsp::FilterDesign<SampleType>::designIIRLowpassHalfBandPolyphaseAllpassMethod (normalizedTransitionWidthUp, stopbandAttenuationdBUp);
        dsp::IIR::Coefficients<SampleType> coeffsUp = getCoefficients (structureUp);
        latency = static_cast<SampleType> (-(coeffsUp.getPhaseForFrequency (0.0001, 1.0)) / (0.0001 * 2 * double_Pi));

        auto structureDown = dsp::FilterDesign<SampleType>::designIIRLowpassHalfBandPolyphaseAllpassMethod (normalizedTransitionWidthDown, stopbandAttenuationdBDown);
        dsp::IIR::Coefficients<SampleType> coeffsDown = getCoefficients (structureDown);
        latency += static_cast<SampleType> (-(coeffsDown.getPhaseForFrequency (0.0001, 1.0)) / (0.0001 * 2 * double_Pi));

        for (auto i = 0; i < structureUp.directPath.size(); i++)
            coefficientsUp.add (structureUp.directPath[i].coefficients[0]);

        for (auto i = 1; i < structureUp.delayedPath.size(); i++)
            coefficientsUp.add (structureUp.delayedPath[i].coefficients[0]);

        for (auto i = 0; i < structureDown.directPath.size(); i++)
            coefficientsDown.add (structureDown.directPath[i].coefficients[0]);

        for (auto i = 1; i < structureDown.delayedPath.size(); i++)
            coefficientsDown.add (structureDown.delayedPath[i].coefficients[0]);

        v1Up.setSize (static_cast<int> (numChannels), coefficientsUp.size());
        v1Down.setSize (static_cast<int> (numChannels), coefficientsDown.size());
        delayDown.resize (static_cast<int> (numChannels));
    }

    ~Oversampling2TimesPolyphaseIIR() {}

    //===============================================================================
    SampleType getLatencyInSamples() override
    {
        return latency;
    }

    void reset() override
    {
        OversamplingEngine<SampleType>::reset();

        v1Up.clear();
        v1Down.clear();
        delayDown.fill (0);
    }

    void processSamplesUp (dsp::AudioBlock<SampleType> &inputBlock) override
    {
        jassert (inputBlock.getNumChannels() <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumChannels()));
        jassert (inputBlock.getNumSamples() * OversamplingEngine<SampleType>::factor <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumSamples()));

        // Initialization
        auto coeffs = coefficientsUp.getRawDataPointer();
        auto numStages = coefficientsUp.size();
        auto delayedStages = numStages / 2;
        auto directStages = numStages - delayedStages;
        auto numSamples = inputBlock.getNumSamples();

        // Processing
        for (size_t channel = 0; channel < inputBlock.getNumChannels(); channel++)
        {
            auto bufferSamples = OversamplingEngine<SampleType>::buffer.getWritePointer (static_cast<int> (channel));
            auto lv1 = v1Up.getWritePointer (static_cast<int> (channel));
            auto samples = inputBlock.getChannelPointer (channel);

            for (size_t i = 0; i < numSamples; i++)
            {
                // Direct path cascaded allpass filters
                auto input = samples[i];
                for (auto n = 0; n < directStages; n++)
                {
                    auto alpha = coeffs[n];
                    auto output = alpha * input + lv1[n];
                    lv1[n] = input - alpha * output;
                    input = output;
                }

                // Output
                bufferSamples[i << 1] = input;

                // Delayed path cascaded allpass filters
                input = samples[i];
                for (auto n = directStages; n < numStages; n++)
                {
                    auto alpha = coeffs[n];
                    auto output = alpha * input + lv1[n];
                    lv1[n] = input - alpha * output;
                    input = output;
                }

                // Output
                bufferSamples[(i << 1) + 1] = input;
            }
        }

        // Snap To Zero
        snapToZero (true);
    }

    void processSamplesDown (dsp::AudioBlock<SampleType> &outputBlock) override
    {
        jassert (outputBlock.getNumChannels() <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumChannels()));
        jassert (outputBlock.getNumSamples() * OversamplingEngine<SampleType>::factor <= static_cast<size_t> (OversamplingEngine<SampleType>::buffer.getNumSamples()));

        // Initialization
        auto coeffs = coefficientsDown.getRawDataPointer();
        auto numStages = coefficientsDown.size();
        auto delayedStages = numStages / 2;
        auto directStages = numStages - delayedStages;
        auto numSamples = outputBlock.getNumSamples();

        // Processing
        for (size_t channel = 0; channel < outputBlock.getNumChannels(); channel++)
        {
            auto bufferSamples = OversamplingEngine<SampleType>::buffer.getWritePointer (static_cast<int> (channel));
            auto lv1 = v1Down.getWritePointer (static_cast<int> (channel));
            auto samples = outputBlock.getChannelPointer (channel);
            auto delay = delayDown.getUnchecked (static_cast<int> (channel));

            for (size_t i = 0; i < numSamples; i++)
            {
                // Direct path cascaded allpass filters
                auto input = bufferSamples[i << 1];
                for (auto n = 0; n < directStages; n++)
                {
                    auto alpha = coeffs[n];
                    auto output = alpha * input + lv1[n];
                    lv1[n] = input - alpha * output;
                    input = output;
                }
                auto directOut = input;

                // Delayed path cascaded allpass filters
                input = bufferSamples[(i << 1) + 1];
                for (auto n = directStages; n < numStages; n++)
                {
                    auto alpha = coeffs[n];
                    auto output = alpha * input + lv1[n];
                    lv1[n] = input - alpha * output;
                    input = output;
                }

                // Output
                samples[i] = (delay + directOut) * static_cast<SampleType> (0.5);
                delay = input;
            }

            delayDown.setUnchecked (static_cast<int> (channel), delay);
        }

        // Snap To Zero
        snapToZero (false);
    }

    void snapToZero (bool snapUpProcessing)
    {
        if (snapUpProcessing)
        {
            for (auto channel = 0; channel < OversamplingEngine<SampleType>::buffer.getNumChannels(); channel++)
            {
                auto lv1 = v1Up.getWritePointer (channel);
                auto numStages = coefficientsUp.size();

                for (auto n = 0; n < numStages; n++)
                    util::snapToZero (lv1[n]);
            }
        }
        else
        {
            for (auto channel = 0; channel < OversamplingEngine<SampleType>::buffer.getNumChannels(); channel++)
            {
                auto lv1 = v1Down.getWritePointer (channel);
                auto numStages = coefficientsDown.size();

                for (auto n = 0; n < numStages; n++)
                    util::snapToZero (lv1[n]);
            }
        }
    }

private:
    //===============================================================================
    /** This function calculates the equivalent high order IIR filter of a given
        polyphase cascaded allpass filters structure.
    */
    const dsp::IIR::Coefficients<SampleType> getCoefficients (typename dsp::FilterDesign<SampleType>::IIRPolyphaseAllpassStructure &structure) const
    {
        dsp::Polynomial<SampleType> numerator1 ({ static_cast<SampleType> (1.0) });
        dsp::Polynomial<SampleType> denominator1 ({ static_cast<SampleType> (1.0) });
        dsp::Polynomial<SampleType> numerator2 ({ static_cast<SampleType> (1.0) });
        dsp::Polynomial<SampleType> denominator2 ({ static_cast<SampleType> (1.0) });

        dsp::Polynomial<SampleType> temp;

        for (auto n = 0; n < structure.directPath.size(); n++)
        {
            auto *coeffs = structure.directPath.getReference (n).getRawCoefficients();

            if (structure.directPath[n].getFilterOrder() == 1)
            {
                temp = dsp::Polynomial<SampleType> ({ coeffs[0], coeffs[1] });
                numerator1 = numerator1.getProductWith (temp);

                temp = dsp::Polynomial<SampleType> ({ static_cast<SampleType> (1.0), coeffs[2] });
                denominator1 = denominator1.getProductWith (temp);
            }
            else
            {
                temp = dsp::Polynomial<SampleType> ({ coeffs[0], coeffs[1], coeffs[2] });
                numerator1 = numerator1.getProductWith (temp);

                temp = dsp::Polynomial<SampleType> ({ static_cast<SampleType> (1.0), coeffs[3], coeffs[4] });
                denominator1 = denominator1.getProductWith (temp);
            }
        }

        for (auto n = 0; n < structure.delayedPath.size(); n++)
        {
            auto *coeffs = structure.delayedPath.getReference (n).getRawCoefficients();

            if (structure.delayedPath[n].getFilterOrder() == 1)
            {
                temp = dsp::Polynomial<SampleType> ({ coeffs[0], coeffs[1] });
                numerator2 = numerator2.getProductWith (temp);

                temp = dsp::Polynomial<SampleType> ({ static_cast<SampleType> (1.0), coeffs[2] });
                denominator2 = denominator2.getProductWith (temp);
            }
            else
            {
                temp = dsp::Polynomial<SampleType> ({ coeffs[0], coeffs[1], coeffs[2] });
                numerator2 = numerator2.getProductWith (temp);

                temp = dsp::Polynomial<SampleType> ({ static_cast<SampleType> (1.0), coeffs[3], coeffs[4] });
                denominator2 = denominator2.getProductWith (temp);
            }
        }

        dsp::Polynomial<SampleType> numeratorf1 = numerator1.getProductWith (denominator2);
        dsp::Polynomial<SampleType> numeratorf2 = numerator2.getProductWith (denominator1);
        dsp::Polynomial<SampleType> numerator = numeratorf1.getSumWith (numeratorf2);
        dsp::Polynomial<SampleType> denominator = denominator1.getProductWith (denominator2);

        dsp::IIR::Coefficients<SampleType> coeffs;

        coeffs.coefficients.clear();
        auto inversion = static_cast<SampleType> (1.0) / denominator[0];

        for (auto i = 0; i <= numerator.getOrder(); i++)
            coeffs.coefficients.add (numerator[i] * inversion);

        for (auto i = 1; i <= denominator.getOrder(); i++)
            coeffs.coefficients.add (denominator[i] * inversion);

        return coeffs;
    }

    //===============================================================================
    Array<SampleType> coefficientsUp, coefficientsDown;
    SampleType latency;

    AudioBuffer<SampleType> v1Up, v1Down;
    Array<SampleType> delayDown;

    //===============================================================================
    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (Oversampling2TimesPolyphaseIIR)
};


//===============================================================================
template <typename SampleType>
Oversampling<SampleType>::Oversampling (size_t newNumChannels, size_t newFactor, FilterType newType, bool newMaxQuality)
{
    jassert (newFactor >= 0 && newFactor <= 4 && newNumChannels > 0);

    factorOversampling = static_cast<size_t> (1) << newFactor;
    isMaximumQuality = newMaxQuality;
    type = newType;
    numChannels = newNumChannels;

    if (newFactor == 0)
    {
        numStages = 1;
        engines.add (new OversamplingDummy<SampleType> (numChannels));
    }
    else if (type == FilterType::filterHalfBandPolyphaseIIR)
    {
        numStages = newFactor;

        for (size_t n = 0; n < numStages; n++)
        {
            auto twUp = (isMaximumQuality ? 0.10f : 0.12f) * (n == 0 ? 0.5f : 1.f);
            auto twDown = (isMaximumQuality ? 0.12f : 0.15f) * (n == 0 ? 0.5f : 1.f);

            auto gaindBStartUp = (isMaximumQuality ? -75.f : -65.f);
            auto gaindBStartDown = (isMaximumQuality ? -70.f : -60.f);
            auto gaindBFactorUp = (isMaximumQuality ? 10.f : 8.f);
            auto gaindBFactorDown = (isMaximumQuality ? 10.f : 8.f);

            engines.add (new Oversampling2TimesPolyphaseIIR<SampleType> (numChannels,
                                                                         twUp, gaindBStartUp + gaindBFactorUp * n,
                                                                         twDown, gaindBStartDown + gaindBFactorDown * n));
        }
    }
    else if (type == FilterType::filterHalfBandFIREquiripple)
    {
        numStages = newFactor;

        for (size_t n = 0; n < numStages; n++)
        {
            auto twUp = (isMaximumQuality ? 0.10f : 0.12f) * (n == 0 ? 0.5f : 1.f);
            auto twDown = (isMaximumQuality ? 0.12f : 0.15f) * (n == 0 ? 0.5f : 1.f);

            auto gaindBStartUp = (isMaximumQuality ? -90.f : -70.f);
            auto gaindBStartDown = (isMaximumQuality ? -70.f : -60.f);
            auto gaindBFactorUp = (isMaximumQuality ? 10.f : 8.f);
            auto gaindBFactorDown = (isMaximumQuality ? 10.f : 8.f);

            engines.add (new Oversampling2TimesEquirippleFIR<SampleType> (numChannels,
                                                                          twUp, gaindBStartUp + gaindBFactorUp * n,
                                                                          twDown, gaindBStartDown + gaindBFactorDown * n));
        }
    }
}

template <typename SampleType>
Oversampling<SampleType>::~Oversampling()
{
    engines.clear();
}

//===============================================================================
template <typename SampleType>
SampleType Oversampling<SampleType>::getLatencyInSamples() noexcept
{
    auto latency = static_cast<SampleType> (0);
    size_t order = 1;

    for (size_t n = 0; n < numStages; n++)
    {
        auto& engine = *engines[static_cast<int> (n)];

        order *= engine.getFactor();
        latency += engine.getLatencyInSamples() / static_cast<SampleType> (order);
    }

    return latency;
}

template <typename SampleType>
size_t Oversampling<SampleType>::getOversamplingFactor() noexcept
{
    return factorOversampling;
}

//===============================================================================
template <typename SampleType>
void Oversampling<SampleType>::initProcessing (size_t maximumNumberOfSamplesBeforeOversampling)
{
    jassert (engines.size() > 0);

    auto currentNumSamples = maximumNumberOfSamplesBeforeOversampling;

    for (size_t n = 0; n < numStages; n++)
    {
        auto& engine = *engines[static_cast<int> (n)];

        engine.initProcessing (currentNumSamples);
        currentNumSamples *= engine.getFactor();
    }
    isReady = true;

    reset();
}

template <typename SampleType>
void Oversampling<SampleType>::reset() noexcept
{
    jassert (engines.size() > 0);

    if (isReady)
        for (auto n = 0; n < engines.size(); n++)
            engines[n]->reset();
}

template <typename SampleType>
typename dsp::AudioBlock<SampleType> Oversampling<SampleType>::processSamplesUp (const dsp::AudioBlock<SampleType> &inputBlock) noexcept
{
    jassert (engines.size() > 0);

    if (! isReady)
        return dsp::AudioBlock<SampleType>();

    dsp::AudioBlock<SampleType> audioBlock = inputBlock;

    for (size_t n = 0; n < numStages; n++)
    {
        auto& engine = *engines[static_cast<int> (n)];
        engine.processSamplesUp (audioBlock);
        audioBlock = engine.getProcessedSamples (audioBlock.getNumSamples() * engine.getFactor());
    }

    return audioBlock;
}

template <typename SampleType>
void Oversampling<SampleType>::processSamplesDown (dsp::AudioBlock<SampleType> &outputBlock) noexcept
{
    jassert (engines.size() > 0);

    if (! isReady)
        return;

    auto currentNumSamples = outputBlock.getNumSamples();

    for (size_t n = 0; n < numStages - 1; n++)
        currentNumSamples *= engines[static_cast<int> (n)]->getFactor();

    for (size_t n = numStages - 1; n > 0; n--)
    {
        auto& engine = *engines[static_cast<int> (n)];

        auto audioBlock = engines[static_cast<int> (n - 1)]->getProcessedSamples (currentNumSamples);
        engine.processSamplesDown (audioBlock);

        currentNumSamples /= engine.getFactor();
    }

    engines[static_cast<int> (0)]->processSamplesDown (outputBlock);
}

template class Oversampling<float>;
template class Oversampling<double>;

} // namespace dsp
} // namespace juce