android-5.1.1_r30/s

/*
 * Copyright (C) 2010 Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1.  Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 * 3.  Neither the name of Apple Computer, Inc. ("Apple") nor the names of
 *     its contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE AND ITS CONTRIBUTORS "AS IS" AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL APPLE OR ITS CONTRIBUTORS BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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 */

#include "config.h"

#if ENABLE(WEB_AUDIO)

#include "platform/audio/AudioBus.h"

#include "platform/audio/AudioFileReader.h"
#include "platform/audio/DenormalDisabler.h"
#include "platform/audio/SincResampler.h"
#include "platform/audio/VectorMath.h"
#include "public/platform/Platform.h"
#include "public/platform/WebAudioBus.h"
#include "wtf/OwnPtr.h"

#include <assert.h>
#include <math.h>
#include <algorithm>

namespace blink {

using namespace VectorMath;

const unsigned MaxBusChannels = 32;

PassRefPtr<AudioBus> AudioBus::create(unsigned numberOfChannels, size_t length, bool allocate)
{
    ASSERT(numberOfChannels <= MaxBusChannels);
    if (numberOfChannels > MaxBusChannels)
        return nullptr;

    return adoptRef(new AudioBus(numberOfChannels, length, allocate));
}

AudioBus::AudioBus(unsigned numberOfChannels, size_t length, bool allocate)
    : m_length(length)
    , m_busGain(1)
    , m_isFirstTime(true)
    , m_sampleRate(0)
{
    m_channels.reserveInitialCapacity(numberOfChannels);

    for (unsigned i = 0; i < numberOfChannels; ++i) {
        PassOwnPtr<AudioChannel> channel = allocate ? adoptPtr(new AudioChannel(length)) : adoptPtr(new AudioChannel(0, length));
        m_channels.append(channel);
    }

    m_layout = LayoutCanonical; // for now this is the only layout we define
}

void AudioBus::setChannelMemory(unsigned channelIndex, float* storage, size_t length)
{
    if (channelIndex < m_channels.size()) {
        channel(channelIndex)->set(storage, length);
        m_length = length; // FIXME: verify that this length matches all the other channel lengths
    }
}

void AudioBus::resizeSmaller(size_t newLength)
{
    ASSERT(newLength <= m_length);
    if (newLength <= m_length)
        m_length = newLength;

    for (unsigned i = 0; i < m_channels.size(); ++i)
        m_channels[i]->resizeSmaller(newLength);
}

void AudioBus::zero()
{
    for (unsigned i = 0; i < m_channels.size(); ++i)
        m_channels[i]->zero();
}

AudioChannel* AudioBus::channelByType(unsigned channelType)
{
    // For now we only support canonical channel layouts...
    if (m_layout != LayoutCanonical)
        return 0;

    switch (numberOfChannels()) {
    case 1: // mono
        if (channelType == ChannelMono || channelType == ChannelLeft)
            return channel(0);
        return 0;

    case 2: // stereo
        switch (channelType) {
        case ChannelLeft: return channel(0);
        case ChannelRight: return channel(1);
        default: return 0;
        }

    case 4: // quad
        switch (channelType) {
        case ChannelLeft: return channel(0);
        case ChannelRight: return channel(1);
        case ChannelSurroundLeft: return channel(2);
        case ChannelSurroundRight: return channel(3);
        default: return 0;
        }

    case 5: // 5.0
        switch (channelType) {
        case ChannelLeft: return channel(0);
        case ChannelRight: return channel(1);
        case ChannelCenter: return channel(2);
        case ChannelSurroundLeft: return channel(3);
        case ChannelSurroundRight: return channel(4);
        default: return 0;
        }

    case 6: // 5.1
        switch (channelType) {
        case ChannelLeft: return channel(0);
        case ChannelRight: return channel(1);
        case ChannelCenter: return channel(2);
        case ChannelLFE: return channel(3);
        case ChannelSurroundLeft: return channel(4);
        case ChannelSurroundRight: return channel(5);
        default: return 0;
        }
    }

    ASSERT_NOT_REACHED();
    return 0;
}

const AudioChannel* AudioBus::channelByType(unsigned type) const
{
    return const_cast<AudioBus*>(this)->channelByType(type);
}

// Returns true if the channel count and frame-size match.
bool AudioBus::topologyMatches(const AudioBus& bus) const
{
    if (numberOfChannels() != bus.numberOfChannels())
        return false; // channel mismatch

    // Make sure source bus has enough frames.
    if (length() > bus.length())
        return false; // frame-size mismatch

    return true;
}

PassRefPtr<AudioBus> AudioBus::createBufferFromRange(const AudioBus* sourceBuffer, unsigned startFrame, unsigned endFrame)
{
    size_t numberOfSourceFrames = sourceBuffer->length();
    unsigned numberOfChannels = sourceBuffer->numberOfChannels();

    // Sanity checking
    bool isRangeSafe = startFrame < endFrame && endFrame <= numberOfSourceFrames;
    ASSERT(isRangeSafe);
    if (!isRangeSafe)
        return nullptr;

    size_t rangeLength = endFrame - startFrame;

    RefPtr<AudioBus> audioBus = create(numberOfChannels, rangeLength);
    audioBus->setSampleRate(sourceBuffer->sampleRate());

    for (unsigned i = 0; i < numberOfChannels; ++i)
        audioBus->channel(i)->copyFromRange(sourceBuffer->channel(i), startFrame, endFrame);

    return audioBus;
}

float AudioBus::maxAbsValue() const
{
    float max = 0.0f;
    for (unsigned i = 0; i < numberOfChannels(); ++i) {
        const AudioChannel* channel = this->channel(i);
        max = std::max(max, channel->maxAbsValue());
    }

    return max;
}

void AudioBus::normalize()
{
    float max = maxAbsValue();
    if (max)
        scale(1.0f / max);
}

void AudioBus::scale(float scale)
{
    for (unsigned i = 0; i < numberOfChannels(); ++i)
        channel(i)->scale(scale);
}

void AudioBus::copyFrom(const AudioBus& sourceBus, ChannelInterpretation channelInterpretation)
{
    if (&sourceBus == this)
        return;

    unsigned numberOfSourceChannels = sourceBus.numberOfChannels();
    unsigned numberOfDestinationChannels = numberOfChannels();

    if (numberOfDestinationChannels == numberOfSourceChannels) {
        for (unsigned i = 0; i < numberOfSourceChannels; ++i)
            channel(i)->copyFrom(sourceBus.channel(i));
    } else {
        switch (channelInterpretation) {
        case Speakers:
            speakersCopyFrom(sourceBus);
            break;
        case Discrete:
            discreteCopyFrom(sourceBus);
            break;
        default:
            ASSERT_NOT_REACHED();
        }
    }
}

void AudioBus::sumFrom(const AudioBus& sourceBus, ChannelInterpretation channelInterpretation)
{
    if (&sourceBus == this)
        return;

    unsigned numberOfSourceChannels = sourceBus.numberOfChannels();
    unsigned numberOfDestinationChannels = numberOfChannels();

    if (numberOfDestinationChannels == numberOfSourceChannels) {
        for (unsigned i = 0; i < numberOfSourceChannels; ++i)
            channel(i)->sumFrom(sourceBus.channel(i));
    } else {
        switch (channelInterpretation) {
        case Speakers:
            speakersSumFrom(sourceBus);
            break;
        case Discrete:
            discreteSumFrom(sourceBus);
            break;
        default:
            ASSERT_NOT_REACHED();
        }
    }
}

void AudioBus::speakersCopyFrom(const AudioBus& sourceBus)
{
    // FIXME: Implement down mixing 5.1 to stereo.
    // https://bugs.webkit.org/show_bug.cgi?id=79192

    unsigned numberOfSourceChannels = sourceBus.numberOfChannels();
    unsigned numberOfDestinationChannels = numberOfChannels();

    if (numberOfDestinationChannels == 2 && numberOfSourceChannels == 1) {
        // Handle mono -> stereo case (for now simply copy mono channel into both left and right)
        // FIXME: Really we should apply an equal-power scaling factor here, since we're effectively panning center...
        const AudioChannel* sourceChannel = sourceBus.channel(0);
        channel(0)->copyFrom(sourceChannel);
        channel(1)->copyFrom(sourceChannel);
    } else if (numberOfDestinationChannels == 1 && numberOfSourceChannels == 2) {
        // Handle stereo -> mono case. output = 0.5 * (input.L + input.R).
        AudioBus& sourceBusSafe = const_cast<AudioBus&>(sourceBus);

        const float* sourceL = sourceBusSafe.channelByType(ChannelLeft)->data();
        const float* sourceR = sourceBusSafe.channelByType(ChannelRight)->data();

        float* destination = channelByType(ChannelLeft)->mutableData();
        vadd(sourceL, 1, sourceR, 1, destination, 1, length());
        float scale = 0.5;
        vsmul(destination, 1, &scale, destination, 1, length());
    } else if (numberOfDestinationChannels == 6 && numberOfSourceChannels == 1) {
        // Handle mono -> 5.1 case, copy mono channel to center.
        channel(2)->copyFrom(sourceBus.channel(0));
        channel(0)->zero();
        channel(1)->zero();
        channel(3)->zero();
        channel(4)->zero();
        channel(5)->zero();
    } else if (numberOfDestinationChannels == 1 && numberOfSourceChannels == 6) {
        // Handle 5.1 -> mono case.
        zero();
        speakersSumFrom5_1_ToMono(sourceBus);
    } else {
        // Fallback for unknown combinations.
        discreteCopyFrom(sourceBus);
    }
}

void AudioBus::speakersSumFrom(const AudioBus& sourceBus)
{
    // FIXME: Implement down mixing 5.1 to stereo.
    // https://bugs.webkit.org/show_bug.cgi?id=79192

    unsigned numberOfSourceChannels = sourceBus.numberOfChannels();
    unsigned numberOfDestinationChannels = numberOfChannels();

    if (numberOfDestinationChannels == 2 && numberOfSourceChannels == 1) {
        // Handle mono -> stereo case (summing mono channel into both left and right).
        const AudioChannel* sourceChannel = sourceBus.channel(0);
        channel(0)->sumFrom(sourceChannel);
        channel(1)->sumFrom(sourceChannel);
    } else if (numberOfDestinationChannels == 1 && numberOfSourceChannels == 2) {
        // Handle stereo -> mono case. output += 0.5 * (input.L + input.R).
        AudioBus& sourceBusSafe = const_cast<AudioBus&>(sourceBus);

        const float* sourceL = sourceBusSafe.channelByType(ChannelLeft)->data();
        const float* sourceR = sourceBusSafe.channelByType(ChannelRight)->data();

        float* destination = channelByType(ChannelLeft)->mutableData();
        float scale = 0.5;
        vsma(sourceL, 1, &scale, destination, 1, length());
        vsma(sourceR, 1, &scale, destination, 1, length());
    } else if (numberOfDestinationChannels == 6 && numberOfSourceChannels == 1) {
        // Handle mono -> 5.1 case, sum mono channel into center.
        channel(2)->sumFrom(sourceBus.channel(0));
    } else if (numberOfDestinationChannels == 1 && numberOfSourceChannels == 6) {
        // Handle 5.1 -> mono case.
        speakersSumFrom5_1_ToMono(sourceBus);
    } else {
        // Fallback for unknown combinations.
        discreteSumFrom(sourceBus);
    }
}

void AudioBus::speakersSumFrom5_1_ToMono(const AudioBus& sourceBus)
{
    AudioBus& sourceBusSafe = const_cast<AudioBus&>(sourceBus);

    const float* sourceL = sourceBusSafe.channelByType(ChannelLeft)->data();
    const float* sourceR = sourceBusSafe.channelByType(ChannelRight)->data();
    const float* sourceC = sourceBusSafe.channelByType(ChannelCenter)->data();
    const float* sourceSL = sourceBusSafe.channelByType(ChannelSurroundLeft)->data();
    const float* sourceSR = sourceBusSafe.channelByType(ChannelSurroundRight)->data();

    float* destination = channelByType(ChannelLeft)->mutableData();

    AudioFloatArray temp(length());
    float* tempData = temp.data();

    // Sum in L and R.
    vadd(sourceL, 1, sourceR, 1, tempData, 1, length());
    float scale = 0.7071;
    vsmul(tempData, 1, &scale, tempData, 1, length());
    vadd(tempData, 1, destination, 1, destination, 1, length());

    // Sum in SL and SR.
    vadd(sourceSL, 1, sourceSR, 1, tempData, 1, length());
    scale = 0.5;
    vsmul(tempData, 1, &scale, tempData, 1, length());
    vadd(tempData, 1, destination, 1, destination, 1, length());

    // Sum in center.
    vadd(sourceC, 1, destination, 1, destination, 1, length());
}

void AudioBus::discreteCopyFrom(const AudioBus& sourceBus)
{
    unsigned numberOfSourceChannels = sourceBus.numberOfChannels();
    unsigned numberOfDestinationChannels = numberOfChannels();

    if (numberOfDestinationChannels < numberOfSourceChannels) {
        // Down-mix by copying channels and dropping the remaining.
        for (unsigned i = 0; i < numberOfDestinationChannels; ++i)
            channel(i)->copyFrom(sourceBus.channel(i));
    } else if (numberOfDestinationChannels > numberOfSourceChannels) {
        // Up-mix by copying as many channels as we have, then zeroing remaining channels.
        for (unsigned i = 0; i < numberOfSourceChannels; ++i)
            channel(i)->copyFrom(sourceBus.channel(i));
        for (unsigned i = numberOfSourceChannels; i < numberOfDestinationChannels; ++i)
            channel(i)->zero();
    }
}

void AudioBus::discreteSumFrom(const AudioBus& sourceBus)
{
    unsigned numberOfSourceChannels = sourceBus.numberOfChannels();
    unsigned numberOfDestinationChannels = numberOfChannels();

    if (numberOfDestinationChannels < numberOfSourceChannels) {
        // Down-mix by summing channels and dropping the remaining.
        for (unsigned i = 0; i < numberOfDestinationChannels; ++i)
            channel(i)->sumFrom(sourceBus.channel(i));
    } else if (numberOfDestinationChannels > numberOfSourceChannels) {
        // Up-mix by summing as many channels as we have.
        for (unsigned i = 0; i < numberOfSourceChannels; ++i)
            channel(i)->sumFrom(sourceBus.channel(i));
    }
}

void AudioBus::copyWithGainFrom(const AudioBus &sourceBus, float* lastMixGain, float targetGain)
{
    if (!topologyMatches(sourceBus)) {
        ASSERT_NOT_REACHED();
        zero();
        return;
    }

    if (sourceBus.isSilent()) {
        zero();
        return;
    }

    unsigned numberOfChannels = this->numberOfChannels();
    ASSERT(numberOfChannels <= MaxBusChannels);
    if (numberOfChannels > MaxBusChannels)
        return;

    // If it is copying from the same bus and no need to change gain, just return.
    if (this == &sourceBus && *lastMixGain == targetGain && targetGain == 1)
        return;

    AudioBus& sourceBusSafe = const_cast<AudioBus&>(sourceBus);
    const float* sources[MaxBusChannels];
    float* destinations[MaxBusChannels];

    for (unsigned i = 0; i < numberOfChannels; ++i) {
        sources[i] = sourceBusSafe.channel(i)->data();
        destinations[i] = channel(i)->mutableData();
    }

    // We don't want to suddenly change the gain from mixing one time slice to the next,
    // so we "de-zipper" by slowly changing the gain each sample-frame until we've achieved the target gain.

    // Take master bus gain into account as well as the targetGain.
    float totalDesiredGain = static_cast<float>(m_busGain * targetGain);

    // First time, snap directly to totalDesiredGain.
    float gain = static_cast<float>(m_isFirstTime ? totalDesiredGain : *lastMixGain);
    m_isFirstTime = false;

    const float DezipperRate = 0.005f;
    unsigned framesToProcess = length();

    // If the gain is within epsilon of totalDesiredGain, we can skip dezippering.
    // FIXME: this value may need tweaking.
    const float epsilon = 0.001f;
    float gainDiff = fabs(totalDesiredGain - gain);

    // Number of frames to de-zipper before we are close enough to the target gain.
    // FIXME: framesToDezipper could be smaller when target gain is close enough within this process loop.
    unsigned framesToDezipper = (gainDiff < epsilon) ? 0 : framesToProcess;

    if (framesToDezipper) {
        if (!m_dezipperGainValues.get() || m_dezipperGainValues->size() < framesToDezipper)
            m_dezipperGainValues = adoptPtr(new AudioFloatArray(framesToDezipper));

        float* gainValues = m_dezipperGainValues->data();
        for (unsigned i = 0; i < framesToDezipper; ++i) {
            gain += (totalDesiredGain - gain) * DezipperRate;

            // FIXME: If we are clever enough in calculating the framesToDezipper value, we can probably get
            // rid of this DenormalDisabler::flushDenormalFloatToZero() call.
            gain = DenormalDisabler::flushDenormalFloatToZero(gain);
            *gainValues++ = gain;
        }

        for (unsigned channelIndex = 0; channelIndex < numberOfChannels; ++channelIndex) {
            vmul(sources[channelIndex], 1, m_dezipperGainValues->data(), 1, destinations[channelIndex], 1, framesToDezipper);
            sources[channelIndex] += framesToDezipper;
            destinations[channelIndex] += framesToDezipper;
        }
    } else
        gain = totalDesiredGain;

    // Apply constant gain after de-zippering has converged on target gain.
    if (framesToDezipper < framesToProcess) {
        for (unsigned channelIndex = 0; channelIndex < numberOfChannels; ++channelIndex)
            vsmul(sources[channelIndex], 1, &gain, destinations[channelIndex], 1, framesToProcess - framesToDezipper);
    }

    // Save the target gain as the starting point for next time around.
    *lastMixGain = gain;
}

void AudioBus::copyWithSampleAccurateGainValuesFrom(const AudioBus &sourceBus, float* gainValues, unsigned numberOfGainValues)
{
    // Make sure we're processing from the same type of bus.
    // We *are* able to process from mono -> stereo
    if (sourceBus.numberOfChannels() != 1 && !topologyMatches(sourceBus)) {
        ASSERT_NOT_REACHED();
        return;
    }

    if (!gainValues || numberOfGainValues > sourceBus.length()) {
        ASSERT_NOT_REACHED();
        return;
    }

    if (sourceBus.length() == numberOfGainValues && sourceBus.length() == length() && sourceBus.isSilent()) {
        zero();
        return;
    }

    // We handle both the 1 -> N and N -> N case here.
    const float* source = sourceBus.channel(0)->data();
    for (unsigned channelIndex = 0; channelIndex < numberOfChannels(); ++channelIndex) {
        if (sourceBus.numberOfChannels() == numberOfChannels())
            source = sourceBus.channel(channelIndex)->data();
        float* destination = channel(channelIndex)->mutableData();
        vmul(source, 1, gainValues, 1, destination, 1, numberOfGainValues);
    }
}

PassRefPtr<AudioBus> AudioBus::createBySampleRateConverting(const AudioBus* sourceBus, bool mixToMono, double newSampleRate)
{
    // sourceBus's sample-rate must be known.
    ASSERT(sourceBus && sourceBus->sampleRate());
    if (!sourceBus || !sourceBus->sampleRate())
        return nullptr;

    double sourceSampleRate = sourceBus->sampleRate();
    double destinationSampleRate = newSampleRate;
    double sampleRateRatio = sourceSampleRate / destinationSampleRate;
    unsigned numberOfSourceChannels = sourceBus->numberOfChannels();

    if (numberOfSourceChannels == 1)
        mixToMono = false; // already mono

    if (sourceSampleRate == destinationSampleRate) {
        // No sample-rate conversion is necessary.
        if (mixToMono)
            return AudioBus::createByMixingToMono(sourceBus);

        // Return exact copy.
        return AudioBus::createBufferFromRange(sourceBus, 0, sourceBus->length());
    }

    if (sourceBus->isSilent()) {
        RefPtr<AudioBus> silentBus = create(numberOfSourceChannels, sourceBus->length() / sampleRateRatio);
        silentBus->setSampleRate(newSampleRate);
        return silentBus;
    }

    // First, mix to mono (if necessary) then sample-rate convert.
    const AudioBus* resamplerSourceBus;
    RefPtr<AudioBus> mixedMonoBus;
    if (mixToMono) {
        mixedMonoBus = AudioBus::createByMixingToMono(sourceBus);
        resamplerSourceBus = mixedMonoBus.get();
    } else {
        // Directly resample without down-mixing.
        resamplerSourceBus = sourceBus;
    }

    // Calculate destination length based on the sample-rates.
    int sourceLength = resamplerSourceBus->length();
    int destinationLength = sourceLength / sampleRateRatio;

    // Create destination bus with same number of channels.
    unsigned numberOfDestinationChannels = resamplerSourceBus->numberOfChannels();
    RefPtr<AudioBus> destinationBus = create(numberOfDestinationChannels, destinationLength);

    // Sample-rate convert each channel.
    for (unsigned i = 0; i < numberOfDestinationChannels; ++i) {
        const float* source = resamplerSourceBus->channel(i)->data();
        float* destination = destinationBus->channel(i)->mutableData();

        SincResampler resampler(sampleRateRatio);
        resampler.process(source, destination, sourceLength);
    }

    destinationBus->clearSilentFlag();
    destinationBus->setSampleRate(newSampleRate);
    return destinationBus;
}

PassRefPtr<AudioBus> AudioBus::createByMixingToMono(const AudioBus* sourceBus)
{
    if (sourceBus->isSilent())
        return create(1, sourceBus->length());

    switch (sourceBus->numberOfChannels()) {
    case 1:
        // Simply create an exact copy.
        return AudioBus::createBufferFromRange(sourceBus, 0, sourceBus->length());
    case 2:
        {
            unsigned n = sourceBus->length();
            RefPtr<AudioBus> destinationBus = create(1, n);

            const float* sourceL = sourceBus->channel(0)->data();
            const float* sourceR = sourceBus->channel(1)->data();
            float* destination = destinationBus->channel(0)->mutableData();

            // Do the mono mixdown.
            for (unsigned i = 0; i < n; ++i)
                destination[i] = (sourceL[i] + sourceR[i]) / 2;

            destinationBus->clearSilentFlag();
            destinationBus->setSampleRate(sourceBus->sampleRate());
            return destinationBus;
        }
    }

    ASSERT_NOT_REACHED();
    return nullptr;
}

bool AudioBus::isSilent() const
{
    for (size_t i = 0; i < m_channels.size(); ++i) {
        if (!m_channels[i]->isSilent())
            return false;
    }
    return true;
}

void AudioBus::clearSilentFlag()
{
    for (size_t i = 0; i < m_channels.size(); ++i)
        m_channels[i]->clearSilentFlag();
}

PassRefPtr<AudioBus> decodeAudioFileData(const char* data, size_t size)
{
    WebAudioBus webAudioBus;
    if (Platform::current()->loadAudioResource(&webAudioBus, data, size))
        return webAudioBus.release();
    return nullptr;
}

PassRefPtr<AudioBus> AudioBus::loadPlatformResource(const char* name, float sampleRate)
{
    const WebData& resource = Platform::current()->loadResource(name);
    if (resource.isEmpty())
        return nullptr;

    RefPtr<AudioBus> audioBus = decodeAudioFileData(resource.data(), resource.size());

    if (!audioBus.get())
        return nullptr;

    // If the bus is already at the requested sample-rate then return as is.
    if (audioBus->sampleRate() == sampleRate)
        return audioBus;

    return AudioBus::createBySampleRateConverting(audioBus.get(), false, sampleRate);
}

PassRefPtr<AudioBus> createBusFromInMemoryAudioFile(const void* data, size_t dataSize, bool mixToMono, float sampleRate)
{
    RefPtr<AudioBus> audioBus = decodeAudioFileData(static_cast<const char*>(data), dataSize);
    if (!audioBus.get())
        return nullptr;

    // If the bus needs no conversion then return as is.
    if ((!mixToMono || audioBus->numberOfChannels() == 1) && audioBus->sampleRate() == sampleRate)
        return audioBus;

    return AudioBus::createBySampleRateConverting(audioBus.get(), mixToMono, sampleRate);
}

} // namespace blink

#endif // ENABLE(WEB_AUDIO)