Source/WebCore/platform/audio/DownSampler.h

/*
 * Copyright (C) 2013 Google Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
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 *     notice, this list of conditions and the following disclaimer.
 * 2.  Redistributions in binary form must reproduce the above copyright
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 *     documentation and/or other materials provided with the distribution.
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 *     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
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#ifndef DownSampler_h
#define DownSampler_h

#include "AudioArray.h"
#include "DirectConvolver.h"

namespace WebCore {

// DownSampler down-samples the source stream by a factor of 2x.

class DownSampler final {
    WTF_MAKE_FAST_ALLOCATED;
public:
    explicit DownSampler(size_t inputBlockSize);

    // The destination buffer |destP| is of size sourceFramesToProcess / 2.
    void process(const float* sourceP, float* destP, size_t sourceFramesToProcess);

    void reset();

    // Latency based on the destination sample-rate.
    size_t latencyFrames() const;

private:
    enum { DefaultKernelSize = 256 };

    size_t m_inputBlockSize;

    // Computes ideal band-limited half-band filter coefficients.
    // In other words, filter out all frequencies higher than 0.25 * Nyquist.
    void initializeKernel();
    AudioFloatArray m_reducedKernel;

    // Half-band filter.
    DirectConvolver m_convolver;

    AudioFloatArray m_tempBuffer;

    // Used as delay-line (FIR filter history) for the input samples to account for the 0.5 term right in the middle of the kernel.
    AudioFloatArray m_inputBuffer;
};

} // namespace WebCore

#endif // DownSampler_h