third_party/WebKit/Source/platform/audio/DownSampler.h

/*
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#ifndef DownSampler_h
#define DownSampler_h

#include "platform/audio/AudioArray.h"
#include "platform/audio/DirectConvolver.h"
#include "wtf/Allocator.h"
#include "wtf/Noncopyable.h"

namespace blink {

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

class PLATFORM_EXPORT DownSampler {
  USING_FAST_MALLOC(DownSampler);
  WTF_MAKE_NONCOPYABLE(DownSampler);

 public:
  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 blink

#endif  // DownSampler_h