message: Use a one-dimensional vector for Block data
  Puts the whole block in contiguous memory and reduce pointer look-up.
  The change has been verified to be bit-exact.
  Cr-Commit-Position: refs/heads/main@{#36983}
diff: namespace webrtc {
  +// Contains one or more channels of 4 milliseconds of audio data.
  +// The audio is split in one or more frequency bands, each with a sampling
  +// rate of 16 kHz.
  class Block {
  public:
  Block(int num_bands, int num_channels, float default_value = 0.0f)
  - : data_(num_bands,
  - std::vector<std::array<float, kBlockSize>>(
  - num_channels,
  - std::array<float, kBlockSize>({default_value}))) {}
  + : num_bands_(num_bands),
  + num_channels_(num_channels),
  + data_(num_bands * num_channels * kBlockSize, default_value) {}
  // Returns the number of bands.
  - int NumBands() const { return data_.size(); }
  + int NumBands() const { return num_bands_; }
  // Returns the number of channels.
  - int NumChannels() const { return data_[0].size(); }
  + int NumChannels() const { return num_channels_; }
  - // Modifies the number of channels.
  + // Modifies the number of channels and sets all samples to zero.
  void SetNumChannels(int num_channels) {
  - for (std::vector<std::array<float, kBlockSize>>& block_band : data_) {
  - block_band.resize(num_channels, std::array<float, kBlockSize>({0.0f}));
  - }
  + num_channels_ = num_channels;
  + data_.resize(num_bands_ * num_channels_ * kBlockSize);
  + std::fill(data_.begin(), data_.end(), 0.0f);
  }
  // Iterators for accessing the data.
  - auto begin(int band, int channel) { return data_[band][channel].begin(); }
  + auto begin(int band, int channel) {
  + return data_.begin() + GetIndex(band, channel);
  + }
  auto begin(int band, int channel) const {
  - return data_[band][channel].begin();
  + return data_.begin() + GetIndex(band, channel);
  }
  - auto end(int band, int channel) { return data_[band][channel].end(); }
  + auto end(int band, int channel) { return begin(band, channel) + kBlockSize; }
  - auto end(int band, int channel) const { return data_[band][channel].end(); }
  + auto end(int band, int channel) const {
  + return begin(band, channel) + kBlockSize;
  + }
  // Access data via ArrayView.
  rtc::ArrayView<float, kBlockSize> View(int band, int channel) {
  - return rtc::ArrayView<float, kBlockSize>(data_[band][channel].data(),
  + return rtc::ArrayView<float, kBlockSize>(&data_[GetIndex(band, channel)],
  kBlockSize);
  }
  rtc::ArrayView<const float, kBlockSize> View(int band, int channel) const {
  - return rtc::ArrayView<const float, kBlockSize>(data_[band][channel].data(),
  - kBlockSize);
  + return rtc::ArrayView<const float, kBlockSize>(
  + &data_[GetIndex(band, channel)], kBlockSize);
  }
  // Lets two Blocks swap audio data.
  - void Swap(Block& b) { data_.swap(b.data_); }
  + void Swap(Block& b) {
  + std::swap(num_bands_, b.num_bands_);
  + std::swap(num_channels_, b.num_channels_);
  + data_.swap(b.data_);
  + }
  private:
  - std::vector<std::vector<std::array<float, kBlockSize>>> data_;
  + // Returns the index of the first sample of the requested |band| and
  + // |channel|.
  + int GetIndex(int band, int channel) const {
  + return (band * num_channels_ + channel) * kBlockSize;
  + }
  +
  + int num_bands_;
  + int num_channels_;
  + std::vector<float> data_;
  };
  } // namespace webrtc
  