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diff --git a/webrtc/modules/audio_processing/aec3/stationarity_estimator.h b/webrtc/modules/audio_processing/aec3/stationarity_estimator.h
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+/*
+ *  Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef MODULES_AUDIO_PROCESSING_AEC3_STATIONARITY_ESTIMATOR_H_
+#define MODULES_AUDIO_PROCESSING_AEC3_STATIONARITY_ESTIMATOR_H_
+
+#include <stddef.h>
+
+#include <array>
+#include <memory>
+
+#include "api/array_view.h"
+#include "modules/audio_processing/aec3/aec3_common.h"  // kFftLengthBy2Plus1...
+#include "modules/audio_processing/aec3/reverb_model.h"
+#include "rtc_base/checks.h"
+
+namespace webrtc {
+
+class ApmDataDumper;
+struct SpectrumBuffer;
+
+class StationarityEstimator {
+ public:
+  StationarityEstimator();
+  ~StationarityEstimator();
+
+  // Reset the stationarity estimator.
+  void Reset();
+
+  // Update just the noise estimator. Usefull until the delay is known
+  void UpdateNoiseEstimator(
+      rtc::ArrayView<const std::array<float, kFftLengthBy2Plus1>> spectrum);
+
+  // Update the flag indicating whether this current frame is stationary. For
+  // getting a more robust estimation, it looks at future and/or past frames.
+  void UpdateStationarityFlags(
+      const SpectrumBuffer& spectrum_buffer,
+      rtc::ArrayView<const float> render_reverb_contribution_spectrum,
+      int idx_current,
+      int num_lookahead);
+
+  // Returns true if the current band is stationary.
+  bool IsBandStationary(size_t band) const {
+    return stationarity_flags_[band] && (hangovers_[band] == 0);
+  }
+
+  // Returns true if the current block is estimated as stationary.
+  bool IsBlockStationary() const;
+
+ private:
+  static constexpr int kWindowLength = 13;
+  // Returns the power of the stationary noise spectrum at a band.
+  float GetStationarityPowerBand(size_t k) const { return noise_.Power(k); }
+
+  // Get an estimation of the stationarity for the current band by looking
+  // at the past/present/future available data.
+  bool EstimateBandStationarity(const SpectrumBuffer& spectrum_buffer,
+                                rtc::ArrayView<const float> average_reverb,
+                                const std::array<int, kWindowLength>& indexes,
+                                size_t band) const;
+
+  // True if all bands at the current point are stationary.
+  bool AreAllBandsStationary();
+
+  // Update the hangover depending on the stationary status of the current
+  // frame.
+  void UpdateHangover();
+
+  // Smooth the stationarity detection by looking at neighbouring frequency
+  // bands.
+  void SmoothStationaryPerFreq();
+
+  class NoiseSpectrum {
+   public:
+    NoiseSpectrum();
+    ~NoiseSpectrum();
+
+    // Reset the noise power spectrum estimate state.
+    void Reset();
+
+    // Update the noise power spectrum with a new frame.
+    void Update(
+        rtc::ArrayView<const std::array<float, kFftLengthBy2Plus1>> spectrum);
+
+    // Get the noise estimation power spectrum.
+    rtc::ArrayView<const float> Spectrum() const { return noise_spectrum_; }
+
+    // Get the noise power spectrum at a certain band.
+    float Power(size_t band) const {
+      RTC_DCHECK_LT(band, noise_spectrum_.size());
+      return noise_spectrum_[band];
+    }
+
+   private:
+    // Get the update coefficient to be used for the current frame.
+    float GetAlpha() const;
+
+    // Update the noise power spectrum at a certain band with a new frame.
+    float UpdateBandBySmoothing(float power_band,
+                                float power_band_noise,
+                                float alpha) const;
+    std::array<float, kFftLengthBy2Plus1> noise_spectrum_;
+    size_t block_counter_;
+  };
+
+  static int instance_count_;
+  std::unique_ptr<ApmDataDumper> data_dumper_;
+  NoiseSpectrum noise_;
+  std::array<int, kFftLengthBy2Plus1> hangovers_;
+  std::array<bool, kFftLengthBy2Plus1> stationarity_flags_;
+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AEC3_STATIONARITY_ESTIMATOR_H_