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diff --git a/webrtc/modules/audio_processing/aec3/residual_echo_estimator.h b/webrtc/modules/audio_processing/aec3/residual_echo_estimator.h
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+/*
+ *  Copyright (c) 2017 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_RESIDUAL_ECHO_ESTIMATOR_H_
+#define MODULES_AUDIO_PROCESSING_AEC3_RESIDUAL_ECHO_ESTIMATOR_H_
+
+#include <array>
+#include <memory>
+
+#include "absl/types/optional.h"
+#include "api/audio/echo_canceller3_config.h"
+#include "modules/audio_processing/aec3/aec3_common.h"
+#include "modules/audio_processing/aec3/aec_state.h"
+#include "modules/audio_processing/aec3/render_buffer.h"
+#include "modules/audio_processing/aec3/reverb_model.h"
+#include "modules/audio_processing/aec3/spectrum_buffer.h"
+#include "rtc_base/checks.h"
+
+namespace webrtc {
+
+class ResidualEchoEstimator {
+ public:
+  ResidualEchoEstimator(const EchoCanceller3Config& config,
+                        size_t num_render_channels);
+  ~ResidualEchoEstimator();
+
+  ResidualEchoEstimator(const ResidualEchoEstimator&) = delete;
+  ResidualEchoEstimator& operator=(const ResidualEchoEstimator&) = delete;
+
+  void Estimate(
+      const AecState& aec_state,
+      const RenderBuffer& render_buffer,
+      rtc::ArrayView<const std::array<float, kFftLengthBy2Plus1>> S2_linear,
+      rtc::ArrayView<const std::array<float, kFftLengthBy2Plus1>> Y2,
+      rtc::ArrayView<std::array<float, kFftLengthBy2Plus1>> R2);
+
+ private:
+  enum class ReverbType { kLinear, kNonLinear };
+
+  // Resets the state.
+  void Reset();
+
+  // Updates estimate for the power of the stationary noise component in the
+  // render signal.
+  void UpdateRenderNoisePower(const RenderBuffer& render_buffer);
+
+  // Adds the estimated unmodelled echo power to the residual echo power
+  // estimate.
+  void AddReverb(ReverbType reverb_type,
+                 const AecState& aec_state,
+                 const RenderBuffer& render_buffer,
+                 rtc::ArrayView<std::array<float, kFftLengthBy2Plus1>> R2);
+
+  // Gets the echo path gain to apply.
+  float GetEchoPathGain(const AecState& aec_state,
+                        bool gain_for_early_reflections) const;
+
+  const EchoCanceller3Config config_;
+  const size_t num_render_channels_;
+  const float early_reflections_transparent_mode_gain_;
+  const float late_reflections_transparent_mode_gain_;
+  const float early_reflections_general_gain_;
+  const float late_reflections_general_gain_;
+  std::array<float, kFftLengthBy2Plus1> X2_noise_floor_;
+  std::array<int, kFftLengthBy2Plus1> X2_noise_floor_counter_;
+  ReverbModel echo_reverb_;
+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AEC3_RESIDUAL_ECHO_ESTIMATOR_H_