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+/*
+ *  Copyright (c) 2012 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_AGC_LOUDNESS_HISTOGRAM_H_
+#define MODULES_AUDIO_PROCESSING_AGC_LOUDNESS_HISTOGRAM_H_
+
+#include <stdint.h>
+
+#include <memory>
+
+namespace webrtc {
+
+// This class implements the histogram of loudness with circular buffers so that
+// the histogram tracks the last T seconds of the loudness.
+class LoudnessHistogram {
+ public:
+  // Create a non-sliding LoudnessHistogram.
+  static LoudnessHistogram* Create();
+
+  // Create a sliding LoudnessHistogram, i.e. the histogram represents the last
+  // `window_size` samples.
+  static LoudnessHistogram* Create(int window_size);
+  ~LoudnessHistogram();
+
+  // Insert RMS and the corresponding activity probability.
+  void Update(double rms, double activity_probability);
+
+  // Reset the histogram, forget the past.
+  void Reset();
+
+  // Current loudness, which is actually the mean of histogram in loudness
+  // domain.
+  double CurrentRms() const;
+
+  // Sum of the histogram content.
+  double AudioContent() const;
+
+  // Number of times the histogram has been updated.
+  int num_updates() const { return num_updates_; }
+
+ private:
+  LoudnessHistogram();
+  explicit LoudnessHistogram(int window);
+
+  // Find the histogram bin associated with the given `rms`.
+  int GetBinIndex(double rms);
+
+  void RemoveOldestEntryAndUpdate();
+  void InsertNewestEntryAndUpdate(int activity_prob_q10, int hist_index);
+  void UpdateHist(int activity_prob_q10, int hist_index);
+  void RemoveTransient();
+
+  // Number of histogram bins.
+  static const int kHistSize = 77;
+
+  // Number of times the histogram is updated
+  int num_updates_;
+  // Audio content, this should be equal to the sum of the components of
+  // `bin_count_q10_`.
+  int64_t audio_content_q10_;
+
+  // LoudnessHistogram of input RMS in Q10 with `kHistSize_` bins. In each
+  // 'Update(),' we increment the associated histogram-bin with the given
+  // probability. The increment is implemented in Q10 to avoid rounding errors.
+  int64_t bin_count_q10_[kHistSize];
+
+  // Circular buffer for probabilities
+  std::unique_ptr<int[]> activity_probability_;
+  // Circular buffer for histogram-indices of probabilities.
+  std::unique_ptr<int[]> hist_bin_index_;
+  // Current index of circular buffer, where the newest data will be written to,
+  // therefore, pointing to the oldest data if buffer is full.
+  int buffer_index_;
+  // Indicating if buffer is full and we had a wrap around.
+  int buffer_is_full_;
+  // Size of circular buffer.
+  int len_circular_buffer_;
+  int len_high_activity_;
+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AGC_LOUDNESS_HISTOGRAM_H_