Add algorithm for Residual Echo Detector.

webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h

+/*
+ *  Copyright (c) 2016 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 WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_MEAN_VARIANCE_ESTIMATOR_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_MEAN_VARIANCE_ESTIMATOR_H_
+
+namespace webrtc {
+
+// This class iteratively estimates the mean and variance of a signal.
+class MeanVarianceEstimator {
+ public:
+  void UpdateEstimate(float value);
+  float std_deviation() const;
+  float mean() const;
+  void Clear();
+
+ private:
+  // Parameter controlling the adaptation speed.
+  static constexpr float kAlpha = 0.01f;

[peah-webrtc, 2016/10/19 14:45:25]

I think it is better to move this into the .cc file instead as it is not used in
the .h file. This makes the code easier to read as the value constant is closer
to, and in the same files as, where it is used.

[ivoc, 2016/10/20 14:04:35]

Done.

+  // Estimate of the expected value of the input values.
+  float mean_ = 0.f;
+  // Estimate of the variance of the input values.
+  float variance_ = 0.f;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_MEAN_VARIANCE_ESTIMATOR_H_