Add algorithm for Residual Echo Detector.

webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h

Index: webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h
diff --git a/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h b/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h
new file mode 100644
index 0000000000000000000000000000000000000000..3c0700fad0eac3a4cfb3fa4b445ad95f79a068d6
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+++ b/webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h
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+/*
+ *  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 Update(float value);
+  float std_deviation() const;
+  float mean() const;
+  void Clear();
+
+ private:
+  // 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_