Add algorithm for Residual Echo Detector.

webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.cc

+/*
+ *  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.
+ */
+
+#include "webrtc/modules/audio_processing/echo_detector/mean_variance_estimator.h"
+
+#include <math.h>
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+void MeanVarianceEstimator::UpdateEstimate(float value) {
+  mean_ = (1.f - kAlpha) * mean_ + kAlpha * value;
+  variance_ =
+      (1.f - kAlpha) * variance_ + kAlpha * (value - mean_) * (value - mean_);
+}
+
+float MeanVarianceEstimator::std_deviation() const {
+  RTC_DCHECK_GE(variance_, 0.f);
+  return sqrtf(variance_);
+}
+
+float MeanVarianceEstimator::mean() const {
+  return mean_;
+}
+
+void MeanVarianceEstimator::Clear() {
+  mean_ = 0.f;
+  variance_ = 0.f;
+}
+
+}  // namespace webrtc