Add algorithm for Residual Echo Detector.

webrtc/modules/audio_processing/echo_detector/normalized_covariance_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/normalized_covariance_estimator.h"
+
+namespace webrtc {
+namespace {
+
+// Parameter controlling the adaptation speed.
+constexpr float kAlpha = 0.01f;
+
+}  // namespace
+
+void NormalizedCovarianceEstimator::Update(float x,
+                                           float x_mean,
+                                           float x_sigma,
+                                           float y,
+                                           float y_mean,
+                                           float y_sigma) {
+  covariance_ =
+      (1.f - kAlpha) * covariance_ + kAlpha * (x - x_mean) * (y - y_mean);
+  normalized_cross_correlation_ = covariance_ / (x_sigma * y_sigma + .0001f);
+}
+
+void NormalizedCovarianceEstimator::Clear() {
+  covariance_ = 0.f;
+  normalized_cross_correlation_ = 0.f;
+}
+
+}  // namespace webrtc