Add algorithm for Residual Echo Detector.

webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.h

Index: webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.h
diff --git a/webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.h b/webrtc/modules/audio_processing/echo_detector/normalized_covariance_estimator.h
new file mode 100644
index 0000000000000000000000000000000000000000..f77a30ec73c25183112ed26bc34d6945fecda43b
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+++ b/webrtc/modules/audio_processing/echo_detector/normalized_covariance_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_NORMALIZED_COVARIANCE_ESTIMATOR_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_NORMALIZED_COVARIANCE_ESTIMATOR_H_
+
+namespace webrtc {
+
+// This class iteratively estimates the normalized covariance between two
+// signals.
+class NormalizedCovarianceEstimator {
+ public:
+  void Update(float x,
+              float x_mean,
+              float x_var,
+              float y,
+              float y_mean,
+              float y_var);
+  // This function returns an estimate of the Pearson product-moment correlation
+  // coefficient of the two signals.
+  float normalized_cross_correlation() const {
+    return normalized_cross_correlation_;
+  }
+  // This function resets the estimated values to zero.
+  void Clear();
+
+ private:
+  float normalized_cross_correlation_ = 0.f;
+  // Estimate of the covariance value.
+  float covariance_ = 0.f;
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_ECHO_DETECTOR_NORMALIZED_COVARIANCE_ESTIMATOR_H_