New module for the adaptive level controlling functionality in the audio processing module

webrtc/modules/audio_processing/level_controller/biquad_filter.cc

Index: webrtc/modules/audio_processing/level_controller/biquad_filter.cc
diff --git a/webrtc/modules/audio_processing/level_controller/biquad_filter.cc b/webrtc/modules/audio_processing/level_controller/biquad_filter.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9c4a4d2e6c123273f259d418eefb4d6b3ae433b6
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+++ b/webrtc/modules/audio_processing/level_controller/biquad_filter.cc
@@ -0,0 +1,35 @@
+/*
+ *  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/level_controller/biquad_filter.h"
+
+namespace webrtc {
+
+// This method applies a biquad filter to an input signal x to produce an
+// output signal y. The biquad coefficients are specified at the construction
+// of the object.
+void BiQuadFilter::Process(rtc::ArrayView<const float> x,
+                           rtc::ArrayView<float> y) {
+  for (size_t k = 0; k < x.size(); ++k) {
+    // Use temporary variable for x[k] to allow in-place function call
+    // (that x and y refer to the same array).
+    const float tmp = x[k];
+    y[k] = coefficients_.b[0] * tmp + coefficients_.b[1] * biquad_state_.b[0] +
+           coefficients_.b[2] * biquad_state_.b[1] -
+           coefficients_.a[0] * biquad_state_.a[0] -
+           coefficients_.a[1] * biquad_state_.a[1];
+    biquad_state_.b[1] = biquad_state_.b[0];
+    biquad_state_.b[0] = tmp;
+    biquad_state_.a[1] = biquad_state_.a[0];
+    biquad_state_.a[0] = y[k];
+  }
+}
+
+}  // namespace webrtc