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..f459ad85d9f283ce61bfefa9b1d06dd6f7e6d56b
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+++ b/webrtc/modules/audio_processing/level_controller/biquad_filter.cc
@@ -0,0 +1,56 @@
+/*
+ *  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"
+
+#include <string.h>
+
+namespace webrtc {
+
+BiQuadFilter::BiQuadFilter(const BiQuadCoefficients& coefficients,
+                           int num_biquads)
+    : num_biquads_(num_biquads),
+      biquad_states_(new std::vector<BiQuadState>(num_biquads_)) {

[hlundin-webrtc, 2016/06/27 11:21:13]

If biquad_states_ was just an std::vector, this would simplify to
biquad_states_(num_biquads_).

[peah-webrtc, 2016/06/27 22:51:46]

Done.

+  for (auto& biquad_state : *biquad_states_) {
+    biquad_state.Reset();
+  }
+  coefficients_ = coefficients;

[hlundin-webrtc, 2016/06/27 11:21:13]

Can't you initialize this in the initializer list? This is just a struct of
arrays of know sizes, so shallow-copy should be sufficient. Also, you can make
coefficients_ const.

[peah-webrtc, 2016/06/27 22:51:46]

Done.

+}
+
+void BiQuadFilter::Process(rtc::ArrayView<const float> x,
+                           rtc::ArrayView<float> y) {
+  ApplyBiQuad(x, y, &(*biquad_states_)[0]);

[hlundin-webrtc, 2016/06/27 11:21:13]

If the size is always 1 (per offline discussion), simplify by folding this
function with ApplyBiQuad.

[peah-webrtc, 2016/06/27 22:51:46]

Done.

+  for (size_t k = 1; k < biquad_states_->size(); ++k) {
+    ApplyBiQuad(y, y, &(*biquad_states_)[k]);
+  }
+}
+
+void BiQuadFilter::ApplyBiQuad(rtc::ArrayView<const float> x,

[hlundin-webrtc, 2016/06/27 11:21:13]

I wouldn't mind a function comment explaining in simple equations what this
does.

[peah-webrtc, 2016/06/27 22:51:46]

Done.

+                               rtc::ArrayView<float> y,
+                               BiQuadState* biquad_state) {
+  const auto coefficients_b = coefficients_.b;

[hlundin-webrtc, 2016/06/27 11:21:13]

There is a very marginal win in defining these local variables. You can skip
them, and just add a '.' in a few places below.

[peah-webrtc, 2016/06/27 22:51:46]

Done.

+  const auto coefficients_a = coefficients_.a;
+  auto memory_b = biquad_state->b;
+  auto memory_a = biquad_state->a;
+  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] * memory_b[0] +
+           coefficients_b[2] * memory_b[1] - coefficients_a[0] * memory_a[0] -
+           coefficients_a[1] * memory_a[1];
+    memory_b[1] = memory_b[0];
+    memory_b[0] = tmp;
+    memory_a[1] = memory_a[0];
+    memory_a[0] = y[k];
+  }
+}
+
+}  // namespace webrtc