Added first layer of the echo canceller 3 functionality

webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.cc

Index: webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.cc
diff --git a/webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.cc b/webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.cc
new file mode 100644
index 0000000000000000000000000000000000000000..539cdf15a108019ebc3a3e7ec2780218388117d0
--- /dev/null
+++ b/webrtc/modules/audio_processing/aec3/cascaded_biquad_filter.cc
@@ -0,0 +1,64 @@
+/*
+ *  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/aec3/cascaded_biquad_filter.h"
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+void CascadedBiQuadFilter::Process(rtc::ArrayView<const float> x,
+                                   rtc::ArrayView<float> y) {
+  ApplyBiQuad(x, y, &biquad_states_[0]);
+  for (size_t k = 1; k < biquad_states_.size(); ++k) {
+    ApplyBiQuad(y, y, &biquad_states_[k]);
+  }
+}
+
+void CascadedBiQuadFilter::Process(rtc::ArrayView<float> y) {
+  for (auto& biquad : biquad_states_) {
+    ApplyBiQuad(y, y, &biquad);
+  }
+}
+
+CascadedBiQuadFilter::CascadedBiQuadFilter(

[hlundin-webrtc, 2016/12/16 10:04:46]

Order of methods is not the same as in h-file. Move the ctor and dtor to the
top.

[peah-webrtc, 2016/12/20 10:10:24]

Done.

+    const CascadedBiQuadFilter::BiQuadCoefficients& coefficients,
+    size_t num_biquads)
+    : num_biquads_(num_biquads),
+      biquad_states_(num_biquads_),
+      coefficients_(coefficients) {
+  for (auto& biquad_state : biquad_states_) {
+    biquad_state.a[0] = biquad_state.a[1] = 0;

[hlundin-webrtc, 2016/12/16 10:04:46]

Probably doesn't matter, but 0.f.

[ivoc, 2016/12/19 11:30:21]

Is it possible to do this initialization in the header? i.e. float a[2] = {0.f,
0.f};

[peah-webrtc, 2016/12/20 10:10:24]

Done.

[peah-webrtc, 2016/12/20 10:10:24]

Not as far as I can see. Afaics brace initialization does not seem to be allowed
in initializer lists. But I moved this to the constructor of the state which
made better code that what it was before (I think).

Done. 

[ivoc, 2016/12/21 13:04:04]

Acknowledged.

[peah-webrtc, 2016/12/21 23:13:48]

Acknowledged.

+    biquad_state.b[0] = biquad_state.b[1] = 0;

[hlundin-webrtc, 2016/12/16 10:04:46]

0.f

[peah-webrtc, 2016/12/20 10:10:24]

Done.

+  }
+}
+
+CascadedBiQuadFilter::~CascadedBiQuadFilter() {}
+
+void CascadedBiQuadFilter::ApplyBiQuad(
+    rtc::ArrayView<const float> x,
+    rtc::ArrayView<float> y,
+    CascadedBiQuadFilter::BiQuadState* biquad_state) {
+  const auto coefficients_b = coefficients_.b;

[hlundin-webrtc, 2016/12/16 10:04:46]

Since you are introducing these local variables to simplify notation, I'd go for
much shorter names. Consider c_a and c_b.

[peah-webrtc, 2016/12/20 10:10:24]

Done.

+  const auto coefficients_a = coefficients_.a;
+  auto memory_b = biquad_state->b;

[hlundin-webrtc, 2016/12/16 10:04:46]

Following the notation of https://en.wikipedia.org/wiki/Digital_biquad_filter, I
would argue that mem_x and mem_y are better.

[peah-webrtc, 2016/12/20 10:10:24]

I'll change to _x and _y. I think that since we anyway abbreviate, I'll use m_
as the abbreviation.

WDYT?

[hlundin-webrtc, 2016/12/20 15:10:34]

Acknowledged.

[peah-webrtc, 2016/12/21 23:13:48]

Acknowledged.

+  auto memory_a = biquad_state->a;
+  for (size_t k = 0; k < x.size(); ++k) {
+    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