Add algorithm for Residual Echo Detector.

webrtc/modules/audio_processing/echo_detector/circular_buffer.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/circular_buffer.h"
+
+#include <algorithm>
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+CircularBuffer::CircularBuffer(size_t size) : buffer_(size) {}
+CircularBuffer::CircularBuffer(CircularBuffer&& other) = default;

[peah-webrtc, 2016/10/19 14:45:25]

I don't think you need this one. My guess is that it is only needed because you
initialize the variable using
CircularBuffer render_buffer_ = CircularBuffer(kRenderBufferSize)

If you instead initialize it as CircularBuffer render_buffer_(kRenderBufferSize)
it should be fine without this one. (you may have to do it in the initializer
list though).

[ivoc, 2016/10/20 14:04:35]

Done.

+CircularBuffer::~CircularBuffer() = default;
+
+void CircularBuffer::Push(float value) {

[peah-webrtc, 2016/10/19 14:45:25]

For me it is very strange to have a circular buffer with a push and pop
function. To me that functionality specifies a stack where the pushed element
goes on top, and the element popped is the most recent element pushed. I think
the code would benefit from another naming of this.

[ivoc, 2016/10/20 14:04:35]

This class behaves like a queue, except that it has a maximum size. For queues
the push/pop naming is pretty common, see for example the c++ standard library:
http://www.cplusplus.com/reference/queue/queue/ 
So I think the names are appropriate here.

[peah-webrtc, 2016/10/20 15:08:23]

I see. Good point. 

+  buffer_[next_insertion_index_] = value;
+  ++next_insertion_index_;
+  next_insertion_index_ %= buffer_.size();
+  RTC_DCHECK_LT(next_insertion_index_, buffer_.size());
+  buffer_size_ = std::min(buffer_size_ + 1, buffer_.size());
+  RTC_DCHECK_LE(buffer_size_, buffer_.size());
+}
+
+rtc::Optional<float> CircularBuffer::Pop() {
+  if (buffer_size_ == 0) {
+    return rtc::Optional<float>();
+  }
+  const size_t index =
+      (buffer_.size() + next_insertion_index_ - buffer_size_) % buffer_.size();
+  RTC_DCHECK_LT(index, buffer_.size());
+  --buffer_size_;
+  return rtc::Optional<float>(buffer_[index]);
+}
+
+void CircularBuffer::Clear() {
+  std::fill(buffer_.begin(), buffer_.end(), 0.f);
+  next_insertion_index_ = 0;
+  buffer_size_ = 0;
+}
+
+}  // namespace webrtc