Calculate the signal level on the media stream local audio track

content/renderer/media/media_stream_audio_level_calculator.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "content/renderer/media/media_stream_audio_level_calculator.h"
+
+#include "base/logging.h"
+#include "base/stl_util.h"
+
+namespace content {
+
+namespace {
+
+int MaxAmplitude(const int16* audio_data, int length) {
+  int max = 0, absolute = 0;
+  for (int i = 0; i < length; ++i) {
+    absolute = std::abs(audio_data[i]);
+    if (absolute > max)
+      max = absolute;
+  }
+  // The range of int16 is [-32768, 32767], verify the |max| should be smaller
+  // than 32768.
+  DCHECK(max <= std::abd(std::numeric_limits<int16>::min()));

[tommi (sloooow) - chröme, 2014/03/01 11:49:33]

a few questions:

* does this compile? (abd?)
* The comment says "smaller than 32768".  The DCHECK allows _equal_ to 32768.
* 32768 is greater than int16 max, so we would be overflowing int16 if that
happened. (it's actually 0x8000 which when stored in an int16 is int16::min()!)
* Why not <= max()?

[no longer working on chromium, 2014/03/01 12:37:00]

No,  it should be abs.
It should be not bigger than 32768.
We are calculating the absolute value of the amplitude, since the minimum value
of int16 is -32768, so the maximum of the absolute value is 32768.

[tommi (sloooow) - chröme, 2014/03/02 10:19:53]

OK, now I get it.  Can you add documentation to MaxAmplitude can return a value
greater than std::numeric_limits<int16>::max()?  My concern was that if we would
at some point generalize this code to support other bits-per-sample situations,
then we can't return the same type as the samples themselves are stored in. 
I.e.

int16 max = MaxAmplitude(my_int16_buffer, length);

can actually end up with a negative number due to overflowing int16.

[no longer working on chromium, 2014/03/03 13:07:12]

Done with adding a comment. 

+
+  return max;
+}
+
+}  // namespace
+
+MediaStreamAudioLevelCalculator::MediaStreamAudioLevelCalculator()
+    : counter_(0),
+      max_amplitude_(0),
+      level_(0) {
+}
+
+MediaStreamAudioLevelCalculator::~MediaStreamAudioLevelCalculator() {
+}
+
+int MediaStreamAudioLevelCalculator::Calculate(const int16* audio_data,
+                                               int number_of_channels,
+                                               int number_of_frames) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  // Permutation of bars that reprents the amplitude level of the audio signal.
+  // The number of elements is 33 because we are indexing them in the range of
+  // [0, 32].
+  static const int kPermutation[33] =
+      {0,1,2,3,4,4,5,5,5,5,6,6,6,6,6,7,7,7,7,8,8,8,9,9,9,9,9,9,9,9,9,9,9};
+
+  // |level_| is updated every 10 callbacks. For the case where callback comes
+  // every 10ms, |level_| will be updated approximately every 100ms.
+  static const int kUpdateFrequency = 10;
+
+  int max = MaxAmplitude(audio_data, number_of_channels * number_of_frames);
+  max_amplitude_ = std::max(max_amplitude_, max);
+
+  if (counter_++ == kUpdateFrequency) {
+    // Divide the max amplitude (32768) by 1000 to get in the range of [0,32]

[tommi (sloooow) - chröme, 2014/03/01 11:49:33]

it can't be 32768.  that's higher than int16 can carry.

[no longer working on chromium, 2014/03/01 12:37:00]

The same explaination, absolute value of <int16>::min() is 32768

+    // which is the range of the permutation array.
+    int index = static_cast<int>(max_amplitude_ / 1000);
+
+    // Make it less likely that the bar stays at position 0. I.e. only if
+    // its in the range 0-250 (instead of 0-1000)
+    if (index == 0 && max_amplitude_ > 250)
+      index = 1;
+
+    // |level_| will be the value in the permutation array that the |index| is
+    // pointing to.
+    level_ = kPermutation[index];
+
+    // Decay the absolute maximum amplitude by 1/4.
+    max_amplitude_ >>= 2;
+
+    // Reset the counter.
+    counter_ = 0;
+  }
+
+  return level_;
+}
+
+}  // namespace content