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 {
+
+static const int kPermutation[33] =

[tommi (sloooow) - chröme, 2014/02/27 08:39:43]

Add documentation. Also document why the size is 33.

[no longer working on chromium, 2014/02/28 08:16:25]

Done.

+    {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};
+static const int kUpdateFrequency = 10;

[tommi (sloooow) - chröme, 2014/02/27 08:39:43]

documentation.  Also, since this is only used in one function, we could define
the constant there.  Same for kPermutation.

[no longer working on chromium, 2014/02/28 08:16:25]

Done.

+static const int k16BitsMaxValue = 32767;

[tommi (sloooow) - chröme, 2014/02/27 08:39:43]

do we need this constant?

Could we instead just use std::numeric_limits<int16>::max() below where
k16BitsMaxValue is used?

[no longer working on chromium, 2014/02/28 08:16:25]

I used it because that is how webrtc code was written, but taking another look
at the problem, if the data type is int16, probably the maximum value will never
be bigger than std::numeric_limits<int16>::max().
So I added a DCHECK here, and will run some tests to verify it.

[no longer working on chromium, 2014/03/01 11:34:28]

AudioBus::ToInterleave guarantees the value in the range of [-32768, 32767], so
the absolute amplitude should not be bigger than 32768, I added a check instead.

+
+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;
+  }
+
+  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(CalledOnValidThread());

[tommi (sloooow) - chröme, 2014/02/27 08:39:43]

nice

+  int max = MaxAmplitude(audio_data, number_of_channels * number_of_frames);
+  max_amplitude_ = std::max(max_amplitude_, std::min(max, k16BitsMaxValue));
+
+  if (counter_++ == kUpdateFrequency) {
+    // Divide k16BitsMaxValue (32767) by 1000 to get in the range of [0,32]
+    // 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