Introduce AudioClock to improve playback delay calculations.

media/filters/audio_clock.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 "media/filters/audio_clock.h"
+
+#include "base/logging.h"
+#include "media/base/buffers.h"
+
+namespace media {
+
+AudioClock::AudioClock(int sample_rate)
+    : sample_rate_(sample_rate), last_endpoint_timestamp_(kNoTimestamp()) {

[DaleCurtis, 2014/04/30 20:36:33]

Instead of storing sample_rate_ you could store a double of
base::Time::kMicrosecondsPerSecond / sample_rate_ as microseconds_per_frame_ for
the calculations below.

[scherkus (not reviewing), 2014/05/02 19:26:05]

opted to combine the expression in CurrentMediaTimestamp() instead

+}
+
+AudioClock::~AudioClock() {
+}
+
+void AudioClock::WroteAudio(int frames,
+                            int delay_frames,
+                            float playback_rate,
+                            base::TimeDelta timestamp) {
+  CHECK_GT(playback_rate, 0);

[DaleCurtis, 2014/04/30 20:36:33]

DCHECK_GE(frames, 0) ? Add to WroteSilence() as well? Otherwise you could
consider using size_t.

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+  CHECK(timestamp != kNoTimestamp());
+
+  if (last_endpoint_timestamp_ == kNoTimestamp())
+    PushBufferedAudio(delay_frames, 0, kNoTimestamp());
+
+  TrimBufferedAudioToMatchDelay(delay_frames);
+  PushBufferedAudio(frames, playback_rate, timestamp);
+
+  last_endpoint_timestamp_ = timestamp;
+}
+
+void AudioClock::WroteSilence(int frames, int delay_frames) {
+  if (last_endpoint_timestamp_ == kNoTimestamp())
+    PushBufferedAudio(delay_frames, 0, kNoTimestamp());
+
+  TrimBufferedAudioToMatchDelay(delay_frames);
+  PushBufferedAudio(frames, 0, kNoTimestamp());
+}
+
+base::TimeDelta AudioClock::CurrentMediaTimestamp() const {
+  base::TimeDelta silence;
+  for (size_t i = 0; i < buffered_audio_.size(); ++i) {
+    // Account for silence ahead of the buffer closest to being played.
+    if (buffered_audio_[i].playback_rate == 0) {
+      silence += base::TimeDelta::FromMicroseconds(

[DaleCurtis, 2014/04/30 20:36:33]

Instead of repeatedly converting this, it'd be better to accumulate frames and
then perform a single conversion later when subtracting the value off
current_media_time. You avoid error due to truncation this way as well as save a
few cycles

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+          base::Time::kMicrosecondsPerSecond * buffered_audio_[i].frames /
+          sample_rate_);
+      continue;
+    }
+
+    // Multiply by playback rate as frames represent time-scaled audio.
+    base::TimeDelta current_media_time = buffered_audio_[i].endpoint_timestamp;

[DaleCurtis, 2014/04/30 20:36:33]

You could write this as a single return statement. That's more legible to me,
but up to you.

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+    current_media_time -= base::TimeDelta::FromMicroseconds(
+        base::Time::kMicrosecondsPerSecond * buffered_audio_[i].frames *
+        buffered_audio_[i].playback_rate / sample_rate_);
+    current_media_time -= silence;
+    return current_media_time;
+  }
+
+  // Either:
+  //   1) AudioClock is uninitialziated and we'll return kNoTimestamp()
+  //   2) All previously buffered audio has been replaced by silence,
+  //      meaning media time is now at the last endpoint
+  return last_endpoint_timestamp_;
+}
+
+base::TimeDelta AudioClock::LastEndpointTimestamp() const {

[DaleCurtis, 2014/04/30 20:36:33]

hacker_style() if you want.

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+  return last_endpoint_timestamp_;
+}
+
+void AudioClock::TrimBufferedAudioToMatchDelay(int delay_frames) {
+  if (buffered_audio_.empty())
+    return;
+
+  size_t i = buffered_audio_.size() - 1;
+  while (true) {
+    if (buffered_audio_[i].frames <= delay_frames) {
+      // Reached the end before accounting for all of |delay_frames|. This

[DaleCurtis, 2014/04/30 20:36:33]

Reflow comment block. Line breaks are off.

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+      // means
+      // we haven't written enough audio data yet to account for hardware
+      // delay.
+      // In this case, do nothing.
+      if (i == 0)
+        return;
+
+      // Keep accounting for |delay_frames|.
+      delay_frames -= buffered_audio_[i].frames;
+      --i;
+      continue;
+    }
+
+    // All of |delay_frames| has been accounted for: adjust amount of frames
+    // left in current buffer. All preceeding elements with index < |i| should
+    // be considered played out and hence discared.

[DaleCurtis, 2014/04/30 20:36:33]

discarded

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+    buffered_audio_[i].frames = delay_frames;

[DaleCurtis, 2014/04/30 20:36:33]

Is this right? delay_frames may be 0 now.  Shouldn't this be -= ?

[scherkus (not reviewing), 2014/05/02 19:26:05]

Say we have: [20, 20, 20] with a delay of 45, we get here with:
i = 0
delay_ms = 5


... which means we end up with [5, 20, 20] to match the delay_ms of 45

If we had [20, 20, 20, 20, 20] with a delay of 60, we get here with:
i = 1
delay_ms = 0

... which means we would end up with [20, 0, 20, 20, 20].  The check below then
increments i by one so that we erase the first two elements to arrive at [20,
20, 20]

[DaleCurtis, 2014/05/02 19:37:06]

Oh I see, I had this turned around and was considering buffers < i to be the
ones that hadn't played out.  I see what you're saying now.  Thanks for the
explanation.

+    break;
+  }
+
+  // At this point |i| points at what will be the new head of

[DaleCurtis, 2014/04/30 20:36:33]

Reflow comment block.

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+  // |buffered_audio_|
+  // however if it contains no audio it should be removed as well.
+  if (buffered_audio_[i].frames == 0)
+    ++i;
+
+  buffered_audio_.erase(buffered_audio_.begin(), buffered_audio_.begin() + i);
+}
+
+void AudioClock::PushBufferedAudio(int frames,
+                                   float playback_rate,
+                                   base::TimeDelta endpoint_timestamp) {
+  DCHECK_EQ(playback_rate == 0, endpoint_timestamp == kNoTimestamp());

[DaleCurtis, 2014/04/30 20:36:33]

These DCHECKS are kind of a pain when they fire. I find it easier to have an if
(xxx) DCHECK()... which should get optimized out on release.

[scherkus (not reviewing), 2014/05/02 19:26:05]

Done.

+
+  if (frames == 0)
+    return;
+
+  // Avoid creating extra elements where possible.
+  if (!buffered_audio_.empty() &&
+      buffered_audio_.back().playback_rate == playback_rate) {
+    buffered_audio_.back().frames += frames;
+    buffered_audio_.back().endpoint_timestamp = endpoint_timestamp;
+    return;
+  }
+
+  buffered_audio_.push_back(
+      BufferedAudio(frames, playback_rate, endpoint_timestamp));
+}
+
+AudioClock::BufferedAudio::BufferedAudio(int frames,
+                                         float playback_rate,
+                                         base::TimeDelta endpoint_timestamp)
+    : frames(frames),
+      playback_rate(playback_rate),
+      endpoint_timestamp(endpoint_timestamp) {
+}
+
+}  // namespace media