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()) {
+}
+
+AudioClock::~AudioClock() {
+}
+
+void AudioClock::WroteAudio(int frames,
+                            int delay_frames,
+                            float playback_rate,
+                            base::TimeDelta timestamp) {
+  CHECK_GT(playback_rate, 0);
+  CHECK(timestamp != kNoTimestamp());
+  DCHECK_GE(frames, 0);
+  DCHECK_GE(delay_frames, 0);
+
+  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) {
+  DCHECK_GE(frames, 0);
+  DCHECK_GE(delay_frames, 0);
+
+  if (last_endpoint_timestamp_ == kNoTimestamp())
+    PushBufferedAudio(delay_frames, 0, kNoTimestamp());
+
+  TrimBufferedAudioToMatchDelay(delay_frames);
+  PushBufferedAudio(frames, 0, kNoTimestamp());
+}
+
+base::TimeDelta AudioClock::CurrentMediaTimestamp() const {
+  int silence_frames = 0;
+  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_frames += buffered_audio_[i].frames;
+      continue;
+    }
+
+    // Multiply by playback rate as frames represent time-scaled audio.
+    return buffered_audio_[i].endpoint_timestamp -
+           base::TimeDelta::FromMicroseconds(
+               ((buffered_audio_[i].frames * buffered_audio_[i].playback_rate) +
+                silence_frames) /
+               sample_rate_ * base::Time::kMicrosecondsPerSecond);
+  }
+
+  // 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+  //   2) All previously buffered audio has been replaced by silence,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_;
+}
+
+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
+      // 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 discarded.
+    buffered_audio_[i].frames = delay_frames;
+    break;
+  }
+
+  // At this point |i| points at what will be the new head of |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) {
+  if (playback_rate == 0)
+    DCHECK(endpoint_timestamp == kNoTimestamp());
+
+  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