Support start trimming post-decoding. Use it with FFmpegDemuxer.

media/base/audio_discard_helper.cc

Index: media/base/audio_discard_helper.cc
diff --git a/media/base/audio_discard_helper.cc b/media/base/audio_discard_helper.cc
index b907fe8e4858d3a4b92fcaa6de5a3eeb17fa65d7..435ce35ec793c3402e109ee7e6eff867d7b8cf98 100644
--- a/media/base/audio_discard_helper.cc
+++ b/media/base/audio_discard_helper.cc
@@ -24,8 +24,9 @@ static void WarnOnNonMonotonicTimestamps(base::TimeDelta last_timestamp,
                 << " diff " << diff.InMicroseconds() << " us";
 }
 
-AudioDiscardHelper::AudioDiscardHelper(int sample_rate)
+AudioDiscardHelper::AudioDiscardHelper(int sample_rate, size_t codec_delay)
     : sample_rate_(sample_rate),
+      codec_delay_(codec_delay),
       timestamp_helper_(sample_rate_),
       discard_frames_(0),
       last_input_timestamp_(kNoTimestamp()) {
@@ -43,6 +44,7 @@ void AudioDiscardHelper::Reset(size_t initial_discard) {
   discard_frames_ = initial_discard;
   last_input_timestamp_ = kNoTimestamp();
   timestamp_helper_.SetBaseTimestamp(kNoTimestamp());
+  delayed_discard_ = NULL;
 }
 
 bool AudioDiscardHelper::ProcessBuffers(
@@ -58,12 +60,26 @@ bool AudioDiscardHelper::ProcessBuffers(
   last_input_timestamp_ = encoded_buffer->timestamp();
 
   // If this is the first buffer seen, setup the timestamp helper.
-  if (!initialized())
+  const bool first_buffer = !initialized();
+  if (first_buffer)
     timestamp_helper_.SetBaseTimestamp(encoded_buffer->timestamp());
   DCHECK(initialized());
 
-  if (!decoded_buffer || !decoded_buffer->frame_count())
+  if (!decoded_buffer) {
+    // If there's a one buffer delay for decoding, we need to save it so it can
+    // be processed with the next decoder buffer.
+    if (first_buffer)
+      delayed_discard_ = encoded_buffer;
     return false;
+  }
+
+  const size_t original_frame_count = decoded_buffer->frame_count();
+
+  // If there's a one buffer delay for decoding, pick up the last encoded buffer
+  // for processing with the current decoded buffer.
+  scoped_refptr<DecoderBuffer> current_encoded_buffer = encoded_buffer;
+  if (delayed_discard_)
+    delayed_discard_.swap(current_encoded_buffer);
 
   if (discard_frames_ > 0) {
     const size_t decoded_frames = decoded_buffer->frame_count();
@@ -71,20 +87,74 @@ bool AudioDiscardHelper::ProcessBuffers(
     discard_frames_ -= frames_to_discard;
 
     // If everything would be discarded, indicate a new buffer is required.
-    if (frames_to_discard == decoded_frames)
+    if (frames_to_discard == decoded_frames) {
+      // For simplicity disallow cases where a buffer with discard padding is
+      // present.  Doing so allows us to avoid complexity around tracking
+      // discards across buffers.
+      DCHECK(current_encoded_buffer->discard_padding().first ==
+             base::TimeDelta());
+      DCHECK(current_encoded_buffer->discard_padding().second ==
+             base::TimeDelta());
       return false;
+    }
 
     decoded_buffer->TrimStart(frames_to_discard);
   }
 
-  // TODO(dalecurtis): Applying the current buffer's discard padding doesn't
-  // make sense in the Vorbis case because there is a delay of one buffer before
-  // decoded buffers are returned. Fix and add support for more than just end
-  // trimming.
-  if (encoded_buffer->discard_padding() > base::TimeDelta()) {
+  // Handle front discard padding.
+  if (current_encoded_buffer->discard_padding().first > base::TimeDelta()) {
+    const size_t decoded_frames = decoded_buffer->frame_count();
+    const size_t start_frames_to_discard =
+        TimeDeltaToFrames(current_encoded_buffer->discard_padding().first);
+
+    // Regardless of the timestamp on the encoded buffer, the corresponding
+    // decoded output will appear |codec_delay_| frames later.
+    size_t discard_start = codec_delay_;
+    if (codec_delay_ > 0) {
+      // If we have a |codec_delay_| and have already discarded frames from this
+      // buffer, the |discard_start| must be adjusted by the number of frames
+      // already discarded.
+      const size_t frames_discarded_so_far =
+          original_frame_count - decoded_buffer->frame_count();
+      CHECK_LE(frames_discarded_so_far, codec_delay_);
+      discard_start -= frames_discarded_so_far;
+    }
+
+    // For simplicity require the start of the discard to be within the current
+    // buffer.  Doing so allows us avoid complexity around tracking discards
+    // across buffers.
+    CHECK_LT(discard_start, decoded_frames);
+
+    const size_t frames_to_discard =
+        std::min(start_frames_to_discard, decoded_frames - discard_start);
+
+    // Carry over any frames which need to be discarded from the front of the
+    // next buffer.
+    DCHECK(!discard_frames_);
+    discard_frames_ = start_frames_to_discard - frames_to_discard;
+
+    // If everything would be discarded, indicate a new buffer is required.
+    if (frames_to_discard == decoded_frames) {
+      // The buffer should not have been marked with end discard if the front
+      // discard removes everything.
+      DCHECK(current_encoded_buffer->discard_padding().second ==
+             base::TimeDelta());
+      return false;
+    }
+
+    decoded_buffer->TrimRange(discard_start, discard_start + frames_to_discard);
+  }
+
+  // Handle end discard padding.
+  if (current_encoded_buffer->discard_padding().second > base::TimeDelta()) {
+    // Limit end discarding to when there is no |codec_delay_|, otherwise it's
+    // non-trivial determining where to start discarding end frames.
+    CHECK(!codec_delay_);
+
     const size_t decoded_frames = decoded_buffer->frame_count();
     const size_t end_frames_to_discard =
-        TimeDeltaToFrames(encoded_buffer->discard_padding());
+        TimeDeltaToFrames(current_encoded_buffer->discard_padding().second);
+
     if (end_frames_to_discard > decoded_frames) {
       DLOG(ERROR) << "Invalid file. Incorrect discard padding value.";
       return false;
@@ -96,7 +166,8 @@ bool AudioDiscardHelper::ProcessBuffers(
 
     decoded_buffer->TrimEnd(end_frames_to_discard);
   } else {
-    DCHECK(encoded_buffer->discard_padding() == base::TimeDelta());
+    DCHECK(current_encoded_buffer->discard_padding().second ==
+           base::TimeDelta());
   }
 
   // Assign timestamp and duration to the buffer.