Parsing of encoded duration for unencrypted opus streams.

media/formats/webm/opus_packet_builder.cc

Index: media/formats/webm/opus_packet_builder.cc
diff --git a/media/formats/webm/opus_packet_builder.cc b/media/formats/webm/opus_packet_builder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..76cf22128f2d0cc06c7a8af16be147ec99e3de00
--- /dev/null
+++ b/media/formats/webm/opus_packet_builder.cc
@@ -0,0 +1,83 @@
+#include "media/formats/webm/opus_packet_builder.h"
+#include "media/formats/webm/webm_cluster_parser.h"
+
+namespace media {
+
+// From Opus RFC. See https://tools.ietf.org/html/rfc6716#page-14
+enum OpusConstants {
+  kNumPossibleOpusConfigs = 32,
+  kMinOpusPacketFrameCount = 1,
+  kMaxOpusPacketFrameCount = 48
+};
+
+OpusPacket::OpusPacket(int config, int frame_count, bool is_VBR) {

[wolenetz, 2015/02/05 23:05:00]

narrow param types (uint8_t config and frame_count?)

likewise, narrow param types of local (frame_count_code)

[chcunningham, 2015/02/06 03:20:09]

Done.

+  duration_ms_ = frame_count *

[wolenetz, 2015/02/05 23:04:59]

nit: protect borders (DCHECK fail for invalid config, frame_count, is_VBR). Or
are invalid OpusPacket creations allowed?

[chcunningham, 2015/02/06 03:20:09]

Done. Decided to not allow invalid packets at this time. Added DCHECKS

+                 WebMClusterParser::kOpusFrameDurationsMu[config] /
+                 static_cast<float>(1000);
+
+  int frame_count_code;
+  uint8_t frame_count_byte;
+
+  if (frame_count == 1) {
+    frame_count_code = 0;
+  } else if (frame_count == 2) {
+    frame_count_code = is_VBR ? 2 : 1;
+  } else {
+    frame_count_code = 3;
+    frame_count_byte = (is_VBR ? 1 << 7 : 0) | frame_count;
+  }
+
+  // All opus packets must have TOC byte.
+  uint8_t opus_toc_byte = (config << 3) | frame_count_code;
+  data_.push_back(opus_toc_byte);
+
+  // For code 3 packets, the number of frames is signaled in the "frame

[wolenetz, 2015/02/05 23:04:59]

For code 2 packets, the compressed length.. is also signaled in the frame count
byte(or 2 bytes). It looks like we're assuming the special "No frame"/DTX/Lost
packet condition for code 2, and somehow subdividing the one byte of frame data
(0x0) in code 1 and 3 into multiple frames. Document these assumptions please.

[chcunningham, 2015/02/06 03:20:09]

Clarified the comment that we're not really attempting to conform to the bit
layout beyond the toc and framecount bytes.

+  // count byte".
+  if (frame_count_code == 3) {
+    data_.push_back(frame_count_byte);
+  }
+
+  // Simulating encoded frame data.
+  data_.push_back(static_cast<uint8_t>(0));
+}
+
+OpusPacket::~OpusPacket() {
+}
+
+const uint8_t* OpusPacket::data() const {
+  return &(data_[0]);
+}
+
+int OpusPacket::size() const {
+  return data_.size();
+}
+
+double OpusPacket::duration_ms() const {
+  return duration_ms_;
+}
+
+ScopedVector<OpusPacket> BuildAllOpusPackets() {

[wolenetz, 2015/02/05 23:04:59]

Are there illegal Opus Packet config + frame_count combos? (such as 48 frames,
each with a config indicating a duration that in aggregate exceeds 120ms) If so,
comment that we create these, and the parser is currently expected to allow them
(with a warning).

[chcunningham, 2015/02/06 03:20:09]

Done.

+  ScopedVector<OpusPacket> opus_packets;
+
+  for (int frame_count = kMinOpusPacketFrameCount;
+       frame_count <= kMaxOpusPacketFrameCount; frame_count++) {
+    for (int opus_config_num = 0; opus_config_num < kNumPossibleOpusConfigs;
+         opus_config_num++) {
+      bool is_VBR = false;

[wolenetz, 2015/02/05 23:04:59]

nit: is_VBR can be inlined in each of the two calls explicitly.

[chcunningham, 2015/02/06 03:20:09]

I like this for self documenting the meaning of the parameter.

[wolenetz, 2015/02/06 19:48:01]

Acknowledged.

+      opus_packets.push_back(
+          new OpusPacket(opus_config_num, frame_count, is_VBR));
+
+      if (frame_count >= 2) {
+        // Add another packet with VBR flag toggled. For frame counts >= 2,
+        // VBR triggers changes to packet framing.
+        is_VBR = true;
+        opus_packets.push_back(
+            new OpusPacket(opus_config_num, frame_count, is_VBR));
+      }
+    }
+  }
+
+  return opus_packets.Pass();
+}
+
+}  // namespace media