Refactor WebRtcOpus_PacketHasFec.

WebRtcOpus_PacketHasFec was written long time ago. see http://webrtc-codereview.appspot.com/7539004.
When revisiting, I notice that adding more comments should help. Code style should be improved a bit too.

Bug: webrtc:10772
Change-Id: If4d60b210e6235b4f787608047e88efc949f6838
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144056
Commit-Queue: Minyue Li <[email protected]>
Reviewed-by: Henrik Lundin <[email protected]>
Reviewed-by: Karl Wiberg <[email protected]>
Cr-Commit-Position: refs/heads/master@{#28479}

modules/audio_coding/codecs/opus/opus_interface.c

@@ -653,59 +653,71 @@ int WebRtcOpus_FecDurationEst(const uint8_t* payload,
   return samples;
 }
 
+// This method is based on Definition of the Opus Audio Codec
+// (https://tools.ietf.org/html/rfc6716). Basically, this method is based on
+// parsing the LP layer of an Opus packet, particularly the LBRR flag.
 int WebRtcOpus_PacketHasFec(const uint8_t* payload,
                             size_t payload_length_bytes) {
-  int frames, channels, payload_length_ms;
-  int n;
+  if (payload == NULL || payload_length_bytes == 0)
+    return 0;
+
+  // In CELT_ONLY mode, packets should not have FEC.
+  if (payload[0] & 0x80)
+    return 0;
+
+  // Max number of frames in an Opus packet is 48.
   opus_int16 frame_sizes[48];
   const unsigned char *frame_data[48];
 
-  if (payload == NULL || payload_length_bytes == 0)
+  // Parse packet to get the frames. But we only care about the first frame,
+  // since we can only decode the FEC from the first one.
+  if (opus_packet_parse(payload, (opus_int32)payload_length_bytes, NULL,
+                        frame_data, frame_sizes, NULL) < 0) {
     return 0;
+  }
 
-  /* In CELT_ONLY mode, packets should not have FEC. */
-  if (payload[0] & 0x80)
+  if (frame_sizes[0] <= 1) {
     return 0;
+  }
 
   // For computing the payload length in ms, the sample rate is not important
   // since it cancels out. We use 48 kHz, but any valid sample rate would work.
-  payload_length_ms = opus_packet_get_samples_per_frame(payload, 48000) / 48;
-  if (10 > payload_length_ms)
+  int payload_length_ms =
+      opus_packet_get_samples_per_frame(payload, 48000) / 48;
+  if (payload_length_ms < 10)
     payload_length_ms = 10;
 
-  channels = opus_packet_get_nb_channels(payload);
-
+  int silk_frames;
   switch (payload_length_ms) {
     case 10:
-    case 20: {
-      frames = 1;
+    case 20:
+      silk_frames = 1;
       break;
-    }
-    case 40: {
-      frames = 2;
+    case 40:
+      silk_frames = 2;
       break;
-    }
-    case 60: {
-      frames = 3;
+    case 60:
+      silk_frames = 3;
       break;
-    }
-    default: {
+    default:
       return 0; // It is actually even an invalid packet.
-    }
-  }
-
-  /* The following is to parse the LBRR flags. */
-  if (opus_packet_parse(payload, (opus_int32)payload_length_bytes, NULL,
-                        frame_data, frame_sizes, NULL) < 0) {
-    return 0;
-  }
-
-  if (frame_sizes[0] <= 1) {
-    return 0;
   }
 
-  for (n = 0; n < channels; n++) {
-    if (frame_data[0][0] & (0x80 >> ((n + 1) * (frames + 1) - 1)))
+  const int channels = opus_packet_get_nb_channels(payload);
+  RTC_DCHECK(channels == 1 || channels == 2);
+
+  // A frame starts with the LP layer. The LP layer begins with two to eight
+  // header bits.These consist of one VAD bit per SILK frame (up to 3),
+  // followed by a single flag indicating the presence of LBRR frames.
+  // For a stereo packet, these first flags correspond to the mid channel, and
+  // a second set of flags is included for the side channel. Because these are
+  // the first symbols decoded by the range coder and because they are coded
+  // as binary values with uniform probability, they can be extracted directly
+  // from the most significant bits of the first byte of compressed data.
+  for (int n = 0; n < channels; n++) {
+    // The LBRR bit for channel 1 is on the (|silk_frames| + 1)-th bit, and
+    // that of channel 2 is on the |(|silk_frames| + 1) * 2 + 1|-th bit.
+    if (frame_data[0][0] & (0x80 >> ((n + 1) * (silk_frames + 1) - 1)))
       return 1;
   }