Adding smarter logic for detecting start of MPEG packet data

Adding test cases to verify the MPEG audio/video header has been properly decoded

Author: benrr101

src/TaglibSharp.Tests/FileFormats/MpgFormatTest.cs

@@ -24,9 +24,19 @@ public void Init ()
 		[Test]
 		public void ReadAudioProperties ()
 		{
+			Assert.AreEqual (128, file.Properties.AudioBitrate);
+			Assert.AreEqual (2, file.Properties.AudioChannels);
 			Assert.AreEqual (44100, file.Properties.AudioSampleRate);
 			// NOTE, with .net core it keeps the decimal places. So, for now, we round to match .net behavior
 			Assert.AreEqual (1391, Math.Round (file.Properties.Duration.TotalMilliseconds));
+
+		}
+
+		[Test]
+		public void ReadVideoProperties ()
+		{
+			Assert.AreEqual (480, file.Properties.VideoHeight);
+			Assert.AreEqual (640, file.Properties.VideoWidth);
 		}
 
 
@@ -68,7 +78,7 @@ public void WritePictures ()
 
 			file.Save ();
 
-			// Read back the Matroska-specific tags 
+			// Read back the Matroska-specific tags
 			file = File.Create (tmp_file);
 			Assert.NotNull (file);
 			pics = file.Tag.Pictures;

src/TaglibSharp/Mpeg/File.cs

@@ -547,8 +547,29 @@ void ReadAudioPacket (ref long position)
 			Seek (position + 4);
 			int length = ReadBlock (2).ToUShort ();
 
-			if (!audio_found)
-				audio_found = AudioHeader.Find (out audio_header, this, position + 15, length - 9);
+			if (!audio_found) {
+				// There is a maximum of 16 stuffing bytes, read up to the PTS/DTS flags
+				ByteVector packetHeaderBytes = this.ReadBlock (19);
+				int i = 0;
+				while (i < packetHeaderBytes.Count && packetHeaderBytes[i] == 0xFF) {
+					// Byte is a stuffing byte
+					i++;
+				}
+
+				if ((packetHeaderBytes[i] & 0x40) != 0) {
+					// STD buffer size is unexpected for audio packets, but whatever
+					i++;
+				}
+
+				// Decode the PTS/DTS flags
+				byte timestampFlags = packetHeaderBytes[i];
+				long dataOffset = 4 + 2 + i +                             // Packet marker + packet length + stuffing bytes/STD buffer size
+				                  ((timestampFlags & 0x20) > 0 ? 4 : 0) + // Presentation timestamp
+				                  ((timestampFlags & 0x10) > 0 ? 4 : 0);  // Decode timestamp
+
+				audio_found = AudioHeader.Find (out audio_header, this, position + dataOffset, length - 9);
+			}
+
 			position += length;
 		}