ogg: Decode Opus packet durations (#231)

Decode the packet duration for Opus streams in OGG.

Co-authored-by: Karl Rikte <karl.rikte@axis.com>

symphonia-format-ogg/src/mappings/opus.rs

@@ -150,9 +150,63 @@ pub fn detect(buf: &[u8]) -> Result<Option<Box<dyn Mapper>>> {
 pub struct OpusPacketParser {}
 
 impl PacketParser for OpusPacketParser {
-    fn parse_next_packet_dur(&mut self, _packet: &[u8]) -> u64 {
-        // TODO: Implement.
-        0
+    fn parse_next_packet_dur(&mut self, packet: &[u8]) -> u64 {
+        // See https://www.rfc-editor.org/rfc/rfc6716
+        // Read TOC (Table Of Contents) byte which is the first byte in the opus data.
+        let toc_byte = match packet.get(0) {
+            Some(b) => b,
+            None => {
+                warn!("opus packet empty");
+                return 0;
+            }
+        };
+        // The configuration number is the 5 most significant bits. Shift out 3 least significant
+        // bits.
+        let configuration_number = toc_byte >> 3; // max 2^5-1 = 31
+
+        // The configuration number maps to packet length according to this lookup table.
+        // See https://www.rfc-editor.org/rfc/rfc6716 top half of page 14.
+        // Numbers are in milliseconds in the rfc. Down below they are in TimeBase units, so
+        // 10ms = 10*48.
+        #[rustfmt::skip]
+        const CONFIGURATION_NUMBER_TO_FRAME_DURATION: [u32; 32] = [
+            10*48, 20*48, 40*48, 60*48,
+            10*48, 20*48, 40*48, 60*48,
+            10*48, 20*48, 40*48, 60*48,
+            10*48, 20*48,
+            10*48, 20*48,
+            (2.5*48.0) as u32, 5*48, 10*48, 20*48,
+            (2.5*48.0) as u32, 5*48, 10*48, 20*48,
+            (2.5*48.0) as u32, 5*48, 10*48, 20*48,
+            (2.5*48.0) as u32, 5*48, 10*48, 20*48,
+        ];
+        // Look up the frame length.
+        let frame_duration =
+            CONFIGURATION_NUMBER_TO_FRAME_DURATION[configuration_number as usize] as u64;
+
+        // Look up the number of frames in the packet.
+        // See https://www.rfc-editor.org/rfc/rfc6716 bottom half of page 14.
+        let c = toc_byte & 0b11; // Note: it's actually called "c" in the rfc.
+        let num_frames = match c {
+            0 => 1,
+            1 | 2 => 2,
+            3 => match packet.get(1) {
+                Some(byte) => {
+                    // TOC byte is followed by number of frames. See page 18 section 3.2.5 code 3
+                    let m = byte & 0b11111; // Note: it's actually called "M" in the rfc.
+                    m as u64
+                }
+                None => {
+                    // What to do here? I'd like to return an error but this is an infalliable
+                    // trait.
+                    warn!("opus code 3 packet with no following byte containing number of frames");
+                    return 0;
+                }
+            },
+            _ => unreachable!("masked 2 bits"),
+        };
+        // Look up the packet length and return it.
+        frame_duration * num_frames
     }
 }
 
@@ -184,7 +238,7 @@ impl Mapper for OpusMapper {
 
     fn map_packet(&mut self, packet: &[u8]) -> Result<MapResult> {
         if !self.need_comment {
-            Ok(MapResult::StreamData { dur: 0 })
+            Ok(MapResult::StreamData { dur: OpusPacketParser {}.parse_next_packet_dur(packet) })
         }
         else {
             let mut reader = BufReader::new(packet);