generator: refactor main processing loop and improve commentary

generator/mpegts_generator.go

@@ -53,14 +53,21 @@ func init() {
 }
 
 const (
-	sdtPid    = 17
-	patPid    = 0
-	pmtPid    = 4096
-	videoPid  = 256
-	streamID  = 0xe0
+	sdtPid   = 17
+	patPid   = 0
+	pmtPid   = 4096
+	videoPid = 256
+	streamID = 0xe0 // First video stream ID.
+)
+
+// Time related constants.
+const (
+	// ptsOffset is the offset added to the clock to determine
+	// the current presentation timestamp,
 	ptsOffset = 700 * time.Millisecond
-	ptsFreq   = 90000 // Hz
-	maxCC     = 0xf
+
+	// pcrFreq is the base Program Clock Reference frequency.
+	pcrFreq = 90000 // Hz
 )
 
 // tsGenerator encapsulates properties of an mpegts generator.
@@ -112,14 +119,46 @@ func (g *tsGenerator) OutputChan() <-chan []byte {
 	return g.outputChan
 }
 
+const (
+	hasPayload         = 0x1
+	hasAdaptationField = 0x2
+)
+
+const (
+	hasDTS = 0x1
+	hasPTS = 0x2
+)
+
 // generate handles the incoming data and generates equivalent mpegts packets -
 // sending them to the output channel
 func (g *tsGenerator) generate() {
 	for {
 		nalu := <-g.nalInputChan
+
+		// Write PAT
+		patPkt := mpegts.Packet{
+			PUSI:    true,
+			PID:     patPid,
+			CC:      g.ccFor(patPid),
+			AFC:     hasPayload,
+			Payload: patTable,
+		}
+		g.outputChan <- patPkt.Bytes()
+
+		// Write PMT.
+		pmtPkt := mpegts.Packet{
+			PUSI:    true,
+			PID:     pmtPid,
+			CC:      g.ccFor(pmtPid),
+			AFC:     hasPayload,
+			Payload: pmtTable,
+		}
+		g.outputChan <- pmtPkt.Bytes()
+
+		// Prepare PES data.
 		pesPkt := pes.Packet{
 			StreamID:     streamID,
-			PDI:          2,
+			PDI:          hasPTS,
 			PTS:          g.pts(),
 			Data:         nalu,
 			HeaderLength: 5,
@@ -133,44 +172,23 @@ func (g *tsGenerator) generate() {
 				PID:  videoPid,
 				RAI:  pusi,
 				CC:   g.ccFor(videoPid),
-				AFC:  3,
+				AFC:  hasAdaptationField | hasPayload,
 				PCRF: pusi,
 			}
 			n := pkt.FillPayload(buf)
 			buf = buf[n:]
 
-			// TODO: create consts for AFC parameters
 			if pusi {
-				// Create pat table
-				patPkt := mpegts.Packet{
-					PUSI:    pusi,
-					PID:     patPid,
-					CC:      g.ccFor(patPid),
-					AFC:     1,
-					Payload: patTable,
-				}
-				g.outputChan <- patPkt.Bytes()
-
-				// Create pmt table
-				pmtPkt := mpegts.Packet{
-					PUSI:    pusi,
-					PID:     pmtPid,
-					CC:      g.ccFor(pmtPid),
-					AFC:     1,
-					Payload: pmtTable,
-				}
-				g.outputChan <- pmtPkt.Bytes()
-
-				// If pusi then we need to gen a pcr
+				// If the packet has a Payload Unit Start Indicator
+				// flag set then we need to write a PCR.
 				pkt.PCR = g.pcr()
-
-				g.tick()
-
 				pusi = false
 			}
 
 			g.outputChan <- pkt.Bytes()
 		}
+
+		g.tick()
 	}
 }
 
@@ -181,17 +199,18 @@ func (g *tsGenerator) tick() {
 
 // pts retuns the current presentation timestamp.
 func (g *tsGenerator) pts() uint64 {
-	return uint64((g.clock + g.ptsOffset).Seconds() * ptsFreq)
+	return uint64((g.clock + g.ptsOffset).Seconds() * pcrFreq)
 }
 
 // pcr returns the current program clock reference.
 func (g *tsGenerator) pcr() uint64 {
-	return uint64(g.clock.Seconds() * ptsFreq)
+	return uint64(g.clock.Seconds() * pcrFreq)
 }
 
 // ccFor returns the next continuity counter for pid.
 func (g *tsGenerator) ccFor(pid int) byte {
 	cc := g.continuity[pid]
-	g.continuity[pid] = (cc + 1) & maxCC
+	const continuityCounterMask = 0xf
+	g.continuity[pid] = (cc + 1) & continuityCounterMask
 	return cc
 }