change rtptime.Encoder signature and initialize time to a random value (#525)

examples/client-record-format-mjpeg-from-image/main.go

@@ -45,7 +45,11 @@ func main() {
 	}
 
 	// setup timestamp generator
-	rtpTime := rtptime.NewEncoder(forma.ClockRate(), 0)
+	rtpTime := &rtptime.Encoder{ClockRate: forma.ClockRate()}
+	err = rtpTime.Initialize()
+	if err != nil {
+		panic(err)
+	}
 	start := time.Now()
 
 	// setup a ticker to sleep between frames
@@ -89,7 +93,7 @@ func main() {
 		}
 
 		// get current timestamp
-		ts := rtpTime.Encode(time.Now().Sub(start))
+		ts := rtpTime.Encode(time.Since(start))
 
 		// write packets to the server
 		for _, pkt := range pkts {

pkg/rtptime/encoder.go

@@ -1,6 +1,7 @@
 package rtptime
 
 import (
+	"crypto/rand"
 	"time"
 )
 
@@ -12,23 +13,47 @@ func divCeil(n, d uint64) uint64 {
 	return v
 }
 
-// Encoder is a RTP timestamp encoder.
-type Encoder struct {
-	clockRate        time.Duration
-	initialTimestamp time.Duration
+func randUint32() (uint32, error) {
+	var b [4]byte
+	_, err := rand.Read(b[:])
+	if err != nil {
+		return 0, err
+	}
+	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3]), nil
 }
 
-// NewEncoder allocates an Encoder.
-func NewEncoder(clockRate int, initialTimestamp uint32) *Encoder {
-	return &Encoder{
-		clockRate: time.Duration(clockRate),
-		// ((2^32) * 1000000000) is less than 2^63
-		initialTimestamp: time.Duration(divCeil(uint64(initialTimestamp)*uint64(time.Second), uint64(clockRate))),
+// Encoder is a RTP timestamp encoder.
+type Encoder struct {
+	// Clock rate.
+	ClockRate int
+
+	// (optional) initial timestamp.
+	// It defaults to a random value.
+	InitialTimestamp *uint32
+
+	clockRateTD        time.Duration
+	initialTimestampTD time.Duration
 }
+
+// Initialize initializes an Encoder.
+func (e *Encoder) Initialize() error {
+	e.clockRateTD = time.Duration(e.ClockRate)
+
+	if e.InitialTimestamp == nil {
+		v, err := randUint32()
+		if err != nil {
+			return err
+		}
+		e.InitialTimestamp = &v
+	}
+
+	e.initialTimestampTD = time.Duration(divCeil(uint64(*e.InitialTimestamp)*uint64(time.Second), uint64(e.ClockRate)))
+
+	return nil
 }
 
 // Encode encodes a timestamp.
 func (e *Encoder) Encode(ts time.Duration) uint32 {
-	ts = e.initialTimestamp + ts
-	return uint32(multiplyAndDivide(ts, e.clockRate, time.Second))
+	ts += e.initialTimestampTD
+	return uint32(multiplyAndDivide(ts, e.clockRateTD, time.Second))
 }

pkg/rtptime/encoder_test.go

@@ -7,8 +7,17 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func uint32Ptr(v uint32) *uint32 {
+	return &v
+}
+
 func TestEncoder(t *testing.T) {
-	e := NewEncoder(90000, 12345)
+	e := &Encoder{
+		ClockRate:        90000,
+		InitialTimestamp: uint32Ptr(12345),
+	}
+	err := e.Initialize()
+	require.NoError(t, err)
 
 	ts := e.Encode(0)
 	require.Equal(t, uint32(12345), ts)
@@ -18,10 +27,13 @@ func TestEncoder(t *testing.T) {
 }
 
 func BenchmarkEncoder(b *testing.B) {
+	e := &Encoder{
+		ClockRate:        90000,
+		InitialTimestamp: uint32Ptr(12345),
+	}
+	e.Initialize() //nolint:errcheck
+
 	for i := 0; i < b.N; i++ {
-		func() {
-			d := NewEncoder(90000, 0)
-			d.Encode(200 * time.Second)
-		}()
+		e.Encode(200 * time.Second)
 	}
 }