hls client: skip packets received before the 1st packet of the leading

track; make sure that the initial DTS is zero

internal/hls/client_processor_mpegts_track.go

@@ -54,5 +54,10 @@ func (t *clientProcessorMPEGTSTrack) processEntry(ctx context.Context, pes *asti
 		return err
 	}
 
+	// silently discard packets prior to the first packet of the leading track
+	if pts < 0 {
+		return nil
+	}
+
 	return t.onEntry(pts, pes.Data)
 }

internal/hls/client_timesync_mpegts.go

@@ -3,31 +3,30 @@ package hls
 import (
 	"context"
 	"fmt"
+	"sync"
 	"time"
 
-	"github.com/aler9/rtsp-simple-server/internal/hls/mpegts"
+	"github.com/aler9/rtsp-simple-server/internal/hls/mpegtstimedec"
 )
 
 type clientTimeSyncMPEGTS struct {
 	startRTC time.Time
-	startDTS int64
-	td       *mpegts.TimeDecoder
+	td       *mpegtstimedec.Decoder
+	mutex    sync.Mutex
 }
 
 func newClientTimeSyncMPEGTS(startDTS int64) *clientTimeSyncMPEGTS {
 	return &clientTimeSyncMPEGTS{
 		startRTC: time.Now(),
-		startDTS: startDTS,
-		td:       mpegts.NewTimeDecoder(),
+		td:       mpegtstimedec.New(startDTS),
 	}
 }
 
 func (ts *clientTimeSyncMPEGTS) convertAndSync(ctx context.Context, rawDTS int64, rawPTS int64) (time.Duration, error) {
-	rawDTS = (rawDTS - ts.startDTS) & 0x1FFFFFFFF
-	rawPTS = (rawPTS - ts.startDTS) & 0x1FFFFFFFF
-
+	ts.mutex.Lock()
 	dts := ts.td.Decode(rawDTS)
 	pts := ts.td.Decode(rawPTS)
+	ts.mutex.Unlock()
 
 	elapsed := time.Since(ts.startRTC)
 	if dts > elapsed {

internal/hls/mpegtstimedec/decoder.go

@@ -0,0 +1,46 @@
+// Package mpegtstimedec contains a MPEG-TS timestamp decoder.
+package mpegtstimedec
+
+import (
+	"time"
+)
+
+const (
+	maximum           = 0x1FFFFFFFF // 33 bits
+	negativeThreshold = 0x1FFFFFFFF / 2
+	clockRate         = 90000
+)
+
+// Decoder is a MPEG-TS timestamp decoder.
+type Decoder struct {
+	overall time.Duration
+	prev    int64
+}
+
+// New allocates a Decoder.
+func New(start int64) *Decoder {
+	return &Decoder{
+		prev: start,
+	}
+}
+
+// Decode decodes a MPEG-TS timestamp.
+func (d *Decoder) Decode(ts int64) time.Duration {
+	diff := (ts - d.prev) & maximum
+
+	// negative difference
+	if diff > negativeThreshold {
+		diff = (d.prev - ts) & maximum
+		d.prev = ts
+		d.overall -= time.Duration(diff)
+	} else {
+		d.prev = ts
+		d.overall += time.Duration(diff)
+	}
+
+	// avoid an int64 overflow and preserve resolution by splitting division into two parts:
+	// first add the integer part, then the decimal part.
+	secs := d.overall / clockRate
+	dec := d.overall % clockRate
+	return secs*time.Second + dec*time.Second/clockRate
+}

internal/hls/mpegtstimedec/decoder_test.go

@@ -0,0 +1,72 @@
+package mpegtstimedec
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNegativeDiff(t *testing.T) {
+	d := New(64523434)
+
+	ts := d.Decode(64523434 - 90000)
+	require.Equal(t, -1*time.Second, ts)
+
+	ts = d.Decode(64523434)
+	require.Equal(t, time.Duration(0), ts)
+
+	ts = d.Decode(64523434 + 90000*2)
+	require.Equal(t, 2*time.Second, ts)
+
+	ts = d.Decode(64523434 + 90000)
+	require.Equal(t, 1*time.Second, ts)
+}
+
+func TestOverflow(t *testing.T) {
+	d := New(0x1FFFFFFFF - 20)
+
+	i := int64(0x1FFFFFFFF - 20)
+	secs := time.Duration(0)
+	const stride = 150
+	lim := int64(uint64(0x1FFFFFFFF - (stride * 90000)))
+
+	for n := 0; n < 100; n++ {
+		// overflow
+		i += 90000 * stride
+		secs += stride
+		ts := d.Decode(i)
+		require.Equal(t, secs*time.Second, ts)
+
+		// reach 2^32 slowly
+		secs += stride
+		i += 90000 * stride
+		for ; i < lim; i += 90000 * stride {
+			ts = d.Decode(i)
+			require.Equal(t, secs*time.Second, ts)
+			secs += stride
+		}
+	}
+}
+
+func TestOverflowAndBack(t *testing.T) {
+	d := New(0x1FFFFFFFF - 90000 + 1)
+
+	ts := d.Decode(0x1FFFFFFFF - 90000 + 1)
+	require.Equal(t, time.Duration(0), ts)
+
+	ts = d.Decode(90000)
+	require.Equal(t, 2*time.Second, ts)
+
+	ts = d.Decode(0x1FFFFFFFF - 90000 + 1)
+	require.Equal(t, time.Duration(0), ts)
+
+	ts = d.Decode(0x1FFFFFFFF - 90000*2 + 1)
+	require.Equal(t, -1*time.Second, ts)
+
+	ts = d.Decode(0x1FFFFFFFF - 90000 + 1)
+	require.Equal(t, time.Duration(0), ts)
+
+	ts = d.Decode(90000)
+	require.Equal(t, 2*time.Second, ts)
+}