fix wrong timestamp computation that happened at 28h28m

this was caused by a int64 overflow.

pkg/rtptimedec/decoder.go

@@ -8,9 +8,8 @@ import (
 // Decoder is a RTP timestamp decoder.
 type Decoder struct {
 	clockRate time.Duration
-	tsAdd     int64
-	tsInitial *int64
-	tsPrev    *int64
+	tsAdd     *int64
+	tsPrev    int64
 }
 
 // New allocates a Decoder.
@@ -24,22 +23,29 @@ func New(clockRate int) *Decoder {
 func (d *Decoder) Decode(ts uint32) time.Duration {
 	ts64 := int64(ts)
 
-	if d.tsPrev != nil {
-		diff := ts64 - *d.tsPrev
+	if d.tsAdd == nil {
+		d.tsPrev = ts64
+		ts64 = -ts64
+		d.tsAdd = &ts64
+		return 0
+	}
+
+	diff := ts64 - d.tsPrev
+	d.tsPrev = ts64
+
 	switch {
 	case diff < -0xFFFFFF: // overflow
-			d.tsAdd += 0x100000000
+		*d.tsAdd += 0x100000000
 
 	case diff > 0xFFFFFF: // timestamp overflowed then went back
-			d.tsAdd -= 0x100000000
-		}
+		*d.tsAdd -= 0x100000000
 	}
 
-	d.tsPrev = &ts64
+	tot := time.Duration(ts64 + *d.tsAdd)
 
-	if d.tsInitial == nil {
-		d.tsInitial = &ts64
-	}
-
-	return time.Duration(ts64+d.tsAdd-*d.tsInitial) * time.Second / d.clockRate
+	// avoid an int64 overflow and preserve resolution by splitting division into two parts:
+	// first add seconds, then the decimal part.
+	secs := tot / d.clockRate
+	dec := tot % d.clockRate
+	return secs*time.Second + dec*time.Second/d.clockRate
 }

pkg/rtptimedec/decoder_test.go

@@ -11,24 +11,29 @@ func TestOverflow(t *testing.T) {
 	d := New(90000)
 
 	i := uint32(4294877296)
+	secs := time.Duration(0)
 	pts := d.Decode(i)
 	require.Equal(t, time.Duration(0), pts)
 
-	// 1st overflow
-	i += 90000 * 2
-	pts = d.Decode(i)
-	require.Equal(t, 2*time.Second, pts)
+	const stride = 150
+	lim := uint32(uint64(4294967296 - (stride * 90000)))
 
-	// reach 4294890000 slowly
-	for ; i < 4294890000; i += 90000 * 10 {
+	for n := 0; n < 100; n++ {
+		// overflow
+		i += 90000 * stride
+		secs += stride
 		pts = d.Decode(i)
-		require.Equal(t, 2*time.Second+time.Second*time.Duration(i-90000)/90000, pts)
+		require.Equal(t, secs*time.Second, pts)
+
+		// reach 2^32 slowly
+		secs += stride
+		i += 90000 * stride
+		for ; i < lim; i += 90000 * stride {
+			pts = d.Decode(i)
+			require.Equal(t, secs*time.Second, pts)
+			secs += stride
+		}
 	}
-
-	// 2nd overflow
-	i += 90000 * 10
-	pts = d.Decode(i)
-	require.Equal(t, 47732*time.Second, pts)
 }
 
 func TestOverflowAndBack(t *testing.T) {