fix: handle stts case with single trailing zero duration

mp4/stts.go

@@ -166,12 +166,15 @@ func (b *SttsBox) Info(w io.Writer, specificBoxLevels, indent, indentStep string
 	return bd.err
 }
 
-// GetSampleNrAtTime - get sample number at or as soon as possible after time
+// GetSampleNrAtTime returns the 1-based sample number at or as soon as possible after time.
+// Match a final single zero duration if present.
+// If time is too big to reach, an error is returned.
 // Time is calculated by summing up durations of previous samples
 func (b *SttsBox) GetSampleNrAtTime(sampleStartTime uint64) (sampleNr uint32, err error) {
 	accTime := uint64(0)
 	accNr := uint32(0)
-	for i := 0; i < len(b.SampleCount); i++ {
+	nrEntries := len(b.SampleCount)
+	for i := 0; i < nrEntries; i++ {
 		timeDelta := uint64(b.SampleTimeDelta[i])
 		if sampleStartTime < accTime+uint64(b.SampleCount[i])*timeDelta {
 			relTime := (sampleStartTime - accTime)
@@ -184,5 +187,10 @@ func (b *SttsBox) GetSampleNrAtTime(sampleStartTime uint64) (sampleNr uint32, er
 		accNr += b.SampleCount[i]
 		accTime += timeDelta * uint64(b.SampleCount[i])
 	}
+	// Check if there is a final single zero duration and time matches.
+	if b.SampleTimeDelta[nrEntries-1] == 0 && b.SampleCount[nrEntries-1] == 1 &&
+		sampleStartTime == accTime {
+		return accNr, nil
+	}
 	return 0, fmt.Errorf("no matching sample found for time=%d", sampleStartTime)
 }

mp4/stts_test.go

@@ -17,26 +17,37 @@ func TestGetSampleNrAtTime(t *testing.T) {
 		SampleTimeDelta: []uint32{10, 14},
 	}
 
+	sttsZero := SttsBox{
+		SampleCount:     []uint32{2, 1},
+		SampleTimeDelta: []uint32{10, 0}, // Single zero duration at end
+	}
+
 	testCases := []struct {
+		stts        SttsBox
 		startTime   uint64
 		sampleNr    uint32
 		expectError bool
 	}{
-		{0, 1, false},
-		{1, 2, false},
-		{10, 2, false},
-		{20, 3, false},
-		{30, 4, false},
-		{31, 5, false},
-		{43, 5, false},
-		{44, 5, false},
-		{45, 6, false},
-		{57, 6, false},
-		{58, 0, true},
+		{stts, 0, 1, false},
+		{stts, 1, 2, false},
+		{stts, 10, 2, false},
+		{stts, 20, 3, false},
+		{stts, 30, 4, false},
+		{stts, 31, 5, false},
+		{stts, 43, 5, false},
+		{stts, 44, 5, false},
+		{stts, 45, 6, false},
+		{stts, 57, 6, false},
+		{stts, 58, 0, true},
+		{sttsZero, 0, 1, false},
+		{sttsZero, 10, 2, false},
+		{sttsZero, 19, 3, false},
+		{sttsZero, 20, 3, false},
+		{sttsZero, 21, 0, true},
 	}
 
 	for _, tc := range testCases {
-		gotNr, err := stts.GetSampleNrAtTime(tc.startTime)
+		gotNr, err := tc.stts.GetSampleNrAtTime(tc.startTime)
 		if tc.expectError {
 			if err == nil {
 				t.Errorf("Did not get error for startTime %d", tc.startTime)