playback: support concatenating segments with long gaps (#5172)

Thanks to the new mtxi MP4 box, it's possible to check whether two
segments are consecutive without involving dates or timestamps.

When the new mtxi box is present in both segments, do not check if
the end of the first segment corresponds to the start of the
second segment.

Author: aler9

internal/playback/segment_fmp4.go

@@ -58,38 +58,22 @@ func findMtxi(userData []amp4.IBox) *recordstore.Mtxi {
 	return nil
 }
 
-func segmentFMP4AreConsecutive(init1 *fmp4.Init, init2 *fmp4.Init) bool {
-	mtxi1 := findMtxi(init1.UserData)
-	mtxi2 := findMtxi(init2.UserData)
-
-	switch {
-	case mtxi1 == nil && mtxi2 != nil:
+func segmentFMP4TracksAreEqual(tracks1 []*fmp4.InitTrack, tracks2 []*fmp4.InitTrack) bool {
+	if len(tracks1) != len(tracks2) {
 		return false
+	}
 
-	case mtxi1 != nil && mtxi2 == nil:
-		return false
+	for i, track1 := range tracks1 {
+		track2 := tracks2[i]
 
-	case mtxi1 == nil && mtxi2 == nil: // legacy method: compare tracks
-		if len(init1.Tracks) != len(init2.Tracks) {
+		if track1.ID != track2.ID ||
+			track1.TimeScale != track2.TimeScale ||
+			reflect.TypeOf(track1.Codec) != reflect.TypeOf(track2.Codec) {
 			return false
 		}
-
-		for i, track1 := range init1.Tracks {
-			track2 := init2.Tracks[i]
-
-			if track1.ID != track2.ID ||
-				track1.TimeScale != track2.TimeScale ||
-				reflect.TypeOf(track1.Codec) != reflect.TypeOf(track2.Codec) {
-				return false
-			}
-		}
-
-		return true
-
-	default:
-		return bytes.Equal(mtxi1.StreamID[:], mtxi2.StreamID[:]) &&
-			(mtxi1.SegmentNumber+1) == mtxi2.SegmentNumber
 	}
+
+	return true
 }
 
 func segmentFMP4CanBeConcatenated(
@@ -98,9 +82,25 @@ func segmentFMP4CanBeConcatenated(
 	curInit *fmp4.Init,
 	curStart time.Time,
 ) bool {
-	return segmentFMP4AreConsecutive(prevInit, curInit) &&
-		!curStart.Before(prevEnd.Add(-concatenationTolerance)) &&
-		!curStart.After(prevEnd.Add(concatenationTolerance))
+	mtxi1 := findMtxi(prevInit.UserData)
+	mtxi2 := findMtxi(curInit.UserData)
+
+	switch {
+	case mtxi1 == nil && mtxi2 != nil:
+		return false
+
+	case mtxi1 != nil && mtxi2 == nil:
+		return false
+
+	case mtxi1 == nil && mtxi2 == nil: // legacy method
+		return segmentFMP4TracksAreEqual(prevInit.Tracks, curInit.Tracks) &&
+			!curStart.Before(prevEnd.Add(-concatenationTolerance)) &&
+			!curStart.After(prevEnd.Add(concatenationTolerance))
+
+	default:
+		return bytes.Equal(mtxi1.StreamID[:], mtxi2.StreamID[:]) &&
+			(mtxi1.SegmentNumber+1) == mtxi2.SegmentNumber
+	}
 }
 
 func segmentFMP4ReadHeader(r io.ReadSeeker) (*fmp4.Init, time.Duration, error) {