diff --git a/examples/add-sidx/main.go b/examples/add-sidx/main.go
index 11ca7f1..3fc37a9 100644
--- a/examples/add-sidx/main.go
+++ b/examples/add-sidx/main.go
@@ -35,7 +35,7 @@ func run(inPath, outPath string, nonZeroEPT bool) error {
 		return err
 	}
 
-	err = updateSidx(inFile, nonZeroEPT)
+	err = inFile.UpdateSidx(true /* addIfNotExists */, nonZeroEPT)
 	if err != nil {
 		return fmt.Errorf("addSidx failed: %w", err)
 	}
@@ -51,152 +51,3 @@ func run(inPath, outPath string, nonZeroEPT bool) error {
 	}
 	return nil
 }
-
-type segData struct {
-	startPos         uint64
-	presentationTime uint64
-	baseDecodeTime   uint64
-	dur              uint32
-	size             uint32
-}
-
-// updateSidx updates or adds a top-level sidx box to a fragmented mp4 file.
-func updateSidx(inFile *mp4.File, nonZeroEPT bool) error {
-
-	if !inFile.IsFragmented() {
-		return fmt.Errorf("input file is not fragmented")
-	}
-
-	initSeg := inFile.Init
-	if initSeg == nil {
-		return fmt.Errorf("input file does not have an init segment")
-	}
-
-	segs := inFile.Segments
-	if len(segs) == 0 {
-		return fmt.Errorf("input file does not have any media segments")
-	}
-	update := inFile.Sidx != nil
-
-	refTrak := findReferenceTrak(initSeg)
-	trex, ok := initSeg.Moov.Mvex.GetTrex(refTrak.Tkhd.TrackID)
-	if !ok {
-		return fmt.Errorf("no trex box found for track %d", refTrak.Tkhd.TrackID)
-	}
-	segDatas, err := findSegmentData(segs, refTrak, trex)
-	if err != nil {
-		return fmt.Errorf("failed to find segment data: %w", err)
-	}
-
-	var sidx *mp4.SidxBox
-	if update {
-		sidx = inFile.Sidx
-	} else {
-		sidx = &mp4.SidxBox{}
-	}
-	fillSidx(sidx, refTrak, segDatas, nonZeroEPT)
-	if !update {
-		err = insertSidx(inFile, segDatas, sidx)
-		if err != nil {
-			return fmt.Errorf("failed to insert sidx box: %w", err)
-		}
-	}
-	return nil
-}
-
-func findReferenceTrak(initSeg *mp4.InitSegment) *mp4.TrakBox {
-	var trak *mp4.TrakBox
-	for _, trak = range initSeg.Moov.Traks {
-		if trak.Mdia.Hdlr.HandlerType == "vide" {
-			return trak
-		}
-	}
-	for _, trak = range initSeg.Moov.Traks {
-		if trak.Mdia.Hdlr.HandlerType == "soun" {
-			return trak
-		}
-	}
-	return initSeg.Moov.Traks[0]
-}
-
-func findSegmentData(segs []*mp4.MediaSegment, refTrak *mp4.TrakBox, trex *mp4.TrexBox) ([]segData, error) {
-	segDatas := make([]segData, 0, len(segs))
-	for _, seg := range segs {
-		frag := seg.Fragments[0]
-		for _, traf := range frag.Moof.Trafs {
-			tfhd := traf.Tfhd
-			if tfhd.TrackID == refTrak.Tkhd.TrackID {
-				// Found the track that the sidx should be based on
-				baseTime := traf.Tfdt.BaseMediaDecodeTime()
-				dur := uint32(0)
-				var firstCompositionTimeOffest int64
-				for i, trun := range traf.Truns {
-					trun.AddSampleDefaultValues(tfhd, trex)
-					samples := trun.GetSamples()
-					for j, sample := range samples {
-						if i == 0 && j == 0 {
-							firstCompositionTimeOffest = int64(sample.CompositionTimeOffset)
-						}
-						dur += sample.Dur
-					}
-				}
-				sd := segData{
-					startPos:         seg.StartPos,
-					presentationTime: uint64(int64(baseTime) + firstCompositionTimeOffest),
-					baseDecodeTime:   baseTime,
-					dur:              dur,
-					size:             uint32(seg.Size()),
-				}
-				segDatas = append(segDatas, sd)
-				break
-			}
-		}
-	}
-	return segDatas, nil
-}
-
-func fillSidx(sidx *mp4.SidxBox, refTrak *mp4.TrakBox, segDatas []segData, nonZeroEPT bool) {
-	ept := uint64(0)
-	if nonZeroEPT {
-		ept = segDatas[0].presentationTime
-	}
-	sidx.Version = 1
-	sidx.Timescale = refTrak.Mdia.Mdhd.Timescale
-	sidx.ReferenceID = 1
-	sidx.EarliestPresentationTime = ept
-	sidx.FirstOffset = 0
-	sidx.SidxRefs = make([]mp4.SidxRef, 0, len(segDatas))
-
-	for _, segData := range segDatas {
-		size := segData.size
-		sidx.SidxRefs = append(sidx.SidxRefs, mp4.SidxRef{
-			ReferencedSize:     size,
-			SubSegmentDuration: segData.dur,
-			StartsWithSAP:      1,
-			SAPType:            1,
-		})
-	}
-}
-
-func insertSidx(inFile *mp4.File, segDatas []segData, sidx *mp4.SidxBox) error {
-	// insert sidx box before first media segment
-	// TODO. Handle case where startPos is not reliable. Maybe first box of first segment
-	firstMediaBox, err := inFile.Segments[0].FirstBox()
-	if err != nil {
-		return fmt.Errorf("could not find position to insert sidx box: %w", err)
-	}
-	var mediaStartIdx = 0
-	for i, ch := range inFile.Children {
-		if ch == firstMediaBox {
-			mediaStartIdx = i
-			break
-		}
-	}
-	if mediaStartIdx == 0 {
-		return fmt.Errorf("could not find position to insert sidx box")
-	}
-	inFile.Children = append(inFile.Children[:mediaStartIdx], append([]mp4.Box{sidx}, inFile.Children[mediaStartIdx:]...)...)
-	inFile.Sidx = sidx
-	inFile.Sidxs = []*mp4.SidxBox{sidx}
-	return nil
-}
diff --git a/mp4/file.go b/mp4/file.go
index 8d6332d..6776ef6 100644
--- a/mp4/file.go
+++ b/mp4/file.go
@@ -697,6 +697,157 @@ func (f *File) CopySampleData(w io.Writer, rs io.ReadSeeker, trak *TrakBox,
 	return nil
 }
 
+func (f *File) UpdateSidx(addIfNotExists, nonZeroEPT bool) error {
+
+	if !f.IsFragmented() {
+		return fmt.Errorf("input file is not fragmented")
+	}
+
+	initSeg := f.Init
+	if initSeg == nil {
+		return fmt.Errorf("input file does not have an init segment")
+	}
+
+	segs := f.Segments
+	if len(segs) == 0 {
+		return fmt.Errorf("input file does not have any media segments")
+	}
+	exists := f.Sidx != nil
+	if !exists && !addIfNotExists {
+		return nil
+	}
+
+	refTrak := findReferenceTrak(initSeg)
+	trex, ok := initSeg.Moov.Mvex.GetTrex(refTrak.Tkhd.TrackID)
+	if !ok {
+		return fmt.Errorf("no trex box found for track %d", refTrak.Tkhd.TrackID)
+	}
+	segDatas, err := findSegmentData(segs, refTrak, trex)
+	if err != nil {
+		return fmt.Errorf("failed to find segment data: %w", err)
+	}
+
+	var sidx *SidxBox
+	if exists {
+		sidx = f.Sidx
+	} else {
+		sidx = &SidxBox{}
+	}
+	fillSidx(sidx, refTrak, segDatas, nonZeroEPT)
+	if !exists {
+		err = insertSidx(f, segDatas, sidx)
+		if err != nil {
+			return fmt.Errorf("failed to insert sidx box: %w", err)
+		}
+	}
+	return nil
+}
+
+func findReferenceTrak(initSeg *InitSegment) *TrakBox {
+	var trak *TrakBox
+	for _, trak = range initSeg.Moov.Traks {
+		if trak.Mdia.Hdlr.HandlerType == "vide" {
+			return trak
+		}
+	}
+	for _, trak = range initSeg.Moov.Traks {
+		if trak.Mdia.Hdlr.HandlerType == "soun" {
+			return trak
+		}
+	}
+	return initSeg.Moov.Traks[0]
+}
+
+type segData struct {
+	startPos         uint64
+	presentationTime uint64
+	baseDecodeTime   uint64
+	dur              uint32
+	size             uint32
+}
+
+func findSegmentData(segs []*MediaSegment, refTrak *TrakBox, trex *TrexBox) ([]segData, error) {
+	segDatas := make([]segData, 0, len(segs))
+	for _, seg := range segs {
+		frag := seg.Fragments[0]
+		for _, traf := range frag.Moof.Trafs {
+			tfhd := traf.Tfhd
+			if tfhd.TrackID == refTrak.Tkhd.TrackID {
+				// Found the track that the sidx should be based on
+				baseTime := traf.Tfdt.BaseMediaDecodeTime()
+				dur := uint32(0)
+				var firstCompositionTimeOffest int64
+				for i, trun := range traf.Truns {
+					trun.AddSampleDefaultValues(tfhd, trex)
+					samples := trun.GetSamples()
+					for j, sample := range samples {
+						if i == 0 && j == 0 {
+							firstCompositionTimeOffest = int64(sample.CompositionTimeOffset)
+						}
+						dur += sample.Dur
+					}
+				}
+				sd := segData{
+					startPos:         seg.StartPos,
+					presentationTime: uint64(int64(baseTime) + firstCompositionTimeOffest),
+					baseDecodeTime:   baseTime,
+					dur:              dur,
+					size:             uint32(seg.Size()),
+				}
+				segDatas = append(segDatas, sd)
+				break
+			}
+		}
+	}
+	return segDatas, nil
+}
+
+func fillSidx(sidx *SidxBox, refTrak *TrakBox, segDatas []segData, nonZeroEPT bool) {
+	ept := uint64(0)
+	if nonZeroEPT {
+		ept = segDatas[0].presentationTime
+	}
+	sidx.Version = 1
+	sidx.Timescale = refTrak.Mdia.Mdhd.Timescale
+	sidx.ReferenceID = 1
+	sidx.EarliestPresentationTime = ept
+	sidx.FirstOffset = 0
+	sidx.SidxRefs = make([]SidxRef, 0, len(segDatas))
+
+	for _, segData := range segDatas {
+		size := segData.size
+		sidx.SidxRefs = append(sidx.SidxRefs, SidxRef{
+			ReferencedSize:     size,
+			SubSegmentDuration: segData.dur,
+			StartsWithSAP:      1,
+			SAPType:            1,
+		})
+	}
+}
+
+func insertSidx(inFile *File, segDatas []segData, sidx *SidxBox) error {
+	// insert sidx box before first media segment
+	// TODO. Handle case where startPos is not reliable. Maybe first box of first segment
+	firstMediaBox, err := inFile.Segments[0].FirstBox()
+	if err != nil {
+		return fmt.Errorf("could not find position to insert sidx box: %w", err)
+	}
+	var mediaStartIdx = 0
+	for i, ch := range inFile.Children {
+		if ch == firstMediaBox {
+			mediaStartIdx = i
+			break
+		}
+	}
+	if mediaStartIdx == 0 {
+		return fmt.Errorf("could not find position to insert sidx box")
+	}
+	inFile.Children = append(inFile.Children[:mediaStartIdx], append([]Box{sidx}, inFile.Children[mediaStartIdx:]...)...)
+	inFile.Sidx = sidx
+	inFile.Sidxs = []*SidxBox{sidx}
+	return nil
+}
+
 func min(a, b int) int {
 	if a < b {
 		return a
diff --git a/mp4/file_test.go b/mp4/file_test.go
index 8e55e24..01b964c 100644
--- a/mp4/file_test.go
+++ b/mp4/file_test.go
@@ -299,3 +299,29 @@ func TestGetSegmentBoundariesFromMfra(t *testing.T) {
 		t.Errorf("not 3 segments in file but %d", len(parsedFile.Segments))
 	}
 }
+
+func TestUpdateSidx(t *testing.T) {
+	file, err := os.Open("./testdata/prog_8s_dec_dashinit.mp4")
+	if err != nil {
+		t.Error(err)
+	}
+
+	parsedFile, err := DecodeFile(file)
+	if err != nil {
+		t.Error(err)
+	}
+	err = parsedFile.UpdateSidx(false, false)
+	if err != nil {
+		t.Error(err)
+	}
+	if parsedFile.Sidx != nil {
+		t.Error("sidx should not be present")
+	}
+	err = parsedFile.UpdateSidx(true, false)
+	if err != nil {
+		t.Error(err)
+	}
+	if parsedFile.Sidx == nil {
+		t.Error("sidx should be present")
+	}
+}