container/mts: BytesForPTSInterval renamed to TrimToPTSRange, and now returns a Clip

It was concluded that it would be more useful if a function that provided a data segment from
the original clip just provided a new Clip, i.e. useful things like PTS and meta is still
available in the segment. So, BytesForPTSInterval was renamed to TrimToPTSRange and now
provides a Clip. The test for this function was updated accordingly.

container/mts/payload.go

@@ -8,8 +8,11 @@ import (
 	"github.com/Comcast/gots/pes"
 )
 
+// TODO: write function for getting subslice of mpegts for meta interval
+
 // Extract extracts the media, PTS, stream ID and meta for an MPEG-TS clip given
 // by p, and returns as a Clip. The MPEG-TS must contain only complete packets.
+// The resultant data is a copy of the original.
 func Extract(p []byte) (*Clip, error) {
 	l := len(p)
 	// Check that clip is divisible by 188, i.e. contains a series of full MPEG-TS clips.
@@ -127,11 +130,12 @@ var (
 	errPTSRange      = errors.New("PTS interval invalid")
 )
 
-// BytesForPTSInterval returns the media data between PTS' from and to. If from
-// sits between two PTS, the Frame posessing lower PTS will be considered the start.
-// The Frame before the Frame corresponding to to will be considered the final
-// Frame.
-func (c *Clip) BytesForPTSInterval(from, to uint64) ([]byte, error) {
+// TrimToPTSRange returns the sub Clip in PTS range defined by from and to.
+// The first Frame in the new Clip will be the Frame for which from corresponds
+// exactly with Frame.PTS, or the Frame in which from lies within. The final
+// Frame in the Clip will be the previous of that for which to coincides with,
+// or the Frame that to lies within.
+func (c *Clip) TrimToPTSRange(from, to uint64) (*Clip, error) {
 	// First check that the interval makes sense.
 	if from >= to {
 		return nil, errPTSRange
@@ -139,7 +143,7 @@ func (c *Clip) BytesForPTSInterval(from, to uint64) ([]byte, error) {
 
 	// Use binary search to find 'from'.
 	n := len(c.frames) - 1
-	idx := sort.Search(
+	startFrameIdx := sort.Search(
 		n,
 		func(i int) bool {
 			if from < c.frames[i+1].PTS {
@@ -148,17 +152,17 @@ func (c *Clip) BytesForPTSInterval(from, to uint64) ([]byte, error) {
 			return false
 		},
 	)
-	if idx == n {
+	if startFrameIdx == n {
 		return nil, errPTSLowerBound
 	}
 
 	// Now get the start index for the backing slice from this Frame.
-	start := c.frames[idx].idx
+	startBackingIdx := c.frames[startFrameIdx].idx
 
 	// Now use binary search again to find 'to'.
-	off := idx + 1
+	off := startFrameIdx + 1
 	n = n - (off)
-	idx = sort.Search(
+	endFrameIdx := sort.Search(
 		n,
 		func(i int) bool {
 			if to <= c.frames[i+off].PTS {
@@ -167,14 +171,18 @@ func (c *Clip) BytesForPTSInterval(from, to uint64) ([]byte, error) {
 			return false
 		},
 	)
-	if idx == n {
+	if endFrameIdx == n {
 		return nil, errPTSUpperBound
 	}
 
-	// Now get the end index for the backing slice from this Frame, and return
-	// segment from backing slice corresponding to start and end.
-	end := c.frames[idx+off-1].idx
-	return c.backing[start : end+len(c.frames[idx+off].Media)], nil
+	// Now get the end index for the backing slice from this Frame.
+	endBackingIdx := c.frames[endFrameIdx+off-1].idx
+
+	// Now return a new clip. NB: data is not copied.
+	return &Clip{
+		frames:  c.frames[startFrameIdx : endFrameIdx+1],
+		backing: c.backing[startBackingIdx : endBackingIdx+len(c.frames[endFrameIdx+off].Media)],
+	}, nil
 }
 
 // Errors that maybe returned from BytesForMetaInterval.

container/mts/payload_test.go

@@ -225,9 +225,9 @@ func genFrames(n, min, max int) [][]byte {
 	return frames
 }
 
-// TestBytesForPTSInterval checks that BytesForPTSInterval can correctly return
-// a slice of media data corresponding to a given PTS interval.
-func TestBytesForPTSInterval(t *testing.T) {
+// TestTrimToPTSRange checks that Clip.TrimToPTSRange will correctly return a
+// sub Clip of the given PTS range.
+func TestTrimToPTSRange(t *testing.T) {
 	const (
 		numOfTestFrames = 10
 		ptsInterval     = 4
@@ -298,7 +298,7 @@ func TestBytesForPTSInterval(t *testing.T) {
 
 	// Run tests.
 	for i, test := range tests {
-		got, err := clip.BytesForPTSInterval(test.from, test.to)
+		got, err := clip.TrimToPTSRange(test.from, test.to)
 
 		// First check the error.
 		if err != nil && err != test.err {
@@ -310,7 +310,7 @@ func TestBytesForPTSInterval(t *testing.T) {
 		}
 
 		// Now check data.
-		if test.err == nil && !bytes.Equal(test.expect, got) {
+		if test.err == nil && !bytes.Equal(test.expect, got.Bytes()) {
 			t.Errorf("did not get expected data for test: %v\n Got: %v\n, Want: %v\n", i, got, test.expect)
 		}
 	}