container/mts: added commenting to SegmentForMeta and accompanying testing

container/mts/mpegts.go

@@ -443,19 +443,25 @@ func TrimToMetaRange(d []byte, key, from, to string) ([]byte, error) {
 	}
 }
 
+// SegmentForMeta returns segments of MTS slice d that correspond to a value of
+// meta for key and val. Therefore, any sequence of packets corresponding to
+// key and val will be appended to the returned [][]byte.
 func SegmentForMeta(d []byte, key, val string) ([][]byte, error) {
 	var (
-		pkt        packet.Packet
+		pkt        packet.Packet // We copy data to this so that we can use comcast gots stuff.
-		segmenting bool
+		segmenting bool          // If true we are currently in a segment corresponsing to given meta.
-		res        [][]byte
+		res        [][]byte      // The resultant [][]byte holding the segments.
-		start      int
+		start      int           // The start index of the current segment.
 	)
 
+	// Go through packets.
 	for i := 0; i < len(d); i += PacketSize {
 		copy(pkt[:], d[i:i+PacketSize])
 		if pkt.PID() == PmtPid {
 			_meta, err := ExtractMeta(pkt[:])
 			switch err {
+			// If there's no meta or a problem with meta, we consider this the end
+			// of the segment.
 			case errNoMeta, meta.ErrUnexpectedMetaFormat:
 				if segmenting {
 					res = append(res, d[start:i])
@@ -467,6 +473,9 @@ func SegmentForMeta(d []byte, key, val string) ([][]byte, error) {
 				return nil, err
 			}
 
+			// If we've got the meta of interest in the PMT and we're not segmenting
+			// then start segmenting. If we don't have the meta of interest in the PMT
+			// and we are segmenting then we want to stop and append the segment to result.
 			if _meta[key] == val && !segmenting {
 				start = i
 				segmenting = true
@@ -477,6 +486,8 @@ func SegmentForMeta(d []byte, key, val string) ([][]byte, error) {
 		}
 	}
 
+	// We've reached the end of the entire MTS clip so if we're segmenting we need
+	// to append current segment to res.
 	if segmenting {
 		res = append(res, d[start:len(d)])
 	}

container/mts/mpegts_test.go

@@ -336,18 +336,20 @@ func TestTrimToMetaRange(t *testing.T) {
 	}
 }
 
+// TestSegmentForMeta checks that SegmentForMeta can correctly segment some MTS
+// data based on a given meta key and value.
 func TestSegmentForMeta(t *testing.T) {
 	Meta = meta.New()
 
 	const (
-		nPSI = 10
+		nPSI = 10  // The number of PSI pairs to write.
-		key  = "n"
+		key  = "n" // The meta key we will work with.
-		val  = "*"
+		val  = "*" // This is the meta value we will look for.
 	)
 
 	tests := []struct {
-		metaVals   [nPSI]string
+		metaVals   [nPSI]string // This represents the meta value for meta pairs (PAT and PMT)
-		expectIdxs []rng
+		expectIdxs []rng        // This gives the expect index ranges for the segments.
 	}{
 		{
 			metaVals: [nPSI]string{"1", "2", val, val, val, "3", val, val, "4", "4"},
@@ -379,7 +381,10 @@ func TestSegmentForMeta(t *testing.T) {
 	var clip bytes.Buffer
 
 	for testn, test := range tests {
+		// We want a clean buffer for each new test, so reset.
 		clip.Reset()
+
+		// Add meta and write PSI to clip.
 		for i := 0; i < nPSI; i++ {
 			if test.metaVals[i] != "" {
 				Meta.Add(key, test.metaVals[i])
@@ -391,26 +396,36 @@ func TestSegmentForMeta(t *testing.T) {
 				t.Fatalf("did not expect to get error writing PSI, error: %v", err)
 			}
 		}
+
+		// Now we get the expected segments using the index ranges from the test.
 		var want [][]byte
 		for _, idxs := range test.expectIdxs {
 			want = append(want, clip.Bytes()[idxs.start:idxs.end])
 		}
+
+		// Now use the function we're testing to get the segments.
 		got, err := SegmentForMeta(clip.Bytes(), key, val)
 		if err != nil {
 			t.Fatalf("unexpected error: %v", err)
 		}
 
+		// Check that segments are OK.
 		if !reflect.DeepEqual(want, got) {
 			t.Errorf("did not get expected result for test %v\nGot: %v\nWant: %v\n", testn, got, want)
 		}
 	}
 }
 
+// rng describes an index range and is used by TestSegmentForMeta.
 type rng struct {
 	start int
 	end   int
 }
 
+// scale takes a PSI index (i.e. first PSI is 0, next is 1) and modifies to be
+// the index of the first byte of the PSI pair (PAT and PMT) in the byte stream.
+// This assumes there are only PSI written consequitively, and is used by
+// TestSegmentForMeta.
 func scale(x, y int) rng {
 	return rng{
 		((x * 2) + 1) * PacketSize,