Support parsing srt without sequence numbers.

srt.go

@@ -51,16 +51,16 @@ func ReadFromSRT(i io.Reader) (o *Subtitles, err error) {
 
 		// Line contains time boundaries
 		if strings.Contains(line, srtTimeBoundariesSeparator) {
-			// Return the wrong number of rows
-			if len(s.Lines) == 0 {
-				err = fmt.Errorf("astisub: line %d: no lines", lineNum)
-				return
+			// Remove last item of previous subtitle since it should be the index.
+			// If the last line is empty then the item is missing an index.
+			var index string
+			if len(s.Lines) != 0 {
+				index := s.Lines[len(s.Lines)-1].String()
+				if index != "" {
+					s.Lines = s.Lines[:len(s.Lines)-1]
+				}
 			}
 
-			// Remove last item of previous subtitle since it's the index
-			index := s.Lines[len(s.Lines)-1]
-			s.Lines = s.Lines[:len(s.Lines)-1]
-
 			// Remove trailing empty lines
 			if len(s.Lines) > 0 {
 				for i := len(s.Lines) - 1; i >= 0; i-- {
@@ -84,7 +84,9 @@ func ReadFromSRT(i io.Reader) (o *Subtitles, err error) {
 			s = &Item{}
 
 			// Fetch Index
-			s.Index, _ = strconv.Atoi(index.String())
+			if index != "" {
+				s.Index, _ = strconv.Atoi(index)
+			}
 
 			// Extract time boundaries
 			s1 := strings.Split(line, srtTimeBoundariesSeparator)

srt_test.go

@@ -27,3 +27,22 @@ func TestSRT(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, string(c), w.String())
 }
+
+func TestSRTMissingSequence(t *testing.T) {
+	// Open
+	s, err := astisub.OpenFile("./testdata/missing-sequence-in.srt")
+	assert.NoError(t, err)
+	assertSubtitleItems(t, s)
+
+	// No subtitles to write
+	w := &bytes.Buffer{}
+	err = astisub.Subtitles{}.WriteToSRT(w)
+	assert.EqualError(t, err, astisub.ErrNoSubtitlesToWrite.Error())
+
+	// Write
+	c, err := ioutil.ReadFile("./testdata/missing-sequence-out.srt")
+	assert.NoError(t, err)
+	err = s.WriteToSRT(w)
+	assert.NoError(t, err)
+	assert.Equal(t, string(c), w.String())
+}

testdata/missing-sequence-in.srt

@@ -0,0 +1,20 @@
+00:01:39 --> 00:01:41,04
+(deep rumbling)
+
+00:02:04,08 --> 00:02:07,12 X1:40 X2:600 Y1:20 Y2:50
+MAN:
+How did we end up here?
+
+00:02:12.16 --> 00:02:15.20
+This place is horrible.
+
+00:02:20.24 --> 00:02:22.28
+Smells like balls.
+
+00:02:28,32 --> 00:02:31,36
+We don't belong
+in this shithole.
+
+00:02:31,40 --> 00:02:33,44
+(computer playing
+electronic melody)

testdata/missing-sequence-out.srt

@@ -0,0 +1,26 @@
+1
+00:01:39,000 --> 00:01:41,040
+(deep rumbling)
+
+2
+00:02:04,080 --> 00:02:07,120
+MAN:
+How did we end up here?
+
+3
+00:02:12,160 --> 00:02:15,200
+This place is horrible.
+
+4
+00:02:20,240 --> 00:02:22,280
+Smells like balls.
+
+5
+00:02:28,320 --> 00:02:31,360
+We don't belong
+in this shithole.
+
+6
+00:02:31,400 --> 00:02:33,440
+(computer playing
+electronic melody)