Fix no newline handling in fragment parser

Correctly process "\ No newline..." marker lines when parsing fragments,
triming the new line from the last read line and advancing the parser.

Also fix the text chunk parser to follow the invariant of not advancing
past the end of the object in the event of an error. The error in
question now has a correct line number as well.

Author: bluekeyes

gitdiff/parser.go

@@ -236,8 +236,14 @@ func (p *parser) ParseTextChunk(frag *Fragment) error {
 		return p.Errorf(0, "no content following fragment header")
 	}
 
+	isNoNewlineLine := func(s string) bool {
+		// test for "\ No newline at end of file" by prefix because the text
+		// changes by locale (git claims all versions are at least 12 chars)
+		return len(s) >= 12 && s[:2] == "\\ "
+	}
+
 	oldLines, newLines := frag.OldLines, frag.NewLines
-	for oldLines > 0 || newLines > 0 {
+	for {
 		line := p.Line(0)
 		switch line[0] {
 		case '\n':
@@ -252,21 +258,28 @@ func (p *parser) ParseTextChunk(frag *Fragment) error {
 			}
 			frag.Lines = append(frag.Lines, FragmentLine{OpContext, line[1:]})
 		case '-':
-			frag.LinesDeleted++
 			oldLines--
+			frag.LinesDeleted++
+			frag.TrailingContext = 0
 			frag.Lines = append(frag.Lines, FragmentLine{OpDelete, line[1:]})
 		case '+':
-			frag.LinesAdded++
 			newLines--
+			frag.LinesAdded++
+			frag.TrailingContext = 0
 			frag.Lines = append(frag.Lines, FragmentLine{OpAdd, line[1:]})
 		default:
-			// this could be "\ No newline at end of file", which we allow
-			// only check the prefix because the text changes by locale
-			// git also asserts that any translation is at least 12 characters
-			if len(line) >= 12 && strings.HasPrefix("\\ ", line) {
+			// this may appear in middle of fragment if it's for a deleted line
+			if isNoNewlineLine(line) {
+				last := len(frag.Lines) - 1
+				frag.Lines[last].Line = strings.TrimSuffix(frag.Lines[last].Line, "\n")
 				break
 			}
-			return p.Errorf(0, "invalid fragment line")
+			return p.Errorf(0, "invalid line operation: %q", line[0])
+		}
+
+		next := p.Line(1)
+		if oldLines <= 0 && newLines <= 0 && !isNoNewlineLine(next) {
+			break
 		}
 
 		if err := p.Next(); err != nil {
@@ -278,7 +291,12 @@ func (p *parser) ParseTextChunk(frag *Fragment) error {
 	}
 
 	if oldLines != 0 || newLines != 0 {
-		return p.Errorf(0, "invalid fragment: remaining lines: %d old, %d new", oldLines, newLines)
+		hdr := max(frag.OldLines-oldLines, frag.NewLines-newLines) + 1
+		return p.Errorf(-hdr, "fragment header miscounts lines: %+d old, %+d new", -oldLines, -newLines)
+	}
+
+	if err := p.Next(); err != nil && err != io.EOF {
+		return err
 	}
 	return nil
 }
@@ -306,3 +324,10 @@ func parseRange(s string) (start int64, end int64, err error) {
 
 	return
 }
+
+func max(a, b int64) int64 {
+	if a > b {
+		return a
+	}
+	return b
+}