Detach parseFragmentHeader from parser struct

This operates on a single line and doesn't need any parser state.

gitdiff/parser.go

@@ -75,7 +75,7 @@ func (p *parser) ParseNextFileHeader() (file *File, err error) {
 		// check for disconnected fragment headers (corrupt patch)
 		if isMaybeFragmentHeader(line) {
 			var frag Fragment
-			if err := p.ParseFragmentHeader(&frag, line); err != nil {
+			if err := parseFragmentHeader(&frag, line); err != nil {
 				// not a valid header, nothing to worry about
 				continue
 			}
@@ -134,41 +134,6 @@ func (p *parser) ParseTraditionalFileHeader(f *File, oldFile, newFile string) er
 	panic("unimplemented")
 }
 
-func (p *parser) ParseFragmentHeader(f *Fragment, header string) error {
-	match := fragmentHeaderRegexp.FindStringSubmatch(header)
-	if len(match) < 5 {
-		return p.Errorf("invalid fragment header")
-	}
-
-	parseInt := func(s string, v *int64) (err error) {
-		if *v, err = strconv.ParseInt(s, 10, 64); err != nil {
-			nerr := err.(*strconv.NumError)
-			return p.Errorf("invalid fragment header value: %s: %v", s, nerr.Err)
-		}
-		return
-	}
-
-	if err := parseInt(match[1], &f.OldPosition); err != nil {
-		return err
-	}
-	if err := parseInt(match[2], &f.OldLines); err != nil {
-		return err
-	}
-
-	if err := parseInt(match[3], &f.NewPosition); err != nil {
-		return err
-	}
-
-	f.NewLines = 1
-	if match[4] != "" {
-		if err := parseInt(match[4], &f.NewLines); err != nil {
-			return err
-		}
-	}
-
-	return nil
-}
-
 // Line reads and returns the next line. The first call to Line after a call to
 // PeekLine will never retrun an error.
 func (p *parser) Line() (line string, err error) {
@@ -202,3 +167,39 @@ func isMaybeFragmentHeader(line string) bool {
 	shortestValidHeader := "@@ -0,0 +1 @@\n"
 	return len(line) >= len(shortestValidHeader) && strings.HasPrefix(line, fragmentHeaderPrefix)
 }
+
+func parseFragmentHeader(f *Fragment, header string) error {
+	// TODO(bkeyes): use strings.FieldsFunc instead of regexp
+	match := fragmentHeaderRegexp.FindStringSubmatch(header)
+	if len(match) < 5 {
+		return fmt.Errorf("invalid fragment header")
+	}
+
+	parseInt := func(s string, v *int64) (err error) {
+		if *v, err = strconv.ParseInt(s, 10, 64); err != nil {
+			nerr := err.(*strconv.NumError)
+			return fmt.Errorf("invalid fragment header value: %s: %v", s, nerr.Err)
+		}
+		return
+	}
+
+	if err := parseInt(match[1], &f.OldPosition); err != nil {
+		return err
+	}
+	if err := parseInt(match[2], &f.OldLines); err != nil {
+		return err
+	}
+
+	if err := parseInt(match[3], &f.NewPosition); err != nil {
+		return err
+	}
+
+	f.NewLines = 1
+	if match[4] != "" {
+		if err := parseInt(match[4], &f.NewLines); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}

gitdiff/parser_test.go

@@ -109,11 +109,8 @@ func TestParseFragmentHeader(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.Name, func(t *testing.T) {
-			p := &parser{r: bufio.NewReader(strings.NewReader(test.Input))}
-			line, _ := p.Line()
-
 			var frag Fragment
-			err := p.ParseFragmentHeader(&frag, line)
+			err := parseFragmentHeader(&frag, test.Input)
 
 			if test.Invalid {
 				if err == nil {