From 699084298b753da13e2d56ca78c2b86302cbd5af Mon Sep 17 00:00:00 2001
From: Billy Keyes <bluekeyes@gmail.com>
Date: Sun, 24 Mar 2019 20:49:27 -0700
Subject: [PATCH] Accept comments when parsing fragment headers

Also parse the header directly instead of using a regexp. This allows
finer-grained errors.
---
 gitdiff/gitdiff.go     |  2 ++
 gitdiff/parser.go      | 70 +++++++++++++++++++++++-------------------
 gitdiff/parser_test.go |  5 +--
 3 files changed, 44 insertions(+), 33 deletions(-)

diff --git a/gitdiff/gitdiff.go b/gitdiff/gitdiff.go
index cea3a89..8c749a9 100644
--- a/gitdiff/gitdiff.go
+++ b/gitdiff/gitdiff.go
@@ -27,6 +27,8 @@ type File struct {
 
 // Fragment describes changed lines starting at a specific line in a text file.
 type Fragment struct {
+	Comment string
+
 	OldPosition int64
 	OldLines    int64
 
diff --git a/gitdiff/parser.go b/gitdiff/parser.go
index c08b0ea..7fea17a 100644
--- a/gitdiff/parser.go
+++ b/gitdiff/parser.go
@@ -4,7 +4,6 @@ import (
 	"bufio"
 	"fmt"
 	"io"
-	"regexp"
 	"strconv"
 	"strings"
 )
@@ -56,11 +55,6 @@ const (
 	devNull = "/dev/null"
 )
 
-var (
-	// TODO(bkeyes): are the boundary conditions necessary?
-	fragmentHeaderRegexp = regexp.MustCompile(`^@@ -(\d+),(\d+) \+(\d+)(?:,(\d+))? @@.*\n`)
-)
-
 // ParseNextFileHeader finds and parses the next file header in the stream. It
 // returns nil if no headers are found before the end of the stream.
 func (p *parser) ParseNextFileHeader() (file *File, err error) {
@@ -166,38 +160,52 @@ func isMaybeFragmentHeader(line string) bool {
 	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 {
+// TODO(bkeyes): fix duplication with isMaybeFragmentHeader
+func parseFragmentHeader(f *Fragment, header string) (err error) {
+	const startMark = "@@ "
+	const endMark = " @@"
+
+	parts := strings.SplitAfterN(header, endMark, 2)
+	if len(parts) < 2 || !strings.HasPrefix(parts[0], startMark) || !strings.HasSuffix(parts[0], endMark) {
 		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
+	header = parts[0][len(startMark) : len(parts[0])-len(endMark)]
+	f.Comment = strings.TrimSpace(parts[1])
+
+	ranges := strings.Split(header, " ")
+	if len(ranges) != 2 {
+		return fmt.Errorf("invalid fragment header")
 	}
 
-	if err := parseInt(match[1], &f.OldPosition); err != nil {
-		return err
+	if !strings.HasPrefix(ranges[0], "-") || !strings.HasPrefix(ranges[1], "+") {
+		return fmt.Errorf("invalid fragment header: bad range marker")
 	}
-	if err := parseInt(match[2], &f.OldLines); err != nil {
-		return err
+	if f.OldPosition, f.OldLines, err = parseRange(ranges[0][1:]); err != nil {
+		return fmt.Errorf("invalid fragment header: %v", err)
 	}
-
-	if err := parseInt(match[3], &f.NewPosition); err != nil {
-		return err
+	if f.NewPosition, f.NewLines, err = parseRange(ranges[1][1:]); err != nil {
+		return fmt.Errorf("invalid fragment header: %v", err)
 	}
-
-	f.NewLines = 1
-	if match[4] != "" {
-		if err := parseInt(match[4], &f.NewLines); err != nil {
-			return err
-		}
-	}
-
 	return nil
 }
+
+func parseRange(s string) (start int64, end int64, err error) {
+	parts := strings.SplitN(s, ",", 2)
+
+	if start, err = strconv.ParseInt(parts[0], 10, 64); err != nil {
+		nerr := err.(*strconv.NumError)
+		return 0, 0, fmt.Errorf("bad start of range: %s: %v", parts[0], nerr.Err)
+	}
+
+	if len(parts) > 1 {
+		if end, err = strconv.ParseInt(parts[1], 10, 64); err != nil {
+			nerr := err.(*strconv.NumError)
+			return 0, 0, fmt.Errorf("bad end of range: %s: %v", parts[1], nerr.Err)
+		}
+	} else {
+		end = 1
+	}
+
+	return
+}
diff --git a/gitdiff/parser_test.go b/gitdiff/parser_test.go
index 6656fba..807d10b 100644
--- a/gitdiff/parser_test.go
+++ b/gitdiff/parser_test.go
@@ -89,9 +89,10 @@ func TestParseFragmentHeader(t *testing.T) {
 				NewLines:    9,
 			},
 		},
-		"trailingWhitespace": {
-			Input: "@@ -21,5 +28,9 @@ \r\n",
+		"trailingComment": {
+			Input: "@@ -21,5 +28,9 @@ func test(n int) {\n",
 			Output: &Fragment{
+				Comment:     "func test(n int) {",
 				OldPosition: 21,
 				OldLines:    5,
 				NewPosition: 28,