Add tests for single fragment parsing

To make output better, also add some (temporary?) String() functions and
fix an assumption about the smallest fragment header I discovered was
wrong while looking at sample patches.

Author: bluekeyes

gitdiff/file_header.go

@@ -61,7 +61,7 @@ func (p *parser) ParseGitFileHeader() (*File, error) {
 }
 
 func (p *parser) ParseTraditionalFileHeader() (*File, error) {
-	const shortestValidFragHeader = "@@ -0,0 +1 @@\n"
+	const shortestValidFragHeader = "@@ -1 +1 @@\n"
 	const (
 		oldPrefix = "--- "
 		newPrefix = "+++ "

gitdiff/file_header_test.go

@@ -1,10 +1,8 @@
 package gitdiff
 
 import (
-	"bufio"
 	"os"
 	"reflect"
-	"strings"
 	"testing"
 )
 
@@ -150,8 +148,7 @@ index deadbeef
 
 	for name, test := range tests {
 		t.Run(name, func(t *testing.T) {
-			p := &parser{r: bufio.NewReader(strings.NewReader(test.Input))}
-			p.Next()
+			p := newTestParser(test.Input, true)
 
 			f, err := p.ParseGitFileHeader()
 			if test.Err {
@@ -256,8 +253,7 @@ context line
 
 	for name, test := range tests {
 		t.Run(name, func(t *testing.T) {
-			p := &parser{r: bufio.NewReader(strings.NewReader(test.Input))}
-			p.Next()
+			p := newTestParser(test.Input, true)
 
 			f, err := p.ParseTraditionalFileHeader()
 			if test.Err {

gitdiff/gitdiff.go

@@ -51,6 +51,10 @@ type FragmentLine struct {
 	Line string
 }
 
+func (fl FragmentLine) String() string {
+	return fl.Op.String() + fl.Line
+}
+
 // LineOp describes the type of a fragment line: context, added, or removed.
 type LineOp int
 
@@ -63,6 +67,18 @@ const (
 	OpAdd
 )
 
+func (op LineOp) String() string {
+	switch op {
+	case OpContext:
+		return " "
+	case OpDelete:
+		return "-"
+	case OpAdd:
+		return "+"
+	}
+	return "?"
+}
+
 // Header returns the cannonical header of this fragment.
 func (f *Fragment) Header() string {
 	return fmt.Sprintf("@@ -%d,%d +%d,%d @@ %s", f.OldPosition, f.OldLines, f.NewPosition, f.NewLines, f.Comment)

gitdiff/parser.go

@@ -245,8 +245,11 @@ func (p *parser) ParseTextChunk(frag *Fragment) error {
 	oldLines, newLines := frag.OldLines, frag.NewLines
 	for {
 		line := p.Line(0)
-		switch line[0] {
+		op, data := line[0], line[1:]
+
+		switch op {
 		case '\n':
+			data = "\n"
 			fallthrough // newer GNU diff versions create empty context lines
 		case ' ':
 			oldLines--
@@ -256,25 +259,25 @@ func (p *parser) ParseTextChunk(frag *Fragment) error {
 			} else {
 				frag.TrailingContext++
 			}
-			frag.Lines = append(frag.Lines, FragmentLine{OpContext, line[1:]})
+			frag.Lines = append(frag.Lines, FragmentLine{OpContext, data})
 		case '-':
 			oldLines--
 			frag.LinesDeleted++
 			frag.TrailingContext = 0
-			frag.Lines = append(frag.Lines, FragmentLine{OpDelete, line[1:]})
+			frag.Lines = append(frag.Lines, FragmentLine{OpDelete, data})
 		case '+':
 			newLines--
 			frag.LinesAdded++
 			frag.TrailingContext = 0
-			frag.Lines = append(frag.Lines, FragmentLine{OpAdd, line[1:]})
+			frag.Lines = append(frag.Lines, FragmentLine{OpAdd, data})
 		default:
 			// 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")
+				last := &frag.Lines[len(frag.Lines)-1]
+				last.Line = strings.TrimSuffix(last.Line, "\n")
 				break
 			}
-			return p.Errorf(0, "invalid line operation: %q", line[0])
+			return p.Errorf(0, "invalid line operation: %q", op)
 		}
 
 		next := p.Line(1)