Fix parsing Windows line directives

Also refactor parsing the lines and add tests

closes #45

main.go

@@ -494,33 +494,9 @@ func (c *config) format(file ast.Node, rwErrs error) (string, error) {
 			return "", err
 		}
 
-		var lines []string
-		scanner := bufio.NewScanner(&buf)
-		for scanner.Scan() {
-			txt := scanner.Text()
-
-			// check for any line directive and store it for next iteration to
-			// re-construct the original file. If it's not a line directive,
-			// continue consturcting the original file
-			if !strings.HasPrefix(txt, "//line") {
-				lines = append(lines, txt)
-				continue
-			}
-
-			splitted := strings.Split(txt, ":")
-			if len(splitted) != 2 {
-				return "", fmt.Errorf("invalid line directive found: %q", txt)
-			}
-			lineNr, err := strconv.Atoi(splitted[1])
-			if err != nil {
-				return "", err
-			}
-
-			for i := len(lines); i < lineNr-1; i++ {
-				lines = append(lines, "")
-			}
-
-			lines = lines[:lineNr-1]
+		lines, err := parseLines(&buf)
+		if err != nil {
+			return "", err
 		}
 
 		// prevent selection to be larger than the actual number of lines
@@ -741,3 +717,60 @@ func (r *rewriteErrors) Append(err error) {
 
 	r.errs = append(r.errs, err)
 }
+
+// parseLines parses the given buffer and returns a slice of lines
+func parseLines(buf io.Reader) ([]string, error) {
+	var lines []string
+	scanner := bufio.NewScanner(buf)
+	for scanner.Scan() {
+		txt := scanner.Text()
+
+		// check for any line directive and store it for next iteration to
+		// re-construct the original file. If it's not a line directive,
+		// continue consturcting the original file
+		if !strings.HasPrefix(txt, "//line") {
+			lines = append(lines, txt)
+			continue
+		}
+
+		lineNr, err := split(txt)
+		if err != nil {
+			return nil, err
+		}
+
+		for i := len(lines); i < lineNr-1; i++ {
+			lines = append(lines, "")
+		}
+
+		lines = lines[:lineNr-1]
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("invalid scanner inputl: %s", err)
+	}
+
+	return lines, nil
+}
+
+// split splits the given line directive and returns the line number
+// see https://golang.org/cmd/compile/#hdr-Compiler_Directives for more
+// information
+// NOTE(arslan): this only splits the line directive that the go.Parser
+// outputs. If the go parser changes the format of the line directive, make
+// sure to fix it in the below function
+func split(line string) (int, error) {
+	for i := len(line) - 1; i >= 0; i-- {
+		if line[i] != ':' {
+			continue
+		}
+
+		nr, err := strconv.Atoi(line[i+1:])
+		if err != nil {
+			return 0, err
+		}
+
+		return nr, nil
+	}
+
+	return 0, fmt.Errorf("couldn't parse line: '%s'", line)
+}

main_test.go

@@ -6,6 +6,7 @@ import (
 	"flag"
 	"fmt"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"reflect"
 	"strings"
@@ -613,3 +614,70 @@ type foo struct {
 		t.Fatal("expected error")
 	}
 }
+
+func TestParseLines(t *testing.T) {
+	var tests = []struct {
+		file string
+	}{
+		{file: "line_directive_unix"},
+		{file: "line_directive_windows"},
+	}
+
+	for _, ts := range tests {
+		ts := ts
+
+		t.Run(ts.file, func(t *testing.T) {
+			filePath := filepath.Join(fixtureDir, fmt.Sprintf("%s.input", ts.file))
+
+			file, err := os.Open(filePath)
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer file.Close()
+
+			out, err := parseLines(file)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			toBytes := func(lines []string) []byte {
+				var buf bytes.Buffer
+				for _, line := range lines {
+					buf.WriteString(line + "\n")
+				}
+				return buf.Bytes()
+			}
+
+			got := toBytes(out)
+
+			// update golden file if necessary
+			golden := filepath.Join(fixtureDir, fmt.Sprintf("%s.golden", ts.file))
+
+			if *update {
+				err := ioutil.WriteFile(golden, got, 0644)
+				if err != nil {
+					t.Error(err)
+				}
+				return
+			}
+
+			// get golden file
+			want, err := ioutil.ReadFile(golden)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			from, err := ioutil.ReadFile(filePath)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			// compare
+			if !bytes.Equal(got, want) {
+				t.Errorf("case %s\ngot:\n====\n\n%s\nwant:\n=====\n\n%s\nfrom:\n=====\n\n%s\n",
+					ts.file, got, want, from)
+			}
+
+		})
+	}
+}

test-fixtures/line_directive_unix.golden

@@ -0,0 +1,5 @@
+package a
+type x struct {
+        Foo int `json:"foo"`
+        bar int
+}

test-fixtures/line_directive_unix.input

@@ -0,0 +1,8 @@
+//line /Users/fatih/test.go:1
+package a
+
+//line /Users/fatih/test.go:2
+type x struct {
+        Foo int `json:"foo"`
+        bar int
+}

test-fixtures/line_directive_windows.golden

@@ -0,0 +1,5 @@
+package a
+type x struct {
+        Foo int `json:"foo"`
+        bar int
+}

test-fixtures/line_directive_windows.input

@@ -0,0 +1,8 @@
+//line c:\\file\\path\\to\\file.go:1
+package a
+
+//line c:\\file\\path\\to\\file.go:2
+type x struct {
+        Foo int `json:"foo"`
+        bar int
+}