Support Apple Diff timestamps (#65)

Apple's diff (at least the one shipping with macOS Venture) does not
include the fractional seconds or Timezone in it's output.

This adds support for that so diffutils does not need to be installed.

I first encountered this problem using golangci-lint on macOS after
upgrading to macOS Ventura.

Author: freeformz

diff/diff.go

@@ -120,6 +120,10 @@ const onlyInMessage = "Only in %s: %s\n"
 // See https://www.gnu.org/software/diffutils/manual/html_node/Detailed-Unified.html.
 const diffTimeParseLayout = "2006-01-02 15:04:05 -0700"
 
+// Apple's diff is based on freebsd diff, which uses a timestamp format that does
+// not include the timezone offset.
+const diffTimeParseWithoutTZLayout = "2006-01-02 15:04:05"
+
 // diffTimeFormatLayout is the layout used to format (i.e., print) the time in unified diff file
 // header timestamps.
 // See https://www.gnu.org/software/diffutils/manual/html_node/Detailed-Unified.html.

diff/diff_test.go

@@ -850,12 +850,14 @@ func TestParseFileDiffAndPrintFileDiff(t *testing.T) {
 
 func TestParseMultiFileDiffAndPrintMultiFileDiff(t *testing.T) {
 	tests := []struct {
-		filename      string
-		wantParseErr  error
-		wantFileDiffs int // How many instances of diff.FileDiff are expected.
+		filename        string
+		wantParseErr    error
+		wantFileDiffs   int    // How many instances of diff.FileDiff are expected.
+		wantOutFileName string // If non-empty, the name of the file containing the expected output.
 	}{
 		{filename: "sample_multi_file.diff", wantFileDiffs: 2},
 		{filename: "sample_multi_file_single.diff", wantFileDiffs: 1},
+		{filename: "sample_multi_file_single_apple_in.diff", wantFileDiffs: 1, wantOutFileName: "sample_multi_file_single_apple_out.diff"},
 		{filename: "sample_multi_file_new.diff", wantFileDiffs: 3},
 		{filename: "sample_multi_file_deleted.diff", wantFileDiffs: 3},
 		{filename: "sample_multi_file_rename.diff", wantFileDiffs: 3},
@@ -892,6 +894,12 @@ func TestParseMultiFileDiffAndPrintMultiFileDiff(t *testing.T) {
 		if err != nil {
 			t.Errorf("%s: PrintMultiFileDiff: %s", test.filename, err)
 		}
+		if test.wantOutFileName != "" {
+			diffData, err = ioutil.ReadFile(filepath.Join("testdata", test.wantOutFileName))
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
 		if !bytes.Equal(printed, diffData) {
 			t.Errorf("%s: printed multi-file diff != original multi-file diff\n\n# PrintMultiFileDiff output - Original:\n%s", test.filename, cmp.Diff(diffData, printed))
 		}

diff/parse.go

@@ -254,7 +254,6 @@ func (r *FileDiffReader) ReadFileHeaders() (origName, newName string, origTimest
 				"", nil, nil, nil
 		}
 	}
-
 	origName, origTimestamp, err = r.readOneFileHeader([]byte("--- "))
 	if err != nil {
 		return "", "", nil, nil, err
@@ -307,10 +306,16 @@ func (r *FileDiffReader) readOneFileHeader(prefix []byte) (filename string, time
 	parts := strings.SplitN(trimmedLine, "\t", 2)
 	filename = parts[0]
 	if len(parts) == 2 {
+		var ts time.Time
 		// Timestamp is optional, but this header has it.
-		ts, err := time.Parse(diffTimeParseLayout, parts[1])
+		ts, err = time.Parse(diffTimeParseLayout, parts[1])
 		if err != nil {
-			return "", nil, err
+			var err1 error
+			ts, err1 = time.Parse(diffTimeParseWithoutTZLayout, parts[1])
+			if err1 != nil {
+				return "", nil, err
+			}
+			err = nil
 		}
 		timestamp = &ts
 	}

diff/testdata/sample_multi_file_single_apple_in.diff

@@ -0,0 +1,29 @@
+diff -u a/oldname1 b/newname1
+--- oldname1	2009-10-11 15:12:20
++++ newname1	2009-10-11 15:12:30
+@@ -1,3 +1,9 @@
++This is an important
++notice! It should
++therefore be located at
++the beginning of this
++document!
++
+ This part of the
+ document has stayed the
+ same from version to
+@@ -5,16 +11,10 @@
+ be shown if it doesn't
+ change.  Otherwise, that
+ would not be helping to
+-compress the size of the
+-changes.
+-
+-This paragraph contains
+-text that is outdated.
+-It will be deleted in the
+-near future.
++compress anything.
+ 
+ It is important to spell
+-check this dokument. On
++check this document. On

diff/testdata/sample_multi_file_single_apple_out.diff

@@ -0,0 +1,29 @@
+diff -u a/oldname1 b/newname1
+--- oldname1	2009-10-11 15:12:20.000000000 +0000
++++ newname1	2009-10-11 15:12:30.000000000 +0000
+@@ -1,3 +1,9 @@
++This is an important
++notice! It should
++therefore be located at
++the beginning of this
++document!
++
+ This part of the
+ document has stayed the
+ same from version to
+@@ -5,16 +11,10 @@
+ be shown if it doesn't
+ change.  Otherwise, that
+ would not be helping to
+-compress the size of the
+-changes.
+-
+-This paragraph contains
+-text that is outdated.
+-It will be deleted in the
+-near future.
++compress anything.
+ 
+ It is important to spell
+-check this dokument. On
++check this document. On