path/filepath: Dir

There was Base but not Dir, so fill in the gap.

R=n13m3y3r, r, rsc, gustavo
CC=golang-dev
https://golang.org/cl/5503067

src/pkg/path/filepath/path.go

@@ -147,6 +147,7 @@ func SplitList(path string) []string {
 // separating it into a directory and file name component.
 // If there is no Separator in path, Split returns an empty dir
 // and file set to path.
+// The returned values have the property that path = dir+file.
 func Split(path string) (dir, file string) {
 	vol := VolumeName(path)
 	i := len(path) - 1
@@ -439,3 +440,21 @@ func Base(path string) string {
 	}
 	return path
 }
+
+// Dir returns the all but the last element of path, typically the path's directory.
+// Trailing path separators are removed before processing.
+// If the path is empty, Dir returns ".".
+// If the path consists entirely of separators, Dir returns a single separator.
+// The returned path does not end in a separator unless it is the root directory.
+func Dir(path string) string {
+	dir, _ := Split(path)
+	dir = Clean(dir)
+	last := len(dir) - 1
+	if last > 0 && os.IsPathSeparator(dir[last]) {
+		dir = dir[:last]
+	}
+	if dir == "" {
+		dir = "."
+	}
+	return dir
+}

src/pkg/path/filepath/path_test.go

@@ -431,6 +431,29 @@ func TestBase(t *testing.T) {
 	}
 }
 
+var dirtests = []PathTest{
+	{"", "."},
+	{".", "."},
+	{"/.", "/"},
+	{"/", "/"},
+	{"////", "/"},
+	{"/foo", "/"},
+	{"x/", "x"},
+	{"abc", "."},
+	{"abc/def", "abc"},
+	{"a/b/.x", "a/b"},
+	{"a/b/c.", "a/b"},
+	{"a/b/c.x", "a/b"},
+}
+
+func TestDir(t *testing.T) {
+	for _, test := range dirtests {
+		if s := filepath.ToSlash(filepath.Dir(test.path)); s != test.result {
+			t.Errorf("Dir(%q) = %q, want %q", test.path, s, test.result)
+		}
+	}
+}
+
 type IsAbsTest struct {
 	path  string
 	isAbs bool