refs #29. implement RightJustify.

README.md

@@ -53,6 +53,7 @@ Here is a list of functions in [strings](http://golang.org/pkg/strings) and [xst
 | [Len](https://godoc.org/github.com/huandu/xstrings#Len) | `mb_strlen` in PHP | [#23](https://github.com/huandu/xstrings/issues/23) |
 | [Partition](https://godoc.org/github.com/huandu/xstrings#Partition) | `str.partition` in Python; `String#partition` in Ruby | [#10](https://github.com/huandu/xstrings/issues/10) |
 | [Reverse](https://godoc.org/github.com/huandu/xstrings#Reverse) | `String#reverse` in Ruby; `strrev` in PHP; `reverse` in Perl | [#7](https://github.com/huandu/xstrings/issues/7) |
+| [RightJustify](https://godoc.org/github.com/huandu/xstrings#RightJustify) | `str.rjust` in Python; `String#rjust` in Ruby | [#29](https://github.com/huandu/xstrings/issues/29) |
 | [RuneWidth](https://godoc.org/github.com/huandu/xstrings#RuneWidth) | - | [#27](https://github.com/huandu/xstrings/issues/27) |
 | [Scrub](https://godoc.org/github.com/huandu/xstrings#Scrub) | `String#scrub` in Ruby | [#20](https://github.com/huandu/xstrings/issues/20) |
 | [Shuffle](https://godoc.org/github.com/huandu/xstrings#Shuffle) | `str_shuffle` in PHP | [#13](https://github.com/huandu/xstrings/issues/13) |

format.go

@@ -112,3 +112,47 @@ func LeftJustify(str string, length int, pad string) string {
 
 	return output.String()
 }
+
+// RightJustify returns a string with pad string at left side if str's rune length is smaller than length.
+// If str's rune length is larger than length, str itself will be returned.
+//
+// If pad is an empty string, str will be returned.
+//
+// Samples:
+//     LeftJustify("hello", 4, " ")    => "hello"
+//     LeftJustify("hello", 10, " ")   => "     hello"
+//     LeftJustify("hello", 10, "123") => "12312hello"
+func RightJustify(str string, length int, pad string) string {
+	l := Len(str)
+
+	if l >= length || pad == "" {
+		return str
+	}
+
+	var r rune
+	var size int
+
+	paddingLen := Len(pad)
+	remains := length - l
+	repeats := remains / paddingLen
+
+	output := &bytes.Buffer{}
+	output.Grow(len(str) + (repeats+1)*len(pad))
+
+	for i := 0; i < repeats; i++ {
+		output.WriteString(pad)
+	}
+
+	remains = remains % paddingLen
+
+	if remains != 0 {
+		for i := 0; i < remains; i++ {
+			r, size = utf8.DecodeRuneInString(pad)
+			output.WriteRune(r)
+			pad = pad[size:]
+		}
+	}
+
+	output.WriteString(str)
+	return output.String()
+}

format_test.go

@@ -56,3 +56,24 @@ func TestLeftJustify(t *testing.T) {
 		sep("hello中文test", "18", ""):   "hello中文test",
 	})
 }
+
+func TestRightJustify(t *testing.T) {
+	runner := func(str string) string {
+		input := strings.Split(str, separator)
+		n, _ := strconv.Atoi(input[1])
+		return RightJustify(input[0], n, input[2])
+	}
+
+	runTestCases(t, runner, _M{
+		sep("hello", "4", " "):    "hello",
+		sep("hello", "10", " "):   "     hello",
+		sep("hello", "10", "123"): "12312hello",
+
+		sep("hello中文test", "4", " "):    "hello中文test",
+		sep("hello中文test", "12", " "):   " hello中文test",
+		sep("hello中文test", "18", "测试！"): "测试！测试！测hello中文test",
+
+		sep("hello中文test", "0", "123"): "hello中文test",
+		sep("hello中文test", "18", ""):   "hello中文test",
+	})
+}