diff --git a/vendor/github.com/xdg/stringprep/bidi.go b/vendor/github.com/xdg/stringprep/bidi.go
new file mode 100644
index 0000000..6f6d321
--- /dev/null
+++ b/vendor/github.com/xdg/stringprep/bidi.go
@@ -0,0 +1,73 @@
+// Copyright 2018 by David A. Golden. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License. You may obtain
+// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
+
+package stringprep
+
+var errHasLCat = "BiDi string can't have runes from category L"
+var errFirstRune = "BiDi string first rune must have category R or AL"
+var errLastRune = "BiDi string last rune must have category R or AL"
+
+// Check for prohibited characters from table C.8
+func checkBiDiProhibitedRune(s string) error {
+	for _, r := range s {
+		if TableC8.Contains(r) {
+			return Error{Msg: errProhibited, Rune: r}
+		}
+	}
+	return nil
+}
+
+// Check for LCat characters from table D.2
+func checkBiDiLCat(s string) error {
+	for _, r := range s {
+		if TableD2.Contains(r) {
+			return Error{Msg: errHasLCat, Rune: r}
+		}
+	}
+	return nil
+}
+
+// Check first and last characters are in table D.1; requires non-empty string
+func checkBadFirstAndLastRandALCat(s string) error {
+	rs := []rune(s)
+	if !TableD1.Contains(rs[0]) {
+		return Error{Msg: errFirstRune, Rune: rs[0]}
+	}
+	n := len(rs) - 1
+	if !TableD1.Contains(rs[n]) {
+		return Error{Msg: errLastRune, Rune: rs[n]}
+	}
+	return nil
+}
+
+// Look for RandALCat characters from table D.1
+func hasBiDiRandALCat(s string) bool {
+	for _, r := range s {
+		if TableD1.Contains(r) {
+			return true
+		}
+	}
+	return false
+}
+
+// Check that BiDi rules are satisfied ; let empty string pass this rule
+func passesBiDiRules(s string) error {
+	if len(s) == 0 {
+		return nil
+	}
+	if err := checkBiDiProhibitedRune(s); err != nil {
+		return err
+	}
+	if hasBiDiRandALCat(s) {
+		if err := checkBiDiLCat(s); err != nil {
+			return err
+		}
+		if err := checkBadFirstAndLastRandALCat(s); err != nil {
+			return err
+		}
+	}
+	return nil
+}