Add util.NthRune

util/strings.go

@@ -69,3 +69,18 @@ func SubstringByRune(s string, low, high int) (string, error) {
 	}
 	return s[bLow:bHigh], nil
 }
+
+// NthRune returns the n-th rune of s.
+func NthRune(s string, n int) (rune, error) {
+	if n < 0 {
+		return 0, IndexOutOfRange
+	}
+	var j int
+	for _, r := range s {
+		if j == n {
+			return r, nil
+		}
+		j++
+	}
+	return 0, IndexOutOfRange
+}

util/strings_test.go

@@ -46,3 +46,24 @@ func TestSubstringByRune(t *testing.T) {
 		}
 	}
 }
+
+var NthRuneTests = []struct {
+	s          string
+	n          int
+	wantedRune rune
+	wantedErr  error
+}{
+	{"你好世界", -1, 0, IndexOutOfRange},
+	{"你好世界", 0, '你', nil},
+	{"你好世界", 4, 0, IndexOutOfRange},
+}
+
+func TestNthRune(t *testing.T) {
+	for _, tt := range NthRuneTests {
+		r, e := NthRune(tt.s, tt.n)
+		if r != tt.wantedRune || e != tt.wantedErr {
+			t.Errorf("NthRune(%q, %v) => (%q, %v), want (%q, %v)",
+				tt.s, tt.n, r, e, tt.wantedRune, tt.wantedErr)
+		}
+	}
+}