2 files changed, 49 insertions, 1 deletions
diff --git a/src/pkg/unicode/utf16/utf16.go b/src/pkg/unicode/utf16/utf16.go
index 903e4012a..c0e47c535 100644
--- a/src/pkg/unicode/utf16/utf16.go
+++ b/src/pkg/unicode/utf16/utf16.go
@@ -36,7 +36,7 @@ func IsSurrogate(r rune) bool {
 // the Unicode replacement code point U+FFFD.
 func DecodeRune(r1, r2 rune) rune {
 	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
-		return (rune(r1)-surr1)<<10 | (rune(r2) - surr2) + 0x10000
+		return (r1-surr1)<<10 | (r2 - surr2) + 0x10000
 	}
 	return replacementChar
 }
diff --git a/src/pkg/unicode/utf16/utf16_test.go b/src/pkg/unicode/utf16/utf16_test.go
index ee16a303d..3dca472bb 100644
--- a/src/pkg/unicode/utf16/utf16_test.go
+++ b/src/pkg/unicode/utf16/utf16_test.go
@@ -99,3 +99,51 @@ func TestDecode(t *testing.T) {
 		}
 	}
 }
+
+var decodeRuneTests = []struct {
+	r1, r2 rune
+	want   rune
+}{
+	{0xd800, 0xdc00, 0x10000},
+	{0xd800, 0xdc01, 0x10001},
+	{0xd808, 0xdf45, 0x12345},
+	{0xdbff, 0xdfff, 0x10ffff},
+	{0xd800, 'a', 0xfffd}, // illegal, replacement rune substituted
+}
+
+func TestDecodeRune(t *testing.T) {
+	for i, tt := range decodeRuneTests {
+		got := DecodeRune(tt.r1, tt.r2)
+		if got != tt.want {
+			t.Errorf("%d: DecodeRune(%q, %q) = %v; want %v", i, tt.r1, tt.r2, got, tt.want)
+		}
+	}
+}
+
+var surrogateTests = []struct {
+	r    rune
+	want bool
+}{
+	// from http://en.wikipedia.org/wiki/UTF-16
+	{'\u007A', false},     // LATIN SMALL LETTER Z
+	{'\u6C34', false},     // CJK UNIFIED IDEOGRAPH-6C34 (water)
+	{'\uFEFF', false},     // Byte Order Mark
+	{'\U00010000', false}, // LINEAR B SYLLABLE B008 A (first non-BMP code point)
+	{'\U0001D11E', false}, // MUSICAL SYMBOL G CLEF
+	{'\U0010FFFD', false}, // PRIVATE USE CHARACTER-10FFFD (last Unicode code point)
+
+	{rune(0xd7ff), false}, // surr1-1
+	{rune(0xd800), true},  // surr1
+	{rune(0xdc00), true},  // surr2
+	{rune(0xe000), false}, // surr3
+	{rune(0xdfff), true},  // surr3-1
+}
+
+func TestIsSurrogate(t *testing.T) {
+	for i, tt := range surrogateTests {
+		got := IsSurrogate(tt.r)
+		if got != tt.want {
+			t.Errorf("%d: IsSurrogate(%q) = %v; want %v", i, tt.r, got, tt.want)
+		}
+	}
+}