1 files changed, 133 insertions, 0 deletions
diff --git a/src/unicode/utf16/utf16.go b/src/unicode/utf16/utf16.go
new file mode 100644
index 0000000..1c6d2c6
--- /dev/null
+++ b/src/unicode/utf16/utf16.go
@@ -0,0 +1,133 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package utf16 implements encoding and decoding of UTF-16 sequences.
+package utf16
+
+// The conditions replacementChar==unicode.ReplacementChar and
+// maxRune==unicode.MaxRune are verified in the tests.
+// Defining them locally avoids this package depending on package unicode.
+
+const (
+	replacementChar = '\uFFFD'     // Unicode replacement character
+	maxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
+)
+
+const (
+	// 0xd800-0xdc00 encodes the high 10 bits of a pair.
+	// 0xdc00-0xe000 encodes the low 10 bits of a pair.
+	// the value is those 20 bits plus 0x10000.
+	surr1 = 0xd800
+	surr2 = 0xdc00
+	surr3 = 0xe000
+
+	surrSelf = 0x10000
+)
+
+// IsSurrogate reports whether the specified Unicode code point
+// can appear in a surrogate pair.
+func IsSurrogate(r rune) bool {
+	return surr1 <= r && r < surr3
+}
+
+// DecodeRune returns the UTF-16 decoding of a surrogate pair.
+// If the pair is not a valid UTF-16 surrogate pair, DecodeRune returns
+// the Unicode replacement code point U+FFFD.
+func DecodeRune(r1, r2 rune) rune {
+	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
+		return (r1-surr1)<<10 | (r2 - surr2) + surrSelf
+	}
+	return replacementChar
+}
+
+// EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
+// If the rune is not a valid Unicode code point or does not need encoding,
+// EncodeRune returns U+FFFD, U+FFFD.
+func EncodeRune(r rune) (r1, r2 rune) {
+	if r < surrSelf || r > maxRune {
+		return replacementChar, replacementChar
+	}
+	r -= surrSelf
+	return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
+}
+
+// Encode returns the UTF-16 encoding of the Unicode code point sequence s.
+func Encode(s []rune) []uint16 {
+	n := len(s)
+	for _, v := range s {
+		if v >= surrSelf {
+			n++
+		}
+	}
+
+	a := make([]uint16, n)
+	n = 0
+	for _, v := range s {
+		switch {
+		case 0 <= v && v < surr1, surr3 <= v && v < surrSelf:
+			// normal rune
+			a[n] = uint16(v)
+			n++
+		case surrSelf <= v && v <= maxRune:
+			// needs surrogate sequence
+			r1, r2 := EncodeRune(v)
+			a[n] = uint16(r1)
+			a[n+1] = uint16(r2)
+			n += 2
+		default:
+			a[n] = uint16(replacementChar)
+			n++
+		}
+	}
+	return a[:n]
+}
+
+// AppendRune appends the UTF-16 encoding of the Unicode code point r
+// to the end of p and returns the extended buffer. If the rune is not
+// a valid Unicode code point, it appends the encoding of U+FFFD.
+func AppendRune(a []uint16, r rune) []uint16 {
+	// This function is inlineable for fast handling of ASCII.
+	switch {
+	case 0 <= r && r < surr1, surr3 <= r && r < surrSelf:
+		// normal rune
+		return append(a, uint16(r))
+	case surrSelf <= r && r <= maxRune:
+		// needs surrogate sequence
+		r1, r2 := EncodeRune(r)
+		return append(a, uint16(r1), uint16(r2))
+	}
+	return append(a, replacementChar)
+}
+
+// Decode returns the Unicode code point sequence represented
+// by the UTF-16 encoding s.
+func Decode(s []uint16) []rune {
+	// Preallocate capacity to hold up to 64 runes.
+	// Decode inlines, so the allocation can live on the stack.
+	buf := make([]rune, 0, 64)
+	return decode(s, buf)
+}
+
+// decode appends to buf the Unicode code point sequence represented
+// by the UTF-16 encoding s and return the extended buffer.
+func decode(s []uint16, buf []rune) []rune {
+	for i := 0; i < len(s); i++ {
+		var ar rune
+		switch r := s[i]; {
+		case r < surr1, surr3 <= r:
+			// normal rune
+			ar = rune(r)
+		case surr1 <= r && r < surr2 && i+1 < len(s) &&
+			surr2 <= s[i+1] && s[i+1] < surr3:
+			// valid surrogate sequence
+			ar = DecodeRune(rune(r), rune(s[i+1]))
+			i++
+		default:
+			// invalid surrogate sequence
+			ar = replacementChar
+		}
+		buf = append(buf, ar)
+	}
+	return buf
+}