1// Copyright 2010 The Go Authors.  All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package utf16 implements encoding and decoding of UTF-16 sequences.
6package utf16
7
8// The conditions replacementChar==unicode.ReplacementChar and
9// maxRune==unicode.MaxRune are verified in the tests.
10// Defining them locally avoids this package depending on package unicode.
11
12const (
13	replacementChar = '\uFFFD'     // Unicode replacement character
14	maxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
15)
16
17const (
18	// 0xd800-0xdc00 encodes the high 10 bits of a pair.
19	// 0xdc00-0xe000 encodes the low 10 bits of a pair.
20	// the value is those 20 bits plus 0x10000.
21	surr1 = 0xd800
22	surr2 = 0xdc00
23	surr3 = 0xe000
24
25	surrSelf = 0x10000
26)
27
28// IsSurrogate reports whether the specified Unicode code point
29// can appear in a surrogate pair.
30func IsSurrogate(r rune) bool {
31	return surr1 <= r && r < surr3
32}
33
34// DecodeRune returns the UTF-16 decoding of a surrogate pair.
35// If the pair is not a valid UTF-16 surrogate pair, DecodeRune returns
36// the Unicode replacement code point U+FFFD.
37func DecodeRune(r1, r2 rune) rune {
38	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
39		return (r1-surr1)<<10 | (r2 - surr2) + 0x10000
40	}
41	return replacementChar
42}
43
44// EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
45// If the rune is not a valid Unicode code point or does not need encoding,
46// EncodeRune returns U+FFFD, U+FFFD.
47func EncodeRune(r rune) (r1, r2 rune) {
48	if r < surrSelf || r > maxRune || IsSurrogate(r) {
49		return replacementChar, replacementChar
50	}
51	r -= surrSelf
52	return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
53}
54
55// Encode returns the UTF-16 encoding of the Unicode code point sequence s.
56func Encode(s []rune) []uint16 {
57	n := len(s)
58	for _, v := range s {
59		if v >= surrSelf {
60			n++
61		}
62	}
63
64	a := make([]uint16, n)
65	n = 0
66	for _, v := range s {
67		switch {
68		case v < 0, surr1 <= v && v < surr3, v > maxRune:
69			v = replacementChar
70			fallthrough
71		case v < surrSelf:
72			a[n] = uint16(v)
73			n++
74		default:
75			r1, r2 := EncodeRune(v)
76			a[n] = uint16(r1)
77			a[n+1] = uint16(r2)
78			n += 2
79		}
80	}
81	return a[0:n]
82}
83
84// Decode returns the Unicode code point sequence represented
85// by the UTF-16 encoding s.
86func Decode(s []uint16) []rune {
87	a := make([]rune, len(s))
88	n := 0
89	for i := 0; i < len(s); i++ {
90		switch r := s[i]; {
91		case surr1 <= r && r < surr2 && i+1 < len(s) &&
92			surr2 <= s[i+1] && s[i+1] < surr3:
93			// valid surrogate sequence
94			a[n] = DecodeRune(rune(r), rune(s[i+1]))
95			i++
96			n++
97		case surr1 <= r && r < surr3:
98			// invalid surrogate sequence
99			a[n] = replacementChar
100			n++
101		default:
102			// normal rune
103			a[n] = rune(r)
104			n++
105		}
106	}
107	return a[0:n]
108}
109