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.
6 package 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 12 const (
13 replacementChar = '\uFFFD' // Unicode replacement character
14 maxRune = '\U0010FFFF' // Maximum valid Unicode code point.
15 )
16 17 const (
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.
30 func 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.
37 func DecodeRune(r1, r2 rune) rune {
38 if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
39 return (r1-surr1)<<10 | (r2 - surr2) + surrSelf
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.
47 func EncodeRune(r rune) (r1, r2 rune) {
48 if r < surrSelf || r > maxRune {
49 return replacementChar, replacementChar
50 }
51 r -= surrSelf
52 return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
53 }
54 55 // RuneLen returns the number of 16-bit words in the UTF-16 encoding of the rune.
56 // It returns -1 if the rune is not a valid value to encode in UTF-16.
57 func RuneLen(r rune) int {
58 switch {
59 case 0 <= r && r < surr1, surr3 <= r && r < surrSelf:
60 return 1
61 case surrSelf <= r && r <= maxRune:
62 return 2
63 default:
64 return -1
65 }
66 }
67 68 // Encode returns the UTF-16 encoding of the Unicode code point sequence s.
69 func Encode(s []rune) []uint16 {
70 n := len(s)
71 for _, v := range s {
72 if v >= surrSelf {
73 n++
74 }
75 }
76 77 a := []uint16{:n}
78 n = 0
79 for _, v := range s {
80 switch RuneLen(v) {
81 case 1: // normal rune
82 a[n] = uint16(v)
83 n++
84 case 2: // needs surrogate sequence
85 r1, r2 := EncodeRune(v)
86 a[n] = uint16(r1)
87 a[n+1] = uint16(r2)
88 n += 2
89 default:
90 a[n] = uint16(replacementChar)
91 n++
92 }
93 }
94 return a[:n]
95 }
96 97 // AppendRune appends the UTF-16 encoding of the Unicode code point r
98 // to the end of p and returns the extended buffer. If the rune is not
99 // a valid Unicode code point, it appends the encoding of U+FFFD.
100 func AppendRune(a []uint16, r rune) []uint16 {
101 // This function is inlineable for fast handling of ASCII.
102 switch {
103 case 0 <= r && r < surr1, surr3 <= r && r < surrSelf:
104 // normal rune
105 return append(a, uint16(r))
106 case surrSelf <= r && r <= maxRune:
107 // needs surrogate sequence
108 r1, r2 := EncodeRune(r)
109 return append(a, uint16(r1), uint16(r2))
110 }
111 return append(a, replacementChar)
112 }
113 114 // Decode returns the Unicode code point sequence represented
115 // by the UTF-16 encoding s.
116 func Decode(s []uint16) []rune {
117 // Preallocate capacity to hold up to 64 runes.
118 // Decode inlines, so the allocation can live on the stack.
119 buf := []rune{:0:64}
120 return decode(s, buf)
121 }
122 123 // decode appends to buf the Unicode code point sequence represented
124 // by the UTF-16 encoding s and return the extended buffer.
125 func decode(s []uint16, buf []rune) []rune {
126 for i := 0; i < len(s); i++ {
127 var ar rune
128 switch r := s[i]; {
129 case r < surr1, surr3 <= r:
130 // normal rune
131 ar = rune(r)
132 case surr1 <= r && r < surr2 && i+1 < len(s) &&
133 surr2 <= s[i+1] && s[i+1] < surr3:
134 // valid surrogate sequence
135 ar = DecodeRune(rune(r), rune(s[i+1]))
136 i++
137 default:
138 // invalid surrogate sequence
139 ar = replacementChar
140 }
141 buf = append(buf, ar)
142 }
143 return buf
144 }
145