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) + 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.
47func 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.
57func 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.
69func Encode(s []rune) []uint16 {
70	n := len(s)
71	for _, v := range s {
72		if v >= surrSelf {
73			n++
74		}
75	}
76
77	a := make([]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.
100func 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.
116func 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 := make([]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.
125func 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