1// Copyright 2023 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// Windows UTF-16 strings can contain unpaired surrogates, which can't be
6// decoded into a valid UTF-8 string. This file defines a set of functions
7// that can be used to encode and decode potentially ill-formed UTF-16 strings
8// by using the [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
9//
10// WTF-8 is a strict superset of UTF-8, i.e. any string that is
11// well-formed in UTF-8 is also well-formed in WTF-8 and the content
12// is unchanged. Also, the conversion never fails and is lossless.
13//
14// The benefit of using WTF-8 instead of UTF-8 when decoding a UTF-16 string
15// is that the conversion is lossless even for ill-formed UTF-16 strings.
16// This property allows to read an ill-formed UTF-16 string, convert it
17// to a Go string, and convert it back to the same original UTF-16 string.
18//
19// See go.dev/issues/59971 for more info.
20
21package syscall
22
23import (
24	"unicode/utf16"
25	"unicode/utf8"
26)
27
28const (
29	surr1 = 0xd800
30	surr2 = 0xdc00
31	surr3 = 0xe000
32
33	tx    = 0b10000000
34	t3    = 0b11100000
35	maskx = 0b00111111
36	mask3 = 0b00001111
37
38	rune1Max = 1<<7 - 1
39	rune2Max = 1<<11 - 1
40)
41
42// encodeWTF16 returns the potentially ill-formed
43// UTF-16 encoding of s.
44func encodeWTF16(s string, buf []uint16) []uint16 {
45	for i := 0; i < len(s); {
46		// Cannot use 'for range s' because it expects valid
47		// UTF-8 runes.
48		r, size := utf8.DecodeRuneInString(s[i:])
49		if r == utf8.RuneError {
50			// Check if s[i:] contains a valid WTF-8 encoded surrogate.
51			if sc := s[i:]; len(sc) >= 3 && sc[0] == 0xED && 0xA0 <= sc[1] && sc[1] <= 0xBF && 0x80 <= sc[2] && sc[2] <= 0xBF {
52				r = rune(sc[0]&mask3)<<12 + rune(sc[1]&maskx)<<6 + rune(sc[2]&maskx)
53				buf = append(buf, uint16(r))
54				i += 3
55				continue
56			}
57		}
58		i += size
59		buf = utf16.AppendRune(buf, r)
60	}
61	return buf
62}
63
64// decodeWTF16 returns the WTF-8 encoding of
65// the potentially ill-formed UTF-16 s.
66func decodeWTF16(s []uint16, buf []byte) []byte {
67	for i := 0; i < len(s); i++ {
68		var ar rune
69		switch r := s[i]; {
70		case r < surr1, surr3 <= r:
71			// normal rune
72			ar = rune(r)
73		case surr1 <= r && r < surr2 && i+1 < len(s) &&
74			surr2 <= s[i+1] && s[i+1] < surr3:
75			// valid surrogate sequence
76			ar = utf16.DecodeRune(rune(r), rune(s[i+1]))
77			i++
78		default:
79			// WTF-8 fallback.
80			// This only handles the 3-byte case of utf8.AppendRune,
81			// as surrogates always fall in that case.
82			ar = rune(r)
83			if ar > utf8.MaxRune {
84				ar = utf8.RuneError
85			}
86			buf = append(buf, t3|byte(ar>>12), tx|byte(ar>>6)&maskx, tx|byte(ar)&maskx)
87			continue
88		}
89		buf = utf8.AppendRune(buf, ar)
90	}
91	return buf
92}
93