1 files changed, 92 insertions, 0 deletions
diff --git a/src/syscall/wtf8_windows.go b/src/syscall/wtf8_windows.go
new file mode 100644
index 0000000..f166021
--- /dev/null
+++ b/src/syscall/wtf8_windows.go
@@ -0,0 +1,92 @@
+// Copyright 2023 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.
+
+// Windows UTF-16 strings can contain unpaired surrogates, which can't be
+// decoded into a valid UTF-8 string. This file defines a set of functions
+// that can be used to encode and decode potentially ill-formed UTF-16 strings
+// by using the [the WTF-8 encoding](https://simonsapin.github.io/wtf-8/).
+//
+// WTF-8 is a strict superset of UTF-8, i.e. any string that is
+// well-formed in UTF-8 is also well-formed in WTF-8 and the content
+// is unchanged. Also, the conversion never fails and is lossless.
+//
+// The benefit of using WTF-8 instead of UTF-8 when decoding a UTF-16 string
+// is that the conversion is lossless even for ill-formed UTF-16 strings.
+// This property allows to read an ill-formed UTF-16 string, convert it
+// to a Go string, and convert it back to the same original UTF-16 string.
+//
+// See go.dev/issues/59971 for more info.
+
+package syscall
+
+import (
+	"unicode/utf16"
+	"unicode/utf8"
+)
+
+const (
+	surr1 = 0xd800
+	surr2 = 0xdc00
+	surr3 = 0xe000
+
+	tx    = 0b10000000
+	t3    = 0b11100000
+	maskx = 0b00111111
+	mask3 = 0b00001111
+
+	rune1Max = 1<<7 - 1
+	rune2Max = 1<<11 - 1
+)
+
+// encodeWTF16 returns the potentially ill-formed
+// UTF-16 encoding of s.
+func encodeWTF16(s string, buf []uint16) []uint16 {
+	for i := 0; i < len(s); {
+		// Cannot use 'for range s' because it expects valid
+		// UTF-8 runes.
+		r, size := utf8.DecodeRuneInString(s[i:])
+		if r == utf8.RuneError {
+			// Check if s[i:] contains a valid WTF-8 encoded surrogate.
+			if sc := s[i:]; len(sc) >= 3 && sc[0] == 0xED && 0xA0 <= sc[1] && sc[1] <= 0xBF && 0x80 <= sc[2] && sc[2] <= 0xBF {
+				r = rune(sc[0]&mask3)<<12 + rune(sc[1]&maskx)<<6 + rune(sc[2]&maskx)
+				buf = append(buf, uint16(r))
+				i += 3
+				continue
+			}
+		}
+		i += size
+		buf = utf16.AppendRune(buf, r)
+	}
+	return buf
+}
+
+// decodeWTF16 returns the WTF-8 encoding of
+// the potentially ill-formed UTF-16 s.
+func decodeWTF16(s []uint16, buf []byte) []byte {
+	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 = utf16.DecodeRune(rune(r), rune(s[i+1]))
+			i++
+		default:
+			// WTF-8 fallback.
+			// This only handles the 3-byte case of utf8.AppendRune,
+			// as surrogates always fall in that case.
+			ar = rune(r)
+			if ar > utf8.MaxRune {
+				ar = utf8.RuneError
+			}
+			buf = append(buf, t3|byte(ar>>12), tx|byte(ar>>6)&maskx, tx|byte(ar)&maskx)
+			continue
+		}
+		buf = utf8.AppendRune(buf, ar)
+	}
+	return buf
+}