--- /dev/null
+package strconv
+
+import (
+ "errors"
+ "unicode/utf8"
+)
+
+// ErrSyntax indicates that a value does not have the right syntax for the target type.
+var ErrSyntax = errors.New("invalid syntax")
+
+// Unquote interprets s as a single-quoted, double-quoted,
+// or backquoted Go string literal, returning the string value
+// that s quotes. (If s is single-quoted, it would be a Go
+// character literal; Unquote returns the corresponding
+// one-character string.)
+func Unquote(s string) (t string, err error) {
+ n := len(s)
+ if n < 2 {
+ return "", ErrSyntax
+ }
+ quote := s[0]
+ if quote != s[n-1] {
+ return "", ErrSyntax
+ }
+ s = s[1 : n-1]
+
+ if quote != '"' {
+ return "", ErrSyntax
+ }
+ if !contains(s, '$') && !contains(s, '{') && contains(s, '\n') {
+ return "", ErrSyntax
+ }
+
+ // Is it trivial? Avoid allocation.
+ if !contains(s, '\\') && !contains(s, quote) && !contains(s, '$') {
+ switch quote {
+ case '"':
+ return s, nil
+ case '\'':
+ r, size := utf8.DecodeRuneInString(s)
+ if size == len(s) && (r != utf8.RuneError || size != 1) {
+ return s, nil
+ }
+ }
+ }
+
+ var runeTmp [utf8.UTFMax]byte
+ buf := make([]byte, 0, 3*len(s)/2) // Try to avoid more allocations.
+ for len(s) > 0 {
+ // If we're starting a '${}' then let it through un-unquoted.
+ // Specifically: we don't unquote any characters within the `${}`
+ // section.
+ if s[0] == '$' && len(s) > 1 && s[1] == '{' {
+ buf = append(buf, '$', '{')
+ s = s[2:]
+
+ // Continue reading until we find the closing brace, copying as-is
+ braces := 1
+ for len(s) > 0 && braces > 0 {
+ r, size := utf8.DecodeRuneInString(s)
+ if r == utf8.RuneError {
+ return "", ErrSyntax
+ }
+
+ s = s[size:]
+
+ n := utf8.EncodeRune(runeTmp[:], r)
+ buf = append(buf, runeTmp[:n]...)
+
+ switch r {
+ case '{':
+ braces++
+ case '}':
+ braces--
+ }
+ }
+ if braces != 0 {
+ return "", ErrSyntax
+ }
+ if len(s) == 0 {
+ // If there's no string left, we're done!
+ break
+ } else {
+ // If there's more left, we need to pop back up to the top of the loop
+ // in case there's another interpolation in this string.
+ continue
+ }
+ }
+
+ if s[0] == '\n' {
+ return "", ErrSyntax
+ }
+
+ c, multibyte, ss, err := unquoteChar(s, quote)
+ if err != nil {
+ return "", err
+ }
+ s = ss
+ if c < utf8.RuneSelf || !multibyte {
+ buf = append(buf, byte(c))
+ } else {
+ n := utf8.EncodeRune(runeTmp[:], c)
+ buf = append(buf, runeTmp[:n]...)
+ }
+ if quote == '\'' && len(s) != 0 {
+ // single-quoted must be single character
+ return "", ErrSyntax
+ }
+ }
+ return string(buf), nil
+}
+
+// contains reports whether the string contains the byte c.
+func contains(s string, c byte) bool {
+ for i := 0; i < len(s); i++ {
+ if s[i] == c {
+ return true
+ }
+ }
+ return false
+}
+
+func unhex(b byte) (v rune, ok bool) {
+ c := rune(b)
+ switch {
+ case '0' <= c && c <= '9':
+ return c - '0', true
+ case 'a' <= c && c <= 'f':
+ return c - 'a' + 10, true
+ case 'A' <= c && c <= 'F':
+ return c - 'A' + 10, true
+ }
+ return
+}
+
+func unquoteChar(s string, quote byte) (value rune, multibyte bool, tail string, err error) {
+ // easy cases
+ switch c := s[0]; {
+ case c == quote && (quote == '\'' || quote == '"'):
+ err = ErrSyntax
+ return
+ case c >= utf8.RuneSelf:
+ r, size := utf8.DecodeRuneInString(s)
+ return r, true, s[size:], nil
+ case c != '\\':
+ return rune(s[0]), false, s[1:], nil
+ }
+
+ // hard case: c is backslash
+ if len(s) <= 1 {
+ err = ErrSyntax
+ return
+ }
+ c := s[1]
+ s = s[2:]
+
+ switch c {
+ case 'a':
+ value = '\a'
+ case 'b':
+ value = '\b'
+ case 'f':
+ value = '\f'
+ case 'n':
+ value = '\n'
+ case 'r':
+ value = '\r'
+ case 't':
+ value = '\t'
+ case 'v':
+ value = '\v'
+ case 'x', 'u', 'U':
+ n := 0
+ switch c {
+ case 'x':
+ n = 2
+ case 'u':
+ n = 4
+ case 'U':
+ n = 8
+ }
+ var v rune
+ if len(s) < n {
+ err = ErrSyntax
+ return
+ }
+ for j := 0; j < n; j++ {
+ x, ok := unhex(s[j])
+ if !ok {
+ err = ErrSyntax
+ return
+ }
+ v = v<<4 | x
+ }
+ s = s[n:]
+ if c == 'x' {
+ // single-byte string, possibly not UTF-8
+ value = v
+ break
+ }
+ if v > utf8.MaxRune {
+ err = ErrSyntax
+ return
+ }
+ value = v
+ multibyte = true
+ case '0', '1', '2', '3', '4', '5', '6', '7':
+ v := rune(c) - '0'
+ if len(s) < 2 {
+ err = ErrSyntax
+ return
+ }
+ for j := 0; j < 2; j++ { // one digit already; two more
+ x := rune(s[j]) - '0'
+ if x < 0 || x > 7 {
+ err = ErrSyntax
+ return
+ }
+ v = (v << 3) | x
+ }
+ s = s[2:]
+ if v > 255 {
+ err = ErrSyntax
+ return
+ }
+ value = v
+ case '\\':
+ value = '\\'
+ case '\'', '"':
+ if c != quote {
+ err = ErrSyntax
+ return
+ }
+ value = rune(c)
+ default:
+ err = ErrSyntax
+ return
+ }
+ tail = s
+ return
+}
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