func UnquoteString()

in confgenerator/filter/internal/ast/ast.go [710:806]
88 lines of code
26 McCabe index (conditional complexity)

func UnquoteString(in string) (string, error) {
	var buf strings.Builder
	buf.Grow(3 * len(in) / 2)

	r := strings.NewReader(in)
	for {
		c, _, err := r.ReadRune()
		if err != nil {
			// EOF is only possible error
			break
		}
		if c != '\\' {
			buf.WriteRune(c)
			continue
		}
		c, _, err = r.ReadRune()
		if err != nil {
			buf.WriteRune('\\')
			break
		}
		switch c {
		case ',', ':', '=', '<', '>', '+', '~', '"', '\\', '.', '*':
			// escaped_char
			buf.WriteRune(c)
		case 'u':
			// unicode_esc
			// [0-9a-f]{4}
			digits := make([]byte, 4)
			n, _ := r.Read(digits)
			digits = digits[:n]
			codepoint, err := strconv.ParseUint(string(digits), 16, 16)
			if n < 4 || err != nil {
				buf.WriteRune('\\')
				buf.WriteRune('u')
				buf.Write(digits)
				break
			}
			buf.WriteRune(rune(codepoint))
		case '0', '1', '2', '3', '4', '5', '6', '7':
			// octal_esc
			// [0-7]{1,2} or [0-3],[0-7]{2}
			digits := []byte{byte(c)}
			for len(digits) < 3 {
				c, err := r.ReadByte()
				if err != nil {
					break
				}
				if c < '0' || c > '7' {
					r.UnreadByte()
					break
				}
				digits = append(digits, c)
				if digits[0] > '3' && len(digits) == 2 {
					break
				}
			}
			codepoint, err := strconv.ParseUint(string(digits), 8, 8)
			if err != nil {
				buf.WriteRune('\\')
				buf.Write(digits)
				break
			}
			buf.WriteRune(rune(codepoint))
		case 'x':
			// hex_esc:
			// 2*hex_digit
			digits := make([]byte, 2)
			n, _ := r.Read(digits)
			digits = digits[:n]
			codepoint, err := strconv.ParseUint(string(digits), 16, 8)
			if n < 2 || err != nil {
				buf.WriteRune('\\')
				buf.WriteRune('x')
				buf.Write(digits)
				break
			}
			buf.WriteRune(rune(codepoint))
		case 'a':
			buf.WriteRune('\a')
		case 'b':
			buf.WriteRune('\b')
		case 'f':
			buf.WriteRune('\f')
		case 'n':
			buf.WriteRune('\n')
		case 'r':
			buf.WriteRune('\r')
		case 't':
			buf.WriteRune('\t')
		case 'v':
			buf.WriteRune('\v')
		default:
			return "", fmt.Errorf(`invalid escape sequence: \%s`, string(c))
		}
	}
	return buf.String(), nil
}