Tries to parse a floating point number located at s. s_end should be a location in the string where reading should absolutely stop. For example at the end of the string, to prevent buffer overflows. Parses the following EBNF grammar: sign = "+" | "-" ; END = ? anything not in digit ? digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ; integer = [sign] , digit , {digit} ;
| 864 | // - parse failure. |
| 865 | // |
| 866 | static bool tryParseDouble(const char *s, const char *s_end, double *result) { |
| 867 | if (s >= s_end) { |
| 868 | return false; |
| 869 | } |
| 870 | |
| 871 | double mantissa = 0.0; |
| 872 | // This exponent is base 2 rather than 10. |
| 873 | // However the exponent we parse is supposed to be one of ten, |
| 874 | // thus we must take care to convert the exponent/and or the |
| 875 | // mantissa to a * 2^E, where a is the mantissa and E is the |
| 876 | // exponent. |
| 877 | // To get the final double we will use ldexp, it requires the |
| 878 | // exponent to be in base 2. |
| 879 | int exponent = 0; |
| 880 | |
| 881 | // NOTE: THESE MUST BE DECLARED HERE SINCE WE ARE NOT ALLOWED |
| 882 | // TO JUMP OVER DEFINITIONS. |
| 883 | char sign = '+'; |
| 884 | char exp_sign = '+'; |
| 885 | char const *curr = s; |
| 886 | |
| 887 | // How many characters were read in a loop. |
| 888 | int read = 0; |
| 889 | // Tells whether a loop terminated due to reaching s_end. |
| 890 | bool end_not_reached = false; |
| 891 | bool leading_decimal_dots = false; |
| 892 | |
| 893 | /* |
| 894 | BEGIN PARSING. |
| 895 | */ |
| 896 | |
| 897 | // Find out what sign we've got. |
| 898 | if (*curr == '+' || *curr == '-') { |
| 899 | sign = *curr; |
| 900 | curr++; |
| 901 | if ((curr != s_end) && (*curr == '.')) { |
| 902 | // accept. Somethig like `.7e+2`, `-.5234` |
| 903 | leading_decimal_dots = true; |
| 904 | } |
| 905 | } else if (IS_DIGIT(*curr)) { /* Pass through. */ |
| 906 | } else if (*curr == '.') { |
| 907 | // accept. Somethig like `.7e+2`, `-.5234` |
| 908 | leading_decimal_dots = true; |
| 909 | } else { |
| 910 | goto fail; |
| 911 | } |
| 912 | |
| 913 | // Read the integer part. |
| 914 | end_not_reached = (curr != s_end); |
| 915 | if (!leading_decimal_dots) { |
| 916 | while (end_not_reached && IS_DIGIT(*curr)) { |
| 917 | mantissa *= 10; |
| 918 | mantissa += static_cast<int>(*curr - 0x30); |
| 919 | curr++; |
| 920 | read++; |
| 921 | end_not_reached = (curr != s_end); |
| 922 | } |
| 923 |