todouble.cpp

double AsDouble(const char *p, const uint32_t len)
{
                const char *it = p, *const e = p + len;
                double sign;

                if (it == e)
                        return 0;
                else if (*it == '-')
                {
                        ++it;
                        sign = -1;
                }
                else if (unlikely(*it == '+'))
                {
                        ++it;
                        sign = 1;
                }
                else
                        sign = 1;

                double v{0};

                do
                {
                        if (*it == '.' || *it == ',')   // support both radix characters
                        {
                                double exp{0};
                                double pow10{1.0}; 

                                for (++it; it != e; ++it)
                                {
                                        if (likely(isdigit(*it)))
                                        {
                                                exp = exp * 10 + (*it- '0');
                                                pow10*=10.0;
                                        }
                                        else
                                                return NAN;
                                        
                                }

                                return (v + (exp * (1.0L / pow10))) * sign;
                        }
                        else if (likely(isdigit(*it)))
                                v = v * 10 + (*(it++) - '0');
                        else
                                return NAN;

                } while (it != e);
}