Function myStod

lib/mathlib.cpp:461–493

in-place conversion of (sub)string to double. Requires no heap.

// in-place conversion of (sub)string to double. Requires no heap.
static double myStod(const std::string& str, std::string::const_iterator from, std::string::const_iterator to, int base)
{
    double result = 0.;
    bool positivesign = true;
    std::string::const_iterator it;
    if ('+' == *from) {
        it = from + 1;
    } else if ('-' == *from) {
        it = from + 1;
        positivesign = false;
    } else
        it = from;
    const std::size_t decimalsep = str.find('.', it-str.begin());
    int distance;
    if (std::string::npos == decimalsep) {
        distance = to - it;
    } else if (decimalsep > (to - str.begin()))
        return 0.; // error handling??
    else
        distance = int(decimalsep)-(from - str.begin());
    auto digitval = [&](char c) {
        if ((10 < base) && (c > '9'))
            return 10 + std::tolower(c) - 'a';
        return c - '0';
    };
    for (; it!=to; ++it) {
        if ('.' == *it)
            continue;
        --distance;
        result += digitval(*it)* std::pow(base, distance);
    }
    return positivesign ? result : -result;
}

// Assuming a limited support of built-in hexadecimal floats (see C99, C++17) that is a fall-back implementation.
// Performance has been optimized WRT to heap activity, however the calculation part is not optimized.