float_to_string.cpp

#include <cmath>
#include <limits>
#include <iostream>
#include "melanolib/math/math.hpp"

using namespace melanolib;


template<class Float>
std::string float_to_string(Float value)
{
    if ( std::isnan(value) )
        return "NaN";

    if ( std::isinf(value) )
    {
        if ( value < 0 )
            return "-Inf";
        else
            return "Inf";
    }

    static const int base = 10;
    int k = math::ceil(math::log(value, base));

    std::string digits;
    Float q = value / math::pow(base, k);
    for ( int i = 0; i < 10; i++ )
    {
        Float scaled = q * base;
        int digit = math::floor(scaled);
        q = scaled - digit;
        digits += '0' + digit;
    }
    int next_digit = math::floor(q * base);
    if ( next_digit >= 5 )
    {
        for ( auto it = digits.rbegin(); it != digits.rend(); ++it )
        {
            *it += 1;
            if ( *it > '9' )
                *it = '0';
            else
                break;
        }
    }

    return "0." + digits + "e" + std::to_string(k);
}