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number-converter_v1.h

#include <map>
#include <stdexcept>
#include <string>

// clang-format off

static std::map<char, unsigned int> roman_map = {
    {'I', 1},
    {'V', 5},
    {'X', 10},
    {'L', 50},
    {'C', 100},
    {'D', 500},
    {'M', 1000},
};

// clang-format on

std::string toRoman(unsigned int value)
{
    if((value > 3000) || (value <= 0))
    {
        throw std::invalid_argument("invalid arabic number value - number is too big");
    }
    const std::string M[] = {"", "M", "MM", "MMM"};
    const std::string C[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
    const std::string X[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
    const std::string I[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};

    return M[value / 1000] + C[(value % 1000) / 100] + X[(value % 100) / 10] + I[(value % 10)];
}

unsigned int fromRoman(const std::string &s)
{
    auto value = [&](const char c) -> unsigned int {
        if(roman_map.count(c) <= 0)
        {
            throw std::invalid_argument("invalid roman number value");
        }

        return roman_map[c];
    };

    unsigned int result = 0;

    for(unsigned int i = 0; i < s.length(); i++)
    {
        auto next_index = i + 1;
        auto curr_value = value(s[i]);

        if((next_index >= s.length()) || (value(s[next_index]) <= curr_value))
        {
            result += curr_value;
            continue;
        }

        result -= curr_value;
    }
    return result;
}

number-converter_v2.h

#include <map>
#include <string>
#include <optional>
#include <iostream>

// clang-format off

static std::map<char, unsigned int> roman_map = {
    {'I', 1},
    {'V', 5},
    {'X', 10},
    {'L', 50},
    {'C', 100},
    {'D', 500},
    {'M', 1000},
};

// clang-format on

std::optional<std::string> toRoman(unsigned int value)
{
    if((value > 3000) || (value <= 0))
    {
         return {};
    }
    const std::string M[] = {"", "M", "MM", "MMM"};
    const std::string C[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
    const std::string X[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
    const std::string I[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};

    return M[value / 1000] + C[(value % 1000) / 100] + X[(value % 100) / 10] + I[(value % 10)];
}

constexpr unsigned int ERROR_CONV = -1;

std::optional<int> fromRoman(const std::string &s)
{
    auto value = [&](const char c) -> unsigned int {
        if(roman_map.count(c) <= 0)
        {
            return {};
        }

        return roman_map[c];
    };

    int result = 0;

    for(unsigned int i = 0; i < s.length(); i++)
    {
        auto next_index = i + 1;
        auto curr_value = value(s[i]);
        if (!curr_value)
        {
            return ERROR_CONV;
        }

        if((next_index >= s.length()) || (value(s[next_index]) <= curr_value))
        {
            result += curr_value;
            continue;
        }

        result -= curr_value;
    }
    return result;
}