// helloworld.cpp

import <iostream>;

int main()
{
	std::cout << "Hello, World!" << std::endl;
	return 0;
}

// interface file for a roman numerals module

export module roman;

import <string>;
import <string_view>;

export std::string to_roman(unsigned int i);
export unsigned int from_roman(std::string_view roman);

// implementation of the to_roman() function

module roman;

std::string to_roman(unsigned int i)
{
   if ( i > 3999 ) return {}; // 3999, or MMMCMXCIX, is the largest standard Roman numeral

   static const std::string ms[] { "","M","MM","MMM" };
   static const std::string cds[] { "","C","CC","CCC","CD","D","DC","DCC","DCCC","CM" };
   static const std::string xls[] { "","X","XX","XXX","XL","L","LX","LXX","LXXX","XC" };
   static const std::string ivs[] { "","I","II","III","IV","V","VI","VII","VIII","IX" };

   return ms[ i / 1000 ] + cds[ (i % 1000) / 100 ] + xls[ (i % 100) / 10 ] + ivs[ i % 10 ];
}

// implementation of the from_roman() function

module roman;

unsigned int from_roman(char c)
{
   switch ( c )
   {
   case 'I': return 1;    case 'V': return 5;   case 'X': return 10;
   case 'L': return 50;   case 'C': return 100; case 'D': return 500;
   case 'M': return 1000; default:  return 0;
   }
}

unsigned int from_roman(std::string_view roman)
{
   unsigned int result {};

   for ( size_t i {}, n { roman.length() }; i < n; ++i )
   {
      const auto j { from_roman(roman[ i ]) };   // integer value of the i'th roman digit

      // look at the next digit (if there is one) to know whether to add or subtract j
      if ( i + 1 == n || j >= from_roman(roman[ i + 1 ]) ) result += j; else result -= j;
   }

   return result;
}

// defining functions in module implementation files

import <iostream>;
import <string>;

import roman;

int main()
{
   std::cout << "1234 in Roman numerals is " << to_roman(1234) << '\n';
   std::cout << "MMXX in Arabic numerals is " << from_roman("MMXX") << '\n';
}