Reply to https://r3dux.org/2017/08/how-to-convert-day-number

// --------------------------------------------------------------------------------
// Reply to https://r3dux.org/2017/08/how-to-convert-day-number-to-month-and-day/
// No error checking, this is just an example implementation of the date algorithms
// from: https://alcor.concordia.ca/~gpkatch/gdate-method.html.
// --------------------------------------------------------------------------------

#include <iostream>
#include <vector>

typedef std::vector<std::string> stringvec;
typedef unsigned long ul;
typedef signed long sl;

// --------------------------------------------------------------------------------
// Given the year, month and day, return the day number.
// --------------------------------------------------------------------------------
ul CalcDayNumFromDate(ul y, ul m, ul d)
{
  m = (m + 9) % 12;
  y -= m / 10;
  ul dn = 365*y + y/4 - y/100 + y/400 + (m*306 + 5)/10 + (d - 1);
  return dn;
}


// --------------------------------------------------------------------------------
// Given a day, calculate and return the date (in a string vector).
// --------------------------------------------------------------------------------
template<typename OUT>
void CalcDateFromDayNum(ul day, OUT &retval)
{
  ul y = (10000*day + 14780) / 3652425;
  sl ddd = day - (365*y + y/4 - y/100 + y/400);
  if (ddd < 0) {
    y--;
    ddd = day - (365*y + y/4 + y/100 + y/400);
  }

  ul mi = (100*ddd + 52) / 3060;
  ul mm = (mi + 2) % 12 + 1;
  y += (mi + 2) / 12;
  ul dd = ddd - (mi*306 + 5) / 10 + 1;

  retval.push_back(std::to_string(y));
  retval.push_back(std::to_string(mm));
  retval.push_back(std::to_string(dd));
}

// --------------------------------------------------------------------------------
// Given the year and a day number, calculate and return the month and day.
// --------------------------------------------------------------------------------
stringvec CalcDayAndMonth(ul year, ul day)
{
  stringvec date;

  ul start_day = CalcDayNumFromDate(year, 1, 1);
  CalcDateFromDayNum(start_day + day - 1, date);

  return date;
}

// --------------------------------------------------------------------------------
// Program entry point.
// --------------------------------------------------------------------------------
int main(int argc, char **argv)
{
  std::string year, day;

  std::string month[] =
      {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul",
       "Aug", "Sep", "Oct", "Nov", "Dec"};

  std::cout << "Enter the year (YYYY): ";
  getline(std::cin, year);
  std::cout << "Enter the day (between 0 and 365): ";
  getline(std::cin, day);

  stringvec date = CalcDayAndMonth(std::atoi(year.c_str()), std::atoi(day.c_str()));

  std::cout << "Month: " << month[std::atoi(date[1].c_str()) - 1]
            << ", Day: " << date[2] << std::endl;

  return 0;
}