#include <stdexcept>

const long   frenchEpoch       = 2375840;
const double daysPerFrenchYear = 1460969.0 / 4000.0;

// The names of the 5 (or 6) additional days at the end of the normal
// calendar (that has 12 months of 30 days each, or 360 days)

static const char * sansCulNames [] = {
  "Invalide",
  "Jour de la Vertu",  "Jour du Genie",        "Jour du Travail",
  "Jour de la Raison", "Jour des Recompenses", "Jour de la Revolution"
};

// The names of the 12 months (coined by Fabre d'Eglantine)

static const char * frenchMonthsNames [] = {
  "Invalide",
  "Vendemiaire", "Brumaire", "Frimaire",  "Nivose",
  "Pluviose",    "Ventose",  "Germinal",  "Floreal",
  "Prairial",    "Messidor", "Thermidor", "Fructidor"
};

long french2jul(
  int fd,
  int fm,
  int fy
) {
  // From Edward M. Reingold and Nachum Dershowitz - "Calendrical
  // Calculations: The Millennium Edition" (Cambridge University
  // Press, 2001) - page 238
  //
  // Essentially, all the days before the given date are summed to the
  // Julian Day number corresponding to the first year of the French
  // Revolutionary Calendar.

  if (fy < 1) {
    throw std::domain_error("Before the French Revolution");
  }

  long result = frenchEpoch - 1;

  result += 365 * --fy;
  result += fy / 4;
  result -= fy / 100;
  result += fy / 400;
  result -= fy / 4000;
  result += 30 * --fm;
  result += fd;

  return result;
}

void jul2french(
  long jd,
  int & fd,
  int & fm,
  int & fy
) {
  // From Edward M. Reingold and Nachum Dershowitz - "Calendrical
  // Calculations: The Millennium Edition" (Cambridge University
  // Press, 2001) - page 238
  //
  // A first approximation of the year in the French Revolutionary
  // Calendar is found, and refined; then, the precise date is
  // computed from the Julian Day number corresponding to the start of
  // the current Revolutionary year.

  if (jd < frenchEpoch) {
    throw std::domain_error("Before the French Revolution");
  }

  long approx = jd - frenchEpoch + 2;

  fy = static_cast<int>(static_cast<double>(approx)/daysPerFrenchYear) + 1;
  long startOfYear = french2jul(1, 1, fy);

  if  (jd < startOfYear) {
    startOfYear = french2jul(1, 1, --fy); // We were wrong by 1 year...
  }
  fm = (jd -= startOfYear) / 30;
  fd = jd - 30 * fm++ + 1;
}