#include <iostream>

long week(
  int day,
  int month,
  int year
){
  // Returns the number of the ISO week in the year (from 1 to 53)
  // containing the given date (assumed valid, and in the Gregorian
  // calendar); more exactly, the returned value is yyyyww, where yyyy
  // is the year and ww the ISO week number.

  int m, start, yearDay, lastDay, lastSunday;

  const int nMonths = 12;

  static int monthDays[nMonths + 1] = {
    0,
    31,  0, 31, 30, 31, 30,
    31, 31, 30, 31, 30, 31
  };

  // - Initializes correctly the number of days in February;
  // - stores in lastDay the total number of days in the given year;
  // - stores in yearDay the total number of days in the year not
  //   after the given date ("day-of-the-year").

  if (isLeap(year)) {
    monthDays[2] = 29;
    lastDay      = 366;
  } else {
    monthDays[2] = 28;
    lastDay      = 365;
  }

  for (m = 0,  yearDay = day;   ++m < month;   ) {
    yearDay += monthDays[m];
  }

  // - Computes in "start" the first day of the first week of this
  //   year ("start" may be negative if the beginning of the first
  //   week is in the previous year);
  // - computes in "lastSunday" the date of the last Sunday in
  //   December.
  //
  // weekDay(day, month, year) is a procedure that computes the day of
  // the week corresponding to a given date, returning 0 for Monday, 1
  // for Tuesday, ..., 6 for Sunday.

  start      = 4 - weekDay(4, 1, year);
  lastSunday = weekDay(31, 12, year);
  lastSunday = lastSunday == 6 ? 31 : 30-lastSunday;

  std::cout << "- The first ISO week of the year " << year
            << " begins on Monday ";
  if (start < 1) {
    std::cout << 31+start << "-Dec-" << year-1 << std::endl;
  } else {
    std::cout << start << "-Jan-" << year << std::endl;
  }

  // After the following printout, "lastSunday" is changed to the
  // day-of-the-year number of the Sunday ending the last ISO week of
  // the current year.

  std::cout << "- The last ISO week of the year " << year
            << " ends on Sunday ";
  if (31-lastSunday < 4) {
    std::cout << lastSunday << "-Dec-" << year << std::endl;
    lastSunday = lastDay - (31 - lastSunday);
  } else {
    std::cout << lastSunday+7-31 << "-Jan-" << year+1 << std::endl;
    lastSunday = lastDay + lastSunday+7-31;
  }

  // - If the day-of-the-year is before "start", we are in the last
  //   week of the previous year (the week of December 31);
  // - if not, and if the given date is after "lastSunday", we are in
  //   the first week of the next year;
  // - in all the other cases, we compute the number of the week
  //   normally.

  if (yearDay < start)      return week(31, 12, year-1);
  if (yearDay > lastSunday) return (year + 1)*100 + 1;

  return year*100 + ((yearDay - start) / 7 + 1);
}