pe019.cpp

#include <iostream>
#include "pe.h"

/*
You are given the following information, but you may prefer to do some research for yourself.

    1 Jan 1900 was a Monday.
    Thirty days has September,
    April, June and November.
    All the rest have thirty-one,
    Saving February alone,
    Which has twenty-eight, rain or shine.
    And on leap years, twenty-nine.
    A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
*/

int num_sundays_first_month_20th_century() {
    int sundays = 0;
    int day = 1; // 1 Jan 1900 was a Monday
    for (int year = 1900; year <= 2000; year++) {
        for (int month = 1; month <= 12; month++) {
            int days_in_month = 31;
            if (month == 4 || month == 6 || month == 9 || month == 11) {
                days_in_month = 30;
            } else if (month == 2) {
                if (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) {
                    days_in_month = 29;
                } else {
                    days_in_month = 28;
                }
            }
            for (int day_of_month = 1; day_of_month <= days_in_month; day_of_month++) {
                if (year > 1900 && day_of_month == 1 && day == 0) {
                    sundays++;
                }
                day++;
                day %= 7;
            }
        }
    }
    return sundays;
}

class pe019 : public pe_base {
    void run_test() {
        check("019", 171, num_sundays_first_month_20th_century());
    }
};

int main(int argc, char** argv) {
    pe019 test;
    test.go();
    std::cout << test.get_message() << std::endl; 
    return test.exit_code();
}