import javax.swing.*;

public class Tiderakning
{
    private static final int[] LENGTHS_OF_MONTHS =
      {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    private static final String[] WEEK_DAYS =
      {"måndag", "tisdag", "onsdag", "torsdag", "fredag", "lördag", "söndag"};


    //Main entry point
    public static void main(String[] args)
    {
        int year;
        String input;

        input = JOptionPane.showInputDialog("Skriv ett datum på formen åååå-mm-dd");
        JOptionPane.showMessageDialog(null, "Det är en " + dateToWeekDay(input));
    }

    //True if 'year' is a multiple of 4 and not a multiple of 100,
    //with the exception of 'year' being a multiple of 400
    private static boolean isLeapYear(int year)
    {
        return (year % 400 == 0 || year % 100 != 0) && year % 4 == 0;
    }

    //returns the number of days into the year the date is
    private static int dayNumber(String date)
    {
       int[] _date = stringToDate(date);
       int month = _date[1];
       int day = _date[2];
       for(int i=0; i<month-1; i++)
           day += LENGTHS_OF_MONTHS[i];

       if(isLeapYear(_date[0]) && month > 2)
           day++;

       return day;
    }

    //returns the week day of the date
    private static String dateToWeekDay(String date)
    {
        int days = 0;
        int year = stringToDate(date)[0]; //The year in the supplied date

        for(int i = 1754; i < year; i++)
            days += isLeapYear(i) ? 366 : 365;
        days += dayNumber(date);

        return WEEK_DAYS[days % 7];
    }

    //Extracts the date and returns an array continaing the year, month and day.
    private static int[] stringToDate(String date)
    {
        int[] out = new int[3];

        String[] splitDate = date.split("-", 3);
        for(int i=0; i<3; i++)
            out[i] = Integer.parseInt(splitDate[i]);

        return out;
    }
}