/**
 * Creates and manipulates dates on the Gregorian calendar, i.e. any 
 * date since October 15, 1582.  Date objects have a month, day, and year
 *
 * @author Greg
 * Copyright:  Greg Shaw, October 15, 2010
 */
public class Date 
{
    // instance var's
    private int day; 		// 1..number of days in the month
    private int month; 		// 1..12
    private int year; 		// >= 1582

    // DAY_NAMES[0] is "Sunday", DAY_NAMES[6] is "Saturday"
    private static final String[] DAY_NAMES = {"Sunday", "Monday", "Tuesday",
                                "Wednesday","Thursday", "Friday", "Saturday"};

    // MONTH_NAMES[1] is "January", MONTH_NAMES[12] is "December"
    private static final String[] MONTH_NAMES = {"", "January", "February",
                         "March", "April", "May", "June","July", "August",
                         "September", "October", "November", "December"};

    // DAYS_IN_MONTH[1] is 31 (January), DAYS_IN_MONTH[12] is 31 (December)
    private static final int[] DAYS_IN_MONTH = { 0, 31, 28, 31, 30, 31, 30,
                                                31, 31, 30, 31, 30, 31 };
    // The first date is 10/15/1582 
    private static final int FIRST_MONTH = 10 ;
    private static final int FIRST_DAY = 15 ;
    private static final int FIRST_YEAR = 1582 ;
    private static final Date FIRST_DATE = new Date
                                           (FIRST_MONTH,FIRST_DAY,FIRST_YEAR) ;
    
    // 10/15/1582 was a Friday
    private static final int START_DAY_INDEX = 5;  // DAY_NAMES[5] = "Friday"

    /**
     * Creates a date object
     * @param month the month
     * @param day the day
     * @param year the year
     * @throws DateException on attempt to create invalid date
     */
    public Date(int month, int day, int year) 
    {
        // create part of Exception message, in case of illegal date
        String s = month + "/" + day + "/" + year;

        // check for illegal month
        if (month > 12 || month < 1) 
        {
            throw new DateException(s + "\nIllegal month: " + month
                    + "  (Month must be between 1 and 12)");
        }

        // check for year < 1582 or date in 1582 but prior to October 15th
        if ((year < 1582)
                || (year == 1582 && month < 10)
                || (year == 1582 && month == 10 && day < 15)) {
            throw new DateException(s
                    + "  (Earliest legal date is October 15, 1582)");
        }

        // check for February 29th and NOT a leap year
        if (month == 2 && day == 29 && !isLeapYear(year)) 
        {
            throw new DateException(s + "  (" + year + " is not a leap year)");
        }

        // check for all 4 possible illegal day values
        //   1. day is <= 0
        //   2. February of a leap year and day > 29
        //   3. February, NOT a leap year and day > 28
        //   4. day > number of days in the month
        if ( day <= 0 
             || month == 2 && isLeapYear(year) && day > 29 
             || month == 2 && !isLeapYear(year) && day > 28
             || month != 2 && day > DAYS_IN_MONTH[month])
        {
            throw new DateException(s + "\nIllegal day: " + day + "\nFor "
               + (month == 2 ? "February " + year : MONTH_NAMES[month])
               + ", must be between 1 and "
               + (isLeapYear(year) && month == 2 ? 29 : DAYS_IN_MONTH[month]));
        }

        // here if month, day, and year are all good
        this.month = month;
        this.day = day;
        this.year = year;
    }

    /**
     * Returns the day portion of this Date, as an int between 1 and the
     * number of days in the month
     * @return the day
     */
    public int getDay() 
    {
        return day;
    }

    /**
     * Returns the month of this Date, as an int between 1 and 12
     * @return the month
     */
    public int getMonth() 
    {
        return month;
    }

    /**
     * Returns the year of this Date
     * @return the year
     */
    public int getYear() 
    {
        return year;
    }

    /**
     * Returns the month as a String (e.g. "March")
     * @return the name of the month
     */
    private String getMonthName() 
    {
        return MONTH_NAMES[month];
    }

    /**
     * Returns the day of the week (e.g. "Saturday")
     * @return the day of the week
     */
    public String getDayOfWeek() 
    {
       // start at 1st date
       Date tempDate = new Date(FIRST_MONTH,FIRST_DAY,FIRST_YEAR) ;  

        int dayNameIndex = START_DAY_INDEX;           // 1st was "Friday"

        // repeat until tempDate reaches date method was called for
        while (!tempDate.equals(this)) 
        {
            tempDate.next();                          // increment tempDate
            dayNameIndex = (dayNameIndex + 1) % 7;    // update day of week
        }
        return DAY_NAMES[dayNameIndex];
    }

    /**
     * Is a specified Date equal to this Date?
     * @param otherDate the Date being tested for equality to this Date
     * @return true if the two dates have the same month, day, and year;
     * false if not
     */
    public boolean equals(Object otherDate) 
    {
        Date other = (Date) otherDate;
        return this.month == other.month &&
               this.day == other.day &&
               this.year == other.year ;
    }

    /**
     * Increases this Date by one day.
     * Month and year roll over as necessary.
     */
    public void next() 
    {
        if (month == 2 && day == 29)        // if it's February 29th...
        {
            day = 1;                        // ...next day is March 1st
            month = 3;
        }
        else if (month == 2 && day == 28)   // if it's February 28th...
        {
            if (this.isLeapYear())          // if it's a leap year...
            {
                day = 29;                   // ...next day is Feb 29th
            }
            else                            // not a leap year...
            {
                day = 1;                    // ...next day is March 1st
                month = 3;
            }
        }
        else if (day == 30)                 // if it's the 30th...
        {
            if (month == 9 || month == 4 ||    // if it's September, April,
                month == 6 || month == 11)     // June, or November...
            {
                day = 1;                       // ...next day is the 1st
                month++;                       //    of the next month
            }
            else                               // it's a 31-day month...
            {
                day = 31;                      // ...next day is the 31st
            }
        }
        else if (day == 31)                    // if it's the 31st...
        {
            if (month == 12)                   // ...of December...
            {
                day = 1;                       // ...Happy New year!
                month = 1;
                year++;
            }
            else                                // not December...
            {
                day = 1;                        // ...next day is 1st day
                month++;                        //    of next month
            }
        }
        else                                    // not last day of month...
        {
            day++;			        // ...next day, same month&year
        }
    }

    /**
     * Decreases this Date by one day.
     * Month and year roll back as necessary.
     */
    public void previous() 
    {
        if (this.equals(FIRST_DATE))  // no previous Gregorian Date
        {
           throw new DateException("Earliest legal date is October 15, 1582");
        }

        if (day != 1)                       // if not the 1st of the month...
        {
            day--;			    // ...prev day, same month & year
        }
        else if (month == 1)                // if it's January 1st...
        {
            day = 31;                       // ...previous day is Dec 31st
            month = 12;                     //    of previous year
            year--;
        }
        else if (month == 3)                // if it's March 1st...
        {
            if (this.isLeapYear())          // if it's a leap year...
            {
                day = 29;                   // ...previous day is Feb 29th
                month = 2;
            }
            else                            // not a leap year...
            {
                day = 28;                   // ...previous day is Feb 28th
                month = 2;
            }
        }
        else                                // the 1st, but not March or Jan...
        {
            if (month == 10 ||              // if previous month has 30 days...
                month == 5 || 
                month == 7 || 
                month == 12)
            {
                day = 30;                   // ...previous day is the 30th
                month-- ;                   //    of the previous month
            } 
            else                            // if previous month has 31 days...
            {
                day = 31;                   // ...previous day is the 31st
                month--;                    //    of the previous month
            }
        }
    }

    /**
     * Adds a specified number of days to this Date
     * @param days the number of days by which to increase the Date
     * (if negative, Date is decreased by that number)
     */
    public void add(int days) 
    {
        if (days < 0) {
            this.subtract(-days);
        } else {
            for (int i = 1; i <= days; i++) {
                this.next();
            }
        }
    }

    /**
     * Subtracts a specified number of days from this Date
     * @param days the number of days by which to decrease the Date
     * (if negative, Date is increased by that number)
     */
    public void subtract(int days) 
    {
        if (days < 0) {
            this.add(-days);
        } else {
            for (int i = 1; i <= days; i++) {
                this.previous();
            }
        }
    }

    /**
     * Returns this Date in short format - numbers only
     * @return the Date as "mm/dd/yyyy"
     */
    public String getShortDate() 
    {
        String out = "";
        if (month <= 9)       // add leading zero if single digit month
        {
            out += "0";
        }
        out += month + "/";
        if (day <= 9)         // add leading zero if single digit day
        {
            out += "0";
        }
        return out + day + "/" + year;
    }

    /**
     * Returns this Date in medium format - with month name
     * @return the Date as "month day, year"
     * (E.g. October 15, 1582)
     */
    public String getMediumDate() 
    {
        return MONTH_NAMES[month] + " " + day + ", " + year;
    }

    /**
     * Returns this Date in long format, with day-of-week,
     * month name, and the ordinal of the day
     * @return the Date as "day-of-week, month day, year"
     * (E.g. "Friday, October 15th, 1582")
     */
    public String getLongDate() 
    {
        // start with day of week and month
        String out = this.getDayOfWeek() + ", " + MONTH_NAMES[month]
                     + " " + day;
        // concatenate ordinal of the day
        switch (day) 
        {
            case 1:
                out += "st, ";
                break;
            case 2:
                out += "nd, ";
                break;
            case 3:
                out += "rd, ";
                break;
            case 21:
                out += "st, ";
                break;
            case 22:
                out += "nd, ";
                break;
            case 23:
                out += "rd, ";
                break;
            case 31:
                out += "st, ";
                break;
            default:
                out += "th, ";
        }
        return out + year;       // concatenate year and return
    }

    // Is this Date in a leap year?
    private boolean isLeapYear() 
    {
        return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0) ;
    }

    // Is a specified year a leap year?
    private static boolean isLeapYear(int year) 
    {
        return (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
    }
}

class DateException extends ArithmeticException 
{
    public DateException() 
    {}

    public DateException(String message) 
    {
        super(message);
    }
}