import java.util.TimeZone;

/**
Objects of the Date class represent calendar dates such as March 14, 1997.<p>

Class invariant: No Date object will ever represent an invalid date, such as one with a negative month or day, or one whose day exceeds the number of days in its month.

@author Marty Stepp
@version 1.0
*/
public class Date {
    // constants
    private static final int JANUARY  =  1;
    private static final int FEBRUARY =  2;
    private static final int DECEMBER = 12;
    private static final int DAYS_PER_WEEK      =   7;
    private static final int DAYS_PER_YEAR      = 365;
    private static final int DAYS_PER_LEAP_YEAR = 366;
    private static final String[] DAY_NAMES = {"SUNDAY", "MONDAY", "TUESDAY",
            "WEDNESDAY", "THURSDAY", "FRIDAY", "SATURDAY"};
    private static final int[] DAYS_PER_MONTH = { -1,
    //   1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12
        31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
    };
    
    // fields
    private int year;
    private int month;
    private int day;

    /**
    Constructs a new object representing the given year, month, and day.
    @throws IllegalArgumentException if month is not between 1 and 12, or if day is not between 1 and the number of days in that month.
    */
    public Date(int year, int month, int day) {
        this.year = year;
        this.month = month;
        this.day = day;
        
        if (month < 1 || month > 12 || day < 1) {
            throw new IllegalArgumentException("Invalid day or month: " + toString());
        }
    }
    
    /** Constructs a new object representing today's date. */
    public Date() {
        this(1970, JANUARY, 1);
        int daysSinceEpoch = (int) ((System.currentTimeMillis() + TimeZone.getDefault().getRawOffset()) / 1000 / 60 / 60 / 24);
        for (int i = 0; i < daysSinceEpoch; i++) {
            nextDay();
        }
    }
    
    /**
    Adds the given number of days to this Date.
    @throws IllegalArgumentException if days < 0.
    */
    public void addDays(int days) {
        for (int i = 1; i <= days; i++) {
            nextDay();
        }
    }
    
    /**
    Returns whether o refers to a Date object representing the same
    year, month, and day as this one.
    */
    public boolean equals(Object o) {
        Date other = (Date) o;
        return day   == other.day &&
               month == other.month &&
               year  == other.year;
    }
    
    /**
    Returns the day represented by this Date object.
    @return the day, between 1 and the number of days in this Date's month
    (which varies from 28 to 31)
    */
    public int getDay() {
        return day;
    }

    /** Returns the number of days in the month represented by the current Date. */
    public int getDaysInMonth() {
        int result = DAYS_PER_MONTH[month];
        if (month == FEBRUARY && isLeapYear()) {
            result++;
        }
        return result;
    }

    /**
    Returns a string such as "Monday" representing what day of the week
    this Date fell on.
    @return the day as either "Sunday", "Monday", "Tuesday, "Wednesday", "Thursday", "Friday", or "Saturday"
    */
    public String getDayOfWeek() {
        int index = 1;
        Date temp = new Date(1753, JANUARY, 1);
        while (temp != this) {
            temp.nextDay();
            index = (index + 1) % DAYS_PER_WEEK;
        }
        
        return DAY_NAMES[index];
    }
    
    /**
    Returns the month represented by this Date object.
    @return the month, between 1 and 12
    */
    public int getMonth() {
        return month;
    }
    
    /**
    Returns the year represented by this Date object.
    @return the year
    */
    public int getYear() {
        return year;
    }
    
    /**
    Returns whether this Date falls on a leap year.
    @return true if in a year divisible by 4 but not by 100 unless by 400
    */
    public boolean isLeapYear() {
        return (year % 400 == 0) || (year % 4 == 0 || year % 100 != 0);
    }
    
    /**
    Advances this Date to the next day.
    If necessary, wraps to the next month and/or year.
    */
    public void nextDay() {
        day++;
        if (day > 31) {
            // wrap to next month
            month++;
            day = 1;
            if (month > DECEMBER) {
                // wrap to next year
                year++;
                month = JANUARY;
            }
        }
    }
    
    /** 
    Returns a String representation of this Date.
    @return a String such as "2007/7/26" for July 26, 2007
    */
    public String toString() {
        return year + "/" + month + "/" + day;
    }
}