package horstmann.ch03_day1;
public class Day
{
	/**
      Constructs a day with a given year, month, and day
      of the Julian/Gregorian calendar. The Julian calendar
      is used for all days before October 15, 1582
      @param aYear a year != 0
      @param aMonth a month between 1 and 12
      @param aDate a date between 1 and 31
	 */
	public Day(int aYear, int aMonth, int aDate)
	{
		year = aYear;
		month = aMonth;
		date = aDate;
	}

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

	/**
      Returns the month of this day
      @return the month
	 */
	public int getMonth()
	{
		return month;
	}

	/**
      Returns the day of the month of this day
      @return the day of the month
	 */
	public int getDate()
	{
		return date;
	}

	/**
      Returns a day that is a certain number of days away from
      this day
      @param n the number of days, can be negative
      @return a day that is n days away from this one
	 */
	public Day addDays(int n)
	{
		Day result = this;
		while (n > 0)
		{
			result = result.nextDay();
			n--;
		}
		while (n < 0)
		{
			result = result.previousDay();
			n++;
		}
		return result;
	}

	/**
      Returns the number of days between this day and another
      day
      @param other the other day
      @return the number of days that this day is away from
      the other (>0 if this day comes later)
	 */
	public int daysFrom(Day other)
	{
		int n = 0;
		Day d = this;
		while (d.compareTo(other) > 0)
		{
			d = d.previousDay();
			n++;
		}
		while (d.compareTo(other) < 0)
		{
			d = d.nextDay();
			n--;
		}
		return n;
	}

	/**
      Compares this day with another day.
      @param other the other day
      @return a positive number if this day comes after the
      other day, a negative number if this day comes before
      the other day, and zero if the days are the same
	 */
	private int compareTo(Day other)
	{
		if (year > other.year) return 1;
		if (year < other.year) return -1;
		if (month > other.month) return 1;
		if (month < other.month) return -1;
		return date - other.date;
	}

	/**
      Computes the next day.
      @return the day following this day
	 */
	private Day nextDay()
	{
		int y = year;
		int m = month;
		int d = date;

		if (y == GREGORIAN_START_YEAR
				&& m == GREGORIAN_START_MONTH
				&& d == JULIAN_END_DAY)
			d = GREGORIAN_START_DAY;
		else if (d < daysPerMonth(y, m))
			d++;
		else
		{
			d = 1;
			m++;
			if (m > DECEMBER)
			{
				m = JANUARY;
				y++;
				if (y == 0) y++;
			}
		}
		return new Day(y, m, d);
	}

	/**
      Computes the previous day.
      @return the day preceding this day
	 */
	private Day previousDay()
	{
		int y = year;
		int m = month;
		int d = date;

		if (y == GREGORIAN_START_YEAR
				&& m == GREGORIAN_START_MONTH
				&& d == GREGORIAN_START_DAY)
			d = JULIAN_END_DAY;
		else if (d > 1)
			d--;
		else
		{
			m--;
			if (m < JANUARY)
			{
				m = DECEMBER;
				y--;
				if (y == 0) y--;
			}
			d = daysPerMonth(y, m);
		}
		return new Day(y, m, d);
	}

	/**
      Gets the days in a given month
      @param y the year
      @param m the month
      @return the last day in the given month
	 */
	private static int daysPerMonth(int y, int m)
	{
		int days = DAYS_PER_MONTH[m - 1];
		if (m == FEBRUARY && isLeapYear(y))
			days++;
		return days;
	}

	/**
      Tests if a year is a leap year
      @param y the year
      @return true if y is a leap year
	 */
	private static boolean isLeapYear(int y)
	{
		if (y % 4 != 0) return false;
		if (y < GREGORIAN_START_YEAR) return true;
		return (y % 100 != 0) || (y % 400 == 0);
	}

	private int year;
	private int month;
	private int date;

	private static final int[] DAYS_PER_MONTH
	= { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

	private static final int GREGORIAN_START_YEAR = 1582;
	private static final int GREGORIAN_START_MONTH = 10;
	private static final int GREGORIAN_START_DAY = 15;
	private static final int JULIAN_END_DAY = 4;

	private static final int JANUARY = 1;
	private static final int FEBRUARY = 2;
	private static final int DECEMBER = 12;
}