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;
}