package horstmann.ch03_day3;
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;
    ymdValid = true;
    julianValid = false;
  }

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

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

  /**
      Returns the day of the month of this day
      @return the day of the month
   */
  public int getDate()
  {
    ensureYmd();
    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)
  {
    ensureJulian();
    return new Day(julian + n);
  }

  /**
      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)
  {
    ensureJulian();
    other.ensureJulian();
    return julian - other.julian;
  }

  private Day(int aJulian)
  {
    julian = aJulian;
    ymdValid = false;
    julianValid = true;
  }

  /**
      Computes the Julian day number of this day if
      necessary
   */
  private void ensureJulian()
  {
    if (julianValid) return;
    julian = toJulian(year, month, date);
    julianValid = true;
  }

  /**
      Converts this Julian day mumber to a calendar date if necessary.
   */
  private void ensureYmd()
  {
    if (ymdValid) return;
    int[] ymd = fromJulian(julian);
    year = ymd[0];
    month = ymd[1];
    date = ymd[2];
    ymdValid = true;
  }

  /**
      Computes the Julian day number of the given day day.

      @param year a year
      @param month a month
      @param date a day of the month
      @return The Julian day number that begins at noon of
      the given day
      Positive year signifies CE, negative year BCE.
      Remember that the year after 1 BCE is 1 CE.

      A convenient reference point is that May 23, 1968 noon
      is Julian day number 2440000.

      Julian day number 0 is a Monday.

      This algorithm is from Press et al., Numerical Recipes
      in C, 2nd ed., Cambridge University Press 1992
   */
  private static int toJulian(int year, int month, int date)
  {
    int jy = year;
    if (year < 0) jy++;
    int jm = month;
    if (month > 2) jm++;
    else
    {
      jy--;
      jm += 13;
    }
    int jul = (int) (java.lang.Math.floor(365.25 * jy)
        + java.lang.Math.floor(30.6001 * jm) + date + 1720995.0);

    int IGREG = 15 + 31 * (10 + 12 * 1582);
    // Gregorian Calendar adopted Oct. 15, 1582

    if (date + 31 * (month + 12 * year) >= IGREG)
      // Change over to Gregorian calendar
    {
      int ja = (int) (0.01 * jy);
      jul += 2 - ja + (int) (0.25 * ja);
    }
    return jul;
  }

  /**
      Converts a Julian day number to a calendar date.

      This algorithm is from Press et al., Numerical Recipes
      in C, 2nd ed., Cambridge University Press 1992

      @param j  the Julian day number
      @return an array whose 0 entry is the year, 1 the month,
      and 2 the day of the month.
   */
  @SuppressWarnings("cast")
  private static int[] fromJulian(int j)
  {
    int ja = j;

    int JGREG = 2299161;
    // The Julian day number of the adoption of the Gregorian calendar

    if (j >= JGREG)
      // Cross-over to Gregorian Calendar produces this correction
    {
      int jalpha = (int) (((float) (j - 1867216) - 0.25)
          / 36524.25);
      ja += 1 + jalpha - (int) (0.25 * jalpha);
    }
    int jb = ja + 1524;
    int jc = (int) (6680.0 + ((float) (jb - 2439870) - 122.1)
        / 365.25);
    int jd = (int) (365 * jc + (0.25 * jc));
    int je = (int) ((jb - jd) / 30.6001);
    int date = jb - jd - (int) (30.6001 * je);
    int month = je - 1;
    if (month > 12) month -= 12;
    int year = jc - 4715;
    if (month > 2) --year;
    if (year <= 0) --year;
    return new int[] { year, month, date };
  }

  private int year;
  private int month;
  private int date;
  private int julian;
  private boolean ymdValid;
  private boolean julianValid;
}