/// <summary>
/// Compute the month and date on which this holiday falls in the year
/// containing the date "date". First figure out which date Easter lands on
/// in this year, and then add the offset for this holiday to get the right
/// date.
/// <p>
/// The algorithm here is taken from the <a
/// href="http://www.faqs.org/faqs/calendars/faq/">Calendar FAQ</a>.
/// </summary>
///
private DateTime ComputeInYear(DateTime date, GregorianCalendar cal)
{
if (cal == null)
{
cal = calendar;
}
lock (cal) {
cal.SetTime(date);
int year = cal.Get(IBM.ICU.Util.Calendar.YEAR);
int g = year % 19; // "Golden Number" of year - 1
int i = 0; // # of days from 3/21 to the Paschal full moon
int j = 0; // Weekday (0-based) of Paschal full moon
if (ILOG.J2CsMapping.Util.DateUtil.After(cal.GetTime(), cal.GetGregorianChange()))
{
// We're past the Gregorian switchover, so use the Gregorian
// rules.
int c = year / 100;
int h = (c - c / 4 - (8 * c + 13) / 25 + 19 * g + 15) % 30;
i = h - (h / 28)
* (1 - (h / 28) * (29 / (h + 1)) * ((21 - g) / 11));
j = (year + year / 4 + i + 2 - c + c / 4) % 7;
}
else
{
// Use the old Julian rules.
i = (19 * g + 15) % 30;
j = (year + year / 4 + i) % 7;
}
int l = i - j;
int m = 3 + (l + 40) / 44; // 1-based month in which Easter falls
int d = l + 28 - 31 * (m / 4); // Date of Easter within that month
cal.Clear();
cal.Set(IBM.ICU.Util.Calendar.ERA, IBM.ICU.Util.GregorianCalendar.AD);
cal.Set(IBM.ICU.Util.Calendar.YEAR, year);
cal.Set(IBM.ICU.Util.Calendar.MONTH, m - 1); // 0-based
cal.Set(IBM.ICU.Util.Calendar.DATE, d);
cal.GetTime(); // JDK 1.1.2 bug workaround
cal.Add(IBM.ICU.Util.Calendar.DATE, daysAfterEaster);
return(cal.GetTime());
}
}
