/* ========================================================================
* JCommon : a free general purpose class library for the Java(tm) platform
* ========================================================================
*
* (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
*
* Project Info: http://www.jfree.org/jcommon/index.html
*
* This library is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or
* (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
* License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
* USA.
*
* [Java is a trademark or registered trademark of Sun Microsystems, Inc.
* in the United States and other countries.]
*
* ---------------------
* EasterSundayRule.java
* ---------------------
* (C) Copyright 2000-2003, by Object Refinery Limited.
*
* Original Author: David Gilbert (for Object Refinery Limited);
* Contributor(s): -;
*
* $Id: EasterSundayRule.java,v 1.4 2005/11/16 15:58:40 taqua Exp $
*
* Changes (from 26-Oct-2001)
* --------------------------
* 26-Oct-2001 : Changed package to com.jrefinery.date.*;
* 03-Oct-2002 : Fixed errors reported by Checkstyle (DG);
* 01-Jun-2005 : Removed the explicit clonable declaration, it is declared
* in the super class.
*/
package org.jfree.date;
/**
* An annual date rule for Easter (Sunday). The algorithm used here was
* obtained from a Calendar FAQ which can be found at:
* <P>
* <a href="http://www.tondering.dk/claus/calendar.html"
* >http://www.tondering.dk/claus/calendar.html</a>.
* <P>
* It is based on an algorithm by Oudin (1940) and quoted in "Explanatory Supplement to the
* Astronomical Almanac", P. Kenneth Seidelmann, editor.
*
* @author David Gilbert
*/
public class EasterSundayRule extends AnnualDateRule {
/**
* Default constructor.
*/
public EasterSundayRule() {
}
/**
* Returns the date of Easter Sunday for the given year. See the class
* description for the source of the algorithm.
* <P>
* This method supports the AnnualDateRule interface.
*
* @param year the year to check.
*
* @return the date of Easter Sunday for the given year.
*/
public SerialDate getDate(final int year) {
final int g = year % 19;
final int c = year / 100;
final int h = (c - c / 4 - (8 * c + 13) / 25 + 19 * g + 15) % 30;
final int i = h - h / 28 * (1 - h / 28 * 29 / (h + 1) * (21 - g) / 11);
final int j = (year + year / 4 + i + 2 - c + c / 4) % 7;
final int l = i - j;
final int month = 3 + (l + 40) / 44;
final int day = l + 28 - 31 * (month / 4);
return SerialDate.createInstance(day, month, year);
}
}