    001/* ========================================================================
    002 * JCommon : a free general purpose class library for the Java(tm) platform
    003 * ========================================================================
    004 *
    005 * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
    006 *
    007 * Project Info:  http://www.jfree.org/jcommon/index.html
    008 *
    009 * This library is free software; you can redistribute it and/or modify it
    010 * under the terms of the GNU Lesser General Public License as published by
    011 * the Free Software Foundation; either version 2.1 of the License, or
    012 * (at your option) any later version.
    013 *
    014 * This library is distributed in the hope that it will be useful, but
    015 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    016 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
    017 * License for more details.
    018 *
    019 * You should have received a copy of the GNU Lesser General Public
    020 * License along with this library; if not, write to the Free Software
    021 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
    022 * USA.
    023 *
    024 * [Java is a trademark or registered trademark of Sun Microsystems, Inc.
    025 * in the United States and other countries.]
    026 *
    027 * ---------------------
    028 * EasterSundayRule.java
    029 * ---------------------
    030 * (C) Copyright 2000-2003, by Object Refinery Limited.
    031 *
    032 * Original Author:  David Gilbert (for Object Refinery Limited);
    033 * Contributor(s):   -;
    034 *
    035 * $Id: EasterSundayRule.java,v 1.4 2005/11/16 15:58:40 taqua Exp $
    036 *
    037 * Changes (from 26-Oct-2001)
    038 * --------------------------
    039 * 26-Oct-2001 : Changed package to com.jrefinery.date.*;
    040 * 03-Oct-2002 : Fixed errors reported by Checkstyle (DG);
    041 * 01-Jun-2005 : Removed the explicit clonable declaration, it is declared
    042 *               in the super class.
    043 */
    044
    045package org.jfree.date;
    046
    047/**
    048 * An annual date rule for Easter (Sunday).  The algorithm used here was
    049 * obtained from a Calendar FAQ which can be found at:
    050 * <P>
    051 * <a href="http://www.tondering.dk/claus/calendar.html"
    052 *     >http://www.tondering.dk/claus/calendar.html</a>.
    053 * <P>
    054 * It is based on an algorithm by Oudin (1940) and quoted in "Explanatory Supplement to the
    055 * Astronomical Almanac", P. Kenneth Seidelmann, editor.
    056 *
    057 * @author David Gilbert
    058 */
    059public class EasterSundayRule extends AnnualDateRule {
    060
    061    /**
    062     * Default constructor.
    063     */
    064    public EasterSundayRule() {
    065    }
    066
    067    /**
    068     * Returns the date of Easter Sunday for the given year.  See the class
    069     * description for the source of the algorithm.
    070     * <P>
    071     * This method supports the AnnualDateRule interface.
    072     *
    073     * @param year  the year to check.
    074     *
    075     * @return the date of Easter Sunday for the given year.
    076     */
    077    public SerialDate getDate(final int year) {
    078        final int g = year % 19;
    079        final int c = year / 100;
    080        final int h = (c - c / 4 - (8 * c + 13) / 25 + 19 * g + 15) % 30;
    081        final int i = h - h / 28 * (1 - h / 28 * 29 / (h + 1) * (21 - g) / 11);
    082        final int j = (year + year / 4 + i + 2 - c + c / 4) % 7;
    083        final int l = i - j;
    084        final int month = 3 + (l + 40) / 44;
    085        final int day = l + 28 - 31 * (month / 4);
    086        return SerialDate.createInstance(day, month, year);
    087    }
    088
    089}