    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
    045    package 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     */
    059    public 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    }