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 }