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}