Source for org.jfree.date.EasterSundayRule

   1: /* ========================================================================
   2:  * JCommon : a free general purpose class library for the Java(tm) platform
   3:  * ========================================================================
   4:  *
   5:  * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
   6:  * 
   7:  * Project Info:  http://www.jfree.org/jcommon/index.html
   8:  *
   9:  * This library is free software; you can redistribute it and/or modify it 
  10:  * under the terms of the GNU Lesser General Public License as published by 
  11:  * the Free Software Foundation; either version 2.1 of the License, or 
  12:  * (at your option) any later version.
  13:  *
  14:  * This library is distributed in the hope that it will be useful, but 
  15:  * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 
  16:  * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public 
  17:  * License for more details.
  18:  *
  19:  * You should have received a copy of the GNU Lesser General Public
  20:  * License along with this library; if not, write to the Free Software
  21:  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, 
  22:  * USA.  
  23:  *
  24:  * [Java is a trademark or registered trademark of Sun Microsystems, Inc. 
  25:  * in the United States and other countries.]
  26:  *
  27:  * ---------------------
  28:  * EasterSundayRule.java
  29:  * ---------------------
  30:  * (C) Copyright 2000-2003, by Object Refinery Limited.
  31:  *
  32:  * Original Author:  David Gilbert (for Object Refinery Limited);
  33:  * Contributor(s):   -;
  34:  *
  35:  * $Id: EasterSundayRule.java,v 1.4 2005/11/16 15:58:40 taqua Exp $
  36:  *
  37:  * Changes (from 26-Oct-2001)
  38:  * --------------------------
  39:  * 26-Oct-2001 : Changed package to com.jrefinery.date.*;
  40:  * 03-Oct-2002 : Fixed errors reported by Checkstyle (DG);
  41:  * 01-Jun-2005 : Removed the explicit clonable declaration, it is declared
  42:  *               in the super class.
  43:  */
  44: 
  45: package org.jfree.date;
  46: 
  47: /**
  48:  * An annual date rule for Easter (Sunday).  The algorithm used here was
  49:  * obtained from a Calendar FAQ which can be found at:
  50:  * <P>
  51:  * <a href="http://www.tondering.dk/claus/calendar.html"
  52:  *     >http://www.tondering.dk/claus/calendar.html</a>.
  53:  * <P>
  54:  * It is based on an algorithm by Oudin (1940) and quoted in "Explanatory Supplement to the
  55:  * Astronomical Almanac", P. Kenneth Seidelmann, editor.
  56:  *
  57:  * @author David Gilbert
  58:  */
  59: public class EasterSundayRule extends AnnualDateRule {
  60: 
  61:     /**
  62:      * Default constructor.
  63:      */
  64:     public EasterSundayRule() {
  65:     }
  66: 
  67:     /**
  68:      * Returns the date of Easter Sunday for the given year.  See the class
  69:      * description for the source of the algorithm.
  70:      * <P>
  71:      * This method supports the AnnualDateRule interface.
  72:      *
  73:      * @param year  the year to check.
  74:      *
  75:      * @return the date of Easter Sunday for the given year.
  76:      */
  77:     public SerialDate getDate(final int year) {
  78:         final int g = year % 19;
  79:         final int c = year / 100;
  80:         final int h = (c - c / 4 - (8 * c + 13) / 25 + 19 * g + 15) % 30;
  81:         final int i = h - h / 28 * (1 - h / 28 * 29 / (h + 1) * (21 - g) / 11);
  82:         final int j = (year + year / 4 + i + 2 - c + c / 4) % 7;
  83:         final int l = i - j;
  84:         final int month = 3 + (l + 40) / 44;
  85:         final int day = l + 28 - 31 * (month / 4);
  86:         return SerialDate.createInstance(day, month, year);
  87:     }
  88: 
  89: }