01: /* ========================================================================
02: * JCommon : a free general purpose class library for the Java(tm) platform
03: * ========================================================================
04: *
05: * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
06: *
07: * Project Info: http://www.jfree.org/jcommon/index.html
08: *
09: * 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
82: * (1 - h / 28 * 29 / (h + 1) * (21 - g) / 11);
83: final int j = (year + year / 4 + i + 2 - c + c / 4) % 7;
84: final int l = i - j;
85: final int month = 3 + (l + 40) / 44;
86: final int day = l + 28 - 31 * (month / 4);
87: return SerialDate.createInstance(day, month, year);
88: }
89:
90: }
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