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jtstand-common is a library derived from jcommon, used by jtstand-chart, which is derived from jfreechart

There is a newer version: 1.5.9

/*
 * Copyright (c) 2009 Albert Kurucz. 
 *
 * This file, EasterSundayRule.java is part of JTStand.
 *
 * JTStand is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * JTStand is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with GTStand.  If not, see .
 */

package org.jfree.date;

/**
 * An annual date rule for Easter (Sunday).  The algorithm used here was
 * obtained from a Calendar FAQ which can be found at:
 * 
 * http://www.tondering.dk/claus/calendar.html.
 * 

 * It is based on an algorithm by Oudin (1940) and quoted in "Explanatory Supplement to the
 * Astronomical Almanac", P. Kenneth Seidelmann, editor.
 *
 * @author David Gilbert
 */
public class EasterSundayRule extends AnnualDateRule {

    /**
     * Default constructor.
     */
    public EasterSundayRule() {
    }

    /**
     * Returns the date of Easter Sunday for the given year.  See the class
     * description for the source of the algorithm.
     * 
     * This method supports the AnnualDateRule interface.
     *
     * @param year  the year to check.
     *
     * @return the date of Easter Sunday for the given year.
     */
    public SerialDate getDate(final int year) {
        final int g = year % 19;
        final int c = year / 100;
        final int h = (c - c / 4 - (8 * c + 13) / 25 + 19 * g + 15) % 30;
        final int i = h - h / 28 * (1 - h / 28 * 29 / (h + 1) * (21 - g) / 11);
        final int j = (year + year / 4 + i + 2 - c + c / 4) % 7;
        final int l = i - j;
        final int month = 3 + (l + 40) / 44;
        final int day = l + 28 - 31 * (month / 4);
        return SerialDate.createInstance(day, month, year);
    }

}