Compute the Day on Which Easter Falls

package com.darwinsys;
 
/*
 * Copyright (c) Ian F. Darwin, http://www.darwinsys.com/, 1996-2002.
 * All rights reserved. Software written by Ian F. Darwin and others.
 * $Id: LICENSE,v 1.8 2004/02/09 03:33:38 ian Exp $
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS''
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 * 
 * Java, the Duke mascot, and all variants of Sun's Java "steaming coffee
 * cup" logo are trademarks of Sun Microsystems. Sun's, and James Gosling's,
 * pioneering role in inventing and promulgating (and standardizing) the Java 
 * language and environment is gratefully acknowledged.
 * 
 * The pioneering role of Dennis Ritchie and Bjarne Stroustrup, of AT&T, for
 * inventing predecessor languages C and C++ is also gratefully acknowledged.
 */
 
import java.util.Calendar;
import java.util.GregorianCalendar;
 
/**
 * Easter - compute the day on which Easter falls.
 * 
 * In the Christian religion, Easter is possibly the most important holiday of the year, so getting its date <I>just so </I> is worthwhile.
 * 
 * @author: Ian F. Darwin, http://www.darwinsys.com/, based on a detailed algorithm in Knuth, vol 1, pg 155.
 * 
 * @Version: $Id: Easter.java,v 1.5 2004/02/09 03:33:46 ian Exp $ Written in C, Toronto, 1988. Java version 1996.
 * 
 * @Note: It's not proven correct, although it gets the right answer for years around the present.
 */
public class Easter {
 
    /*
     * Compute the day of the year that Easter falls on. Step names E1 E2 etc., are direct references to Knuth, Vol 1, p 155. @exception IllegalArgumentexception If the year is before 1582 (since the
     * algorithm only works on the Gregorian calendar).
     */
    public static final Calendar findHolyDay(int year) {
	if (year <= 1582) {
	    throw new IllegalArgumentException("Algorithm invalid before April 1583");
	}
	int golden, century, x, z, d, epact, n;
 
	golden = (year % 19) + 1; /* E1: metonic cycle */
	century = (year / 100) + 1; /* E2: e.g. 1984 was in 20th C */
	x = (3 * century / 4) - 12; /* E3: leap year correction */
	z = ((8 * century + 5) / 25) - 5; /* E3: sync with moon's orbit */
	d = (5 * year / 4) - x - 10;
	epact = (11 * golden + 20 + z - x) % 30; /* E5: epact */
	if ((epact == 25 && golden > 11) || epact == 24)
	    epact++;
	n = 44 - epact;
	n += 30 * (n < 21 ? 1 : 0); /* E6: */
	n += 7 - ((d + n) % 7);
	if (n > 31) /* E7: */
	    return new GregorianCalendar(year, 4 - 1, n - 31); /* April */
	else
	    return new GregorianCalendar(year, 3 - 1, n); /* March */
    }
 
    /** Main program, when used as a standalone application */
    public static void main(String[] argv) {
 
	if (argv.length == 0) {
	    int thisYear = new GregorianCalendar().get(Calendar.YEAR);
	    Calendar c = Easter.findHolyDay(thisYear);
	    System.out.println(c.getTime());
	} else
	    for (int i = 0; i < argv.length; i++) {
		int year = 0;
		try {
		    year = Integer.parseInt(argv[i]);
		    System.out.println(Easter.findHolyDay(year).getTime());
		} catch (IllegalArgumentException e) {
		    System.err.println("Year " + argv[i] + " invalid (" + e.getMessage() + ").");
		}
	    }
    }
}