Easter - compute the day on which Easter falls
/*
* 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() + ").");
}
}
}
}
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