1205821Sedwin/*-
217849Swosch * Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
3181322Sedwin * All rights reserved.
413840Swosch *
517849Swosch * Redistribution and use in source and binary forms, with or without
617849Swosch * modification, are permitted provided that the following conditions
717849Swosch * are met:
817849Swosch * 1. Redistributions of source code must retain the above copyright
917849Swosch *    notice, this list of conditions and the following disclaimer.
1017849Swosch * 2. Redistributions in binary form must reproduce the above copyright
1117849Swosch *    notice, this list of conditions and the following disclaimer in the
1217849Swosch *    documentation and/or other materials provided with the distribution.
1317849Swosch *
1417849Swosch * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
1517849Swosch * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
1617849Swosch * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
1717849Swosch * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
1817849Swosch * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
1917849Swosch * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
2017849Swosch * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
2117849Swosch * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
2217849Swosch * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
2317849Swosch * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
2417849Swosch * SUCH DAMAGE.
2517849Swosch */
2613840Swosch
2787235Smarkm#include <sys/cdefs.h>
2887235Smarkm__FBSDID("$FreeBSD$");
2987235Smarkm
3015737Sache#include <stdio.h>
3115737Sache#include <stdlib.h>
3213840Swosch#include <string.h>
3313840Swosch
3415720Sache#include "calendar.h"
3515720Sache
3613840Swosch/* return year day for Easter */
3713840Swosch
3870598Sdwmalone/*
3970598Sdwmalone * This code is based on the Calendar FAQ's code for how to calculate
4070598Sdwmalone * easter is. This is the Gregorian calendar version. They refer to
4170598Sdwmalone * the Algorithm of Oudin in the "Explanatory Supplement to the
4270598Sdwmalone * Astronomical Almanac".
4370598Sdwmalone */
4470598Sdwmalone
45169343Sdwmaloneint
46169343Sdwmaloneeaster(int year) /* 0 ... abcd, NOT since 1900 */
4713840Swosch{
48181322Sedwin	int G,	/* Golden number - 1 */
49181322Sedwin	    C,	/* Century */
50181322Sedwin	    H,	/* 23 - epact % 30 */
51181322Sedwin	    I,	/* days from 21 March to Paschal full moon */
52181322Sedwin	    J,	/* weekday of full moon */
53181322Sedwin	    L;	/* days from 21 March to Sunday on of before full moon */
5413840Swosch
55181322Sedwin	G = year % 19;
56181322Sedwin	C = year / 100;
57181322Sedwin	H = (C - C / 4 - (8 * C + 13) / 25 + 19 * G + 15) % 30;
58181322Sedwin	I = H - (H / 28) * (1 - (H / 28) * (29 / (H + 1)) * ((21 - G) / 11));
59181322Sedwin	J = (year + year / 4 + I + 2 - C + C / 4) % 7;
6013840Swosch
61181322Sedwin	L = I - J;
6213840Swosch
63205821Sedwin	if (isleap(year))
64181322Sedwin		return 31 + 29 + 21 + L + 7;
65181322Sedwin	else
66181322Sedwin		return 31 + 28 + 21 + L + 7;
6713840Swosch}
68