easter.c revision 31529 
1/*-
2 * Copyright (c) 1997 Wolfgang Helbig
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 *
14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 * SUCH DAMAGE.
25 *
26 *	$Id$
27 */
28
29#include "calendar.h"
30
31/* Compute Easter Sunday in Gregorian Calendar */
32date *
33easterg(int y, date *dt)
34{
35	int c, i, j, k, l, n;
36
37	n = y % 19;
38	c = y / 100;
39	k = (c - 17) / 25;
40	i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
41	i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
42	j = (y + y/4 + i + 2 - c + c/4) % 7;
43	l = i - j;
44	dt->m = 3 + (l + 40) / 44;
45	dt->d = l + 28 - 31*(dt->m / 4);
46	dt->y = y;
47	return (dt);
48}
49
50/* Compute Easter Sunday in Julian Calendar */
51date   *
52easterj(int y, date * dt)
53{
54
55	/*
56	 * Table for the easter limits in one metonic (19-year) cycle. 21
57	 * to 31 is in March, 1 through 18 in April. Easter is the first
58	 * sunday after the easter limit.
59	 */
60	int     mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
61		    24, 12, 1, 21, 9, 29, 17};
62
63	/* Offset from a weekday to next sunday */
64	int     ns[] = {6, 5, 4, 3, 2, 1, 7};
65	int     dn;
66
67	/* Assign the easter limit of y to *dt */
68	dt->d = mc[y % 19];
69
70	if (dt->d < 21)
71		dt->m = 4;
72	else
73		dt->m = 3;
74
75	dt->y = y;
76
77	/* Compute the next sunday after the easter limit */
78	dn = ndaysj(dt);
79	dn += ns[weekday(dn)];
80
81	return (jdate(dn, dt));
82}
83