easter.c revision 31611 
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: easter.c,v 1.1.1.1 1997/12/04 10:41:49 helbig Exp $
27 */
28
29#include "calendar.h"
30
31static int	 easterodn(int y);
32
33/* Compute Easter Sunday in Gregorian Calendar */
34date *
35easterg(int y, date *dt)
36{
37	int c, i, j, k, l, n;
38
39	n = y % 19;
40	c = y / 100;
41	k = (c - 17) / 25;
42	i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
43	i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
44	j = (y + y/4 + i + 2 - c + c/4) % 7;
45	l = i - j;
46	dt->m = 3 + (l + 40) / 44;
47	dt->d = l + 28 - 31*(dt->m / 4);
48	dt->y = y;
49	return (dt);
50}
51
52/* Compute the Gregorian date of Easter Sunday in Julian Calendar */
53date *
54easterog(int y, date *dt)
55{
56
57	return (gdate(easterodn(y), dt));
58}
59
60/* Compute the Julian date of Easter Sunday in Julian Calendar */
61date   *
62easteroj(int y, date * dt)
63{
64
65	return (jdate(easterodn(y), dt));
66}
67
68/* Compute the day number of Easter Sunday in Julian Calendar */
69static int
70easterodn(int y)
71{
72	/*
73	 * Table for the easter limits in one metonic (19-year) cycle. 21
74	 * to 31 is in March, 1 through 18 in April. Easter is the first
75	 * sunday after the easter limit.
76	 */
77	int     mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
78		    24, 12, 1, 21, 9, 29, 17};
79
80	/* Offset from a weekday to next sunday */
81	int     ns[] = {6, 5, 4, 3, 2, 1, 7};
82	date	dt;
83	int     dn;
84
85	/* Assign the easter limit of y to dt */
86	dt.d = mc[y % 19];
87
88	if (dt.d < 21)
89		dt.m = 4;
90	else
91		dt.m = 3;
92
93	dt.y = y;
94
95	/* Return the next sunday after the easter limit */
96	dn = ndaysj(&dt);
97	return (dn + ns[weekday(dn)]);
98}
99