easter.c revision 50476 
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 * $FreeBSD: head/lib/libcalendar/easter.c 50476 1999-08-28 00:22:10Z peter $
27 */
28
29#include "calendar.h"
30
31typedef struct date date;
32
33static int	 easterodn(int y);
34
35/* Compute Easter Sunday in Gregorian Calendar */
36date *
37easterg(int y, date *dt)
38{
39	int c, i, j, k, l, n;
40
41	n = y % 19;
42	c = y / 100;
43	k = (c - 17) / 25;
44	i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
45	i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
46	j = (y + y/4 + i + 2 - c + c/4) % 7;
47	l = i - j;
48	dt->m = 3 + (l + 40) / 44;
49	dt->d = l + 28 - 31*(dt->m / 4);
50	dt->y = y;
51	return (dt);
52}
53
54/* Compute the Gregorian date of Easter Sunday in Julian Calendar */
55date *
56easterog(int y, date *dt)
57{
58
59	return (gdate(easterodn(y), dt));
60}
61
62/* Compute the Julian date of Easter Sunday in Julian Calendar */
63date   *
64easteroj(int y, date * dt)
65{
66
67	return (jdate(easterodn(y), dt));
68}
69
70/* Compute the day number of Easter Sunday in Julian Calendar */
71static int
72easterodn(int y)
73{
74	/*
75	 * Table for the easter limits in one metonic (19-year) cycle. 21
76	 * to 31 is in March, 1 through 18 in April. Easter is the first
77	 * sunday after the easter limit.
78	 */
79	int     mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
80		    24, 12, 1, 21, 9, 29, 17};
81
82	/* Offset from a weekday to next sunday */
83	int     ns[] = {6, 5, 4, 3, 2, 1, 7};
84	date	dt;
85	int     dn;
86
87	/* Assign the easter limit of y to dt */
88	dt.d = mc[y % 19];
89
90	if (dt.d < 21)
91		dt.m = 4;
92	else
93		dt.m = 3;
94
95	dt.y = y;
96
97	/* Return the next sunday after the easter limit */
98	dn = ndaysj(&dt);
99	return (dn + ns[weekday(dn)]);
100}
101