1/*-
2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3 *
4 * Copyright (c) 1997 Wolfgang Helbig
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 *    notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 *    notice, this list of conditions and the following disclaimer in the
14 *    documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29#include <sys/cdefs.h>
30__FBSDID("$FreeBSD$");
31
32#include "calendar.h"
33
34typedef struct date date;
35
36static int	 easterodn(int y);
37
38/* Compute Easter Sunday in Gregorian Calendar */
39date *
40easterg(int y, date *dt)
41{
42	int c, i, j, k, l, n;
43
44	n = y % 19;
45	c = y / 100;
46	k = (c - 17) / 25;
47	i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
48	i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
49	j = (y + y/4 + i + 2 - c + c/4) % 7;
50	l = i - j;
51	dt->m = 3 + (l + 40) / 44;
52	dt->d = l + 28 - 31*(dt->m / 4);
53	dt->y = y;
54	return (dt);
55}
56
57/* Compute the Gregorian date of Easter Sunday in Julian Calendar */
58date *
59easterog(int y, date *dt)
60{
61
62	return (gdate(easterodn(y), dt));
63}
64
65/* Compute the Julian date of Easter Sunday in Julian Calendar */
66date   *
67easteroj(int y, date * dt)
68{
69
70	return (jdate(easterodn(y), dt));
71}
72
73/* Compute the day number of Easter Sunday in Julian Calendar */
74static int
75easterodn(int y)
76{
77	/*
78	 * Table for the easter limits in one metonic (19-year) cycle. 21
79	 * to 31 is in March, 1 through 18 in April. Easter is the first
80	 * sunday after the easter limit.
81	 */
82	int     mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
83		    24, 12, 1, 21, 9, 29, 17};
84
85	/* Offset from a weekday to next sunday */
86	int     ns[] = {6, 5, 4, 3, 2, 1, 7};
87	date	dt;
88	int     dn;
89
90	/* Assign the easter limit of y to dt */
91	dt.d = mc[y % 19];
92
93	if (dt.d < 21)
94		dt.m = 4;
95	else
96		dt.m = 3;
97
98	dt.y = y;
99
100	/* Return the next sunday after the easter limit */
101	dn = ndaysj(&dt);
102	return (dn + ns[weekday(dn)]);
103}
104