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: src/lib/libcalendar/easter.c,v 1.4 1999/08/28 00:04:04 peter Exp $
27 * $DragonFly: src/lib/libcalendar/easter.c,v 1.2 2003/06/17 04:26:48 dillon Exp $
28 */
29
30 #include "calendar.h"
31
32 typedef struct date date;
33
34 static int easterodn(int y);
35
36 /* Compute Easter Sunday in Gregorian Calendar */
37 date *
easterg(int y,date * dt)38 easterg(int y, date *dt)
39 {
40 int c, i, j, k, l, n;
41
42 n = y % 19;
43 c = y / 100;
44 k = (c - 17) / 25;
45 i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
46 i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
47 j = (y + y/4 + i + 2 - c + c/4) % 7;
48 l = i - j;
49 dt->m = 3 + (l + 40) / 44;
50 dt->d = l + 28 - 31*(dt->m / 4);
51 dt->y = y;
52 return (dt);
53 }
54
55 /* Compute the Gregorian date of Easter Sunday in Julian Calendar */
56 date *
easterog(int y,date * dt)57 easterog(int y, date *dt)
58 {
59
60 return (gdate(easterodn(y), dt));
61 }
62
63 /* Compute the Julian date of Easter Sunday in Julian Calendar */
64 date *
easteroj(int y,date * dt)65 easteroj(int y, date * dt)
66 {
67
68 return (jdate(easterodn(y), dt));
69 }
70
71 /* Compute the day number of Easter Sunday in Julian Calendar */
72 static int
easterodn(int y)73 easterodn(int y)
74 {
75 /*
76 * Table for the easter limits in one metonic (19-year) cycle. 21
77 * to 31 is in March, 1 through 18 in April. Easter is the first
78 * sunday after the easter limit.
79 */
80 int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
81 24, 12, 1, 21, 9, 29, 17};
82
83 /* Offset from a weekday to next sunday */
84 int ns[] = {6, 5, 4, 3, 2, 1, 7};
85 date dt;
86 int dn;
87
88 /* Assign the easter limit of y to dt */
89 dt.d = mc[y % 19];
90
91 if (dt.d < 21)
92 dt.m = 4;
93 else
94 dt.m = 3;
95
96 dt.y = y;
97
98 /* Return the next sunday after the easter limit */
99 dn = ndaysj(&dt);
100 return (dn + ns[weekday(dn)]);
101 }
102