1 /* $OpenBSD: ostern.c,v 1.10 2024/04/23 13:34:50 jsg Exp $ */
2
3 /*
4 * Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
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 <stdio.h>
30 #include <time.h>
31
32 #include "calendar.h"
33
34 /* return year day for Easter */
35
36 int
easter(int year)37 easter(int year) /* 0 ... abcd, NOT since 1900 */
38 {
39 int e_a, e_b, e_c, e_d, e_e,e_f, e_g, e_h, e_i, e_k;
40 int e_l, e_m, e_n, e_p, e_q;
41
42 /* silly, but it works */
43 e_a = year % 19;
44 e_b = year / 100;
45 e_c = year % 100;
46
47 e_d = e_b / 4;
48 e_e = e_b % 4;
49 e_f = (e_b + 8) / 25;
50 e_g = (e_b + 1 - e_f) / 3;
51 e_h = ((19 * e_a) + 15 + e_b - (e_d + e_g)) % 30;
52 e_i = e_c / 4;
53 e_k = e_c % 4;
54 e_l = (32 + 2 * e_e + 2 * e_i - (e_h + e_k)) % 7;
55 e_m = (e_a + 11 * e_h + 22 * e_l) / 451;
56 e_n = (e_h + e_l + 114 - (7 * e_m)) / 31;
57 e_p = (e_h + e_l + 114 - (7 * e_m)) % 31;
58 e_p = e_p + 1;
59
60 e_q = 31 + 28 + e_p;
61 if (isleap(year))
62 e_q++;
63
64 if (e_n == 4)
65 e_q += 31;
66
67 #if DEBUG
68 printf("%d %d %d %d %d %d %d %d %d %d %d %d %d %d %d\n",
69 e_a, e_b, e_c, e_d, e_e, e_f, e_g, e_h,
70 e_i, e_k, e_l, e_m, e_n, e_p, e_q);
71 #endif
72
73 return (e_q);
74 }
75