xref: /freebsd/usr.bin/calendar/paskha.c (revision a0b9e2e854027e6ff61fb075a1309dbc71c42b54)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3  *
4  * Copyright (C) 1993-1996 by Andrey A. Chernov, Moscow, Russia.
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 ``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 REGENTS 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 <stdio.h>
33 #include <stdlib.h>
34 #include <string.h>
35 
36 #include "calendar.h"
37 
38 #define	PASKHA		"paskha"
39 #define	PASKHALEN	(sizeof(PASKHA) - 1)
40 
41 /* return difference in days between Julian and Gregorian calendars */
42 int
43 j2g(int year)
44 {
45 	return (year < 1500) ?
46 		0 :
47 		10 + (year/100 - 16) - ((year/100 - 16) / 4);
48 }
49 
50 /* return year day for Orthodox Easter using Gauss formula */
51 /* (new style result) */
52 
53 int
54 paskha(int R) /*year*/
55 {
56 	int a, b, c, d, e;
57 	static int x = 15;
58 	static int y = 6;
59 	int *cumday;
60 
61 	a = R % 19;
62 	b = R % 4;
63 	c = R % 7;
64 	d = (19 * a + x) % 30;
65 	e = (2 * b + 4 * c + 6 * d + y) % 7;
66 	cumday = cumdaytab[isleap(R)];
67 	return (((cumday[3] + 1) + 22) + (d + e) + j2g(R));
68 }
69