xref: /freebsd/usr.bin/calendar/ostern.c (revision 3ff369fed2a08f32dda232c10470b949bef9489f)
1 /*
2  * Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
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 
27 #include <sys/cdefs.h>
28 __FBSDID("$FreeBSD$");
29 
30 #include <stdio.h>
31 #include <stdlib.h>
32 #include <string.h>
33 #include <time.h>
34 
35 #include "calendar.h"
36 
37 /* return year day for Easter */
38 
39 /*
40  * This code is based on the Calendar FAQ's code for how to calculate
41  * easter is. This is the Gregorian calendar version. They refer to
42  * the Algorithm of Oudin in the "Explanatory Supplement to the
43  * Astronomical Almanac".
44  */
45 
46 int easter (year)
47     int year;            /* 0 ... abcd, NOT since 1900 */
48 {
49     int G,	/* Golden number - 1 */
50 	C,	/* Century */
51 	H,	/* 23 - epact % 30 */
52 	I,	/* days from 21 March to Paschal full moon */
53 	J,	/* weekday of full moon */
54 	L;	/* days from 21 March to Sunday on of before full moon */
55 
56     G = year % 19;
57     C = year / 100;
58     H = (C - C/4 - (8*C+13)/25 + 19*G + 15) % 30;
59     I = H - (H/28)*(1 - (H/28)*(29/(H + 1))*((21 - G)/11));
60     J = (year + year/4 + I + 2 - C + C/4) % 7;
61 
62     L = I - J;
63 
64     if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
65 	return 31 + 29 + 21 + L + 7;
66     else
67 	return 31 + 28 + 21 + L + 7;
68 }
69 
70 /* return year day for  Easter or easter depending days
71  * Match: Easter([+-][0-9]+)?
72  * e.g: Easter-2 is  Good Friday (2 days before Easter)
73  */
74 
75 int
76 geteaster(s, year)
77 	char *s;
78         int year;
79 {
80 	int offset = 0;
81 	extern struct fixs neaster;
82 
83 #define EASTER "easter"
84 #define EASTERNAMELEN (sizeof(EASTER) - 1)
85 
86 	if (strncasecmp(s, EASTER, EASTERNAMELEN) == 0)
87 	    s += EASTERNAMELEN;
88 	else if (   neaster.name != NULL
89 		 && strncasecmp(s, neaster.name, neaster.len) == 0
90 		)
91 	    s += neaster.len;
92 	else
93 	    return(0);
94 
95 #if DEBUG
96 	printf("%s %d %d\n", s, year, EASTERNAMELEN);
97 #endif
98 
99 	/* Easter+1  or Easter-2
100 	 *       ^            ^   */
101 
102 	switch(*s) {
103 
104 	case '-':
105 	case '+':
106 	    offset = atoi(s);
107 	    break;
108 
109 	default:
110 	    offset = 0;
111 	}
112 
113 	return (easter(year) + offset);
114 }
115