ostern.c revision 87235 
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
29__FBSDID("$FreeBSD: head/usr.bin/calendar/ostern.c 87235 2001-12-02 22:44:14Z markm $");
30
31#include <stdio.h>
32#include <stdlib.h>
33#include <string.h>
34#include <time.h>
35
36#include "calendar.h"
37
38/* return year day for Easter */
39
40/*
41 * This code is based on the Calendar FAQ's code for how to calculate
42 * easter is. This is the Gregorian calendar version. They refer to
43 * the Algorithm of Oudin in the "Explanatory Supplement to the
44 * Astronomical Almanac".
45 */
46
47int easter (year)
48    int year;            /* 0 ... abcd, NOT since 1900 */
49{
50    int G,	/* Golden number - 1 */
51	C,	/* Century */
52	H,	/* 23 - epact % 30 */
53	I,	/* days from 21 March to Paschal full moon */
54	J,	/* weekday of full moon */
55	L;	/* days from 21 March to Sunday on of before full moon */
56
57    G = year % 19;
58    C = year / 100;
59    H = (C - C/4 - (8*C+13)/25 + 19*G + 15) % 30;
60    I = H - (H/28)*(1 - (H/28)*(29/(H + 1))*((21 - G)/11));
61    J = (year + year/4 + I + 2 - C + C/4) % 7;
62
63    L = I - J;
64
65    if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
66	return 31 + 29 + 21 + L + 7;
67    else
68	return 31 + 28 + 21 + L + 7;
69}
70
71/* return year day for  Easter or easter depending days
72 * Match: Easter([+-][0-9]+)?
73 * e.g: Easter-2 is  Good Friday (2 days before Easter)
74 */
75
76int
77geteaster(s, year)
78	char *s;
79        int year;
80{
81	int offset = 0;
82	extern struct fixs neaster;
83
84#define EASTER "easter"
85#define EASTERNAMELEN (sizeof(EASTER) - 1)
86
87	if (strncasecmp(s, EASTER, EASTERNAMELEN) == 0)
88	    s += EASTERNAMELEN;
89	else if (   neaster.name != NULL
90		 && strncasecmp(s, neaster.name, neaster.len) == 0
91		)
92	    s += neaster.len;
93	else
94	    return(0);
95
96#if DEBUG
97	printf("%s %d %d\n", s, year, EASTERNAMELEN);
98#endif
99
100	/* Easter+1  or Easter-2
101	 *       ^            ^   */
102
103	switch(*s) {
104
105	case '-':
106	case '+':
107	    offset = atoi(s);
108	    break;
109
110	default:
111	    offset = 0;
112	}
113
114	return (easter(year) + offset);
115}
116