ostern.c revision 200470 
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: head/usr.bin/calendar/ostern.c 200470 2009-12-13 04:50:11Z delphij $");
29
30#include <stdio.h>
31#include <stdlib.h>
32#include <string.h>
33
34#include "calendar.h"
35
36/* return year day for Easter */
37
38/*
39 * This code is based on the Calendar FAQ's code for how to calculate
40 * easter is. This is the Gregorian calendar version. They refer to
41 * the Algorithm of Oudin in the "Explanatory Supplement to the
42 * Astronomical Almanac".
43 */
44
45int
46easter(int year) /* 0 ... abcd, NOT since 1900 */
47{
48	int G,	/* Golden number - 1 */
49	    C,	/* Century */
50	    H,	/* 23 - epact % 30 */
51	    I,	/* days from 21 March to Paschal full moon */
52	    J,	/* weekday of full moon */
53	    L;	/* days from 21 March to Sunday on of before full moon */
54
55	G = year % 19;
56	C = year / 100;
57	H = (C - C / 4 - (8 * C + 13) / 25 + 19 * G + 15) % 30;
58	I = H - (H / 28) * (1 - (H / 28) * (29 / (H + 1)) * ((21 - G) / 11));
59	J = (year + year / 4 + I + 2 - C + C / 4) % 7;
60
61	L = I - J;
62
63	if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
64		return 31 + 29 + 21 + L + 7;
65	else
66		return 31 + 28 + 21 + L + 7;
67}
68
69/* return year day for  Easter or easter depending days
70 * Match: Easter([+-][0-9]+)?
71 * e.g: Easter-2 is  Good Friday (2 days before Easter)
72 */
73
74int
75geteaster(char *s, int year)
76{
77	int offset = 0;
78
79#define	EASTER "easter"
80#define	EASTERNAMELEN (sizeof(EASTER) - 1)
81
82	if (strncasecmp(s, EASTER, EASTERNAMELEN) == 0)
83		s += EASTERNAMELEN;
84	else if (neaster.name != NULL
85	    && strncasecmp(s, neaster.name, neaster.len) == 0)
86		s += neaster.len;
87	else
88		return (0);
89
90#if DEBUG
91	printf("%s %d %d\n", s, year, EASTERNAMELEN);
92#endif
93
94	/* Easter+1  or Easter-2
95	 *       ^            ^   */
96
97	switch (*s) {
98
99	case '-':
100	case '+':
101		offset = atoi(s);
102		break;
103
104	default:
105		offset = 0;
106	}
107
108	return (easter(year) + offset);
109}
110