freebsd-src/usr.bin/calendar/ostern.c
David Malone db3626fb72 Use a slightly better documented way of calculating easter and
include a reference to the Calendar FAQ. Also make argument parsing
a little more robust.

PR:		23881
2001-01-02 11:35:57 +00:00

114 lines
3.2 KiB
C

/*
* Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* $FreeBSD$
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include "calendar.h"
/* return year day for Easter */
/*
* This code is based on the Calendar FAQ's code for how to calculate
* easter is. This is the Gregorian calendar version. They refer to
* the Algorithm of Oudin in the "Explanatory Supplement to the
* Astronomical Almanac".
*/
int easter (year)
int year; /* 0 ... abcd, NOT since 1900 */
{
int G, /* Golden number - 1 */
C, /* Century */
H, /* 23 - epact % 30 */
I, /* days from 21 March to Paschal full moon */
J, /* weekday of full moon */
L; /* days from 21 March to Sunday on of before full moon */
G = year % 19;
C = year / 100;
H = (C - C/4 - (8*C+13)/25 + 19*G + 15) % 30;
I = H - (H/28)*(1 - (H/28)*(29/(H + 1))*((21 - G)/11));
J = (year + year/4 + I + 2 - C + C/4) % 7;
L = I - J;
if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
return 31 + 29 + 21 + L + 7;
else
return 31 + 28 + 21 + L + 7;
}
/* return year day for Easter or easter depending days
* Match: Easter([+-][0-9]+)?
* e.g: Easter-2 is Good Friday (2 days before Easter)
*/
int
geteaster(s, year)
char *s;
int year;
{
register int offset = 0;
extern struct fixs neaster;
#define EASTER "easter"
#define EASTERNAMELEN (sizeof(EASTER) - 1)
if (strncasecmp(s, EASTER, EASTERNAMELEN) == 0)
s += EASTERNAMELEN;
else if ( neaster.name != NULL
&& strncasecmp(s, neaster.name, neaster.len) == 0
)
s += neaster.len;
else
return(0);
#if DEBUG
printf("%s %d %d\n", s, year, EASTERNAMELEN);
#endif
/* Easter+1 or Easter-2
* ^ ^ */
switch(*s) {
case '-':
case '+':
offset = atoi(s);
break;
default:
offset = 0;
}
return (easter(year) + offset);
}