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/*-
* Copyright (c) 1997 Wolfgang Helbig
* 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 "calendar.h"
typedef struct date date;
static int easterodn(int y);
/* Compute Easter Sunday in Gregorian Calendar */
date *
easterg(int y, date *dt)
{
int c, i, j, k, l, n;
n = y % 19;
c = y / 100;
k = (c - 17) / 25;
i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
j = (y + y/4 + i + 2 - c + c/4) % 7;
l = i - j;
dt->m = 3 + (l + 40) / 44;
dt->d = l + 28 - 31*(dt->m / 4);
dt->y = y;
return (dt);
}
/* Compute the Gregorian date of Easter Sunday in Julian Calendar */
date *
easterog(int y, date *dt)
{
return (gdate(easterodn(y), dt));
}
/* Compute the Julian date of Easter Sunday in Julian Calendar */
date *
easteroj(int y, date * dt)
{
return (jdate(easterodn(y), dt));
}
/* Compute the day number of Easter Sunday in Julian Calendar */
static int
easterodn(int y)
{
/*
* Table for the easter limits in one metonic (19-year) cycle. 21
* to 31 is in March, 1 through 18 in April. Easter is the first
* sunday after the easter limit.
*/
int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
24, 12, 1, 21, 9, 29, 17};
/* Offset from a weekday to next sunday */
int ns[] = {6, 5, 4, 3, 2, 1, 7};
date dt;
int dn;
/* Assign the easter limit of y to dt */
dt.d = mc[y % 19];
if (dt.d < 21)
dt.m = 4;
else
dt.m = 3;
dt.y = y;
/* Return the next sunday after the easter limit */
dn = ndaysj(&dt);
return (dn + ns[weekday(dn)]);
}
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