104 lines
2.9 KiB
C
104 lines
2.9 KiB
C
/*-
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* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
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*
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* Copyright (c) 1997 Wolfgang Helbig
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include <sys/cdefs.h>
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__FBSDID("$FreeBSD: head/lib/libcalendar/easter.c 326219 2017-11-26 02:00:33Z pfg $");
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#include "calendar.h"
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typedef struct date date;
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static int easterodn(int y);
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/* Compute Easter Sunday in Gregorian Calendar */
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date *
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easterg(int y, date *dt)
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{
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int c, i, j, k, l, n;
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n = y % 19;
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c = y / 100;
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k = (c - 17) / 25;
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i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
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i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
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j = (y + y/4 + i + 2 - c + c/4) % 7;
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l = i - j;
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dt->m = 3 + (l + 40) / 44;
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dt->d = l + 28 - 31*(dt->m / 4);
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dt->y = y;
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return (dt);
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}
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/* Compute the Gregorian date of Easter Sunday in Julian Calendar */
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date *
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easterog(int y, date *dt)
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{
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return (gdate(easterodn(y), dt));
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}
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/* Compute the Julian date of Easter Sunday in Julian Calendar */
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date *
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easteroj(int y, date * dt)
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{
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return (jdate(easterodn(y), dt));
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}
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/* Compute the day number of Easter Sunday in Julian Calendar */
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static int
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easterodn(int y)
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{
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/*
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* Table for the easter limits in one metonic (19-year) cycle. 21
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* to 31 is in March, 1 through 18 in April. Easter is the first
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* sunday after the easter limit.
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*/
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int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
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24, 12, 1, 21, 9, 29, 17};
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/* Offset from a weekday to next sunday */
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int ns[] = {6, 5, 4, 3, 2, 1, 7};
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date dt;
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int dn;
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/* Assign the easter limit of y to dt */
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dt.d = mc[y % 19];
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if (dt.d < 21)
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dt.m = 4;
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else
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dt.m = 3;
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dt.y = y;
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/* Return the next sunday after the easter limit */
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dn = ndaysj(&dt);
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return (dn + ns[weekday(dn)]);
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}
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