1/*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 *    notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 *    notice, this list of conditions and the following disclaimer in the
14 *    documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28
29#include <sys/cdefs.h>
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 (isleap(year))
64		return 31 + 29 + 21 + L + 7;
65	else
66		return 31 + 28 + 21 + L + 7;
67}
68