1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD 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 __FBSDID("$FreeBSD$"); 31 32 #include <stdio.h> 33 #include <stdlib.h> 34 #include <string.h> 35 36 #include "calendar.h" 37 38 /* return year day for Easter */ 39 40 /* 41 * This code is based on the Calendar FAQ's code for how to calculate 42 * easter is. This is the Gregorian calendar version. They refer to 43 * the Algorithm of Oudin in the "Explanatory Supplement to the 44 * Astronomical Almanac". 45 */ 46 47 int 48 easter(int year) /* 0 ... abcd, NOT since 1900 */ 49 { 50 int G, /* Golden number - 1 */ 51 C, /* Century */ 52 H, /* 23 - epact % 30 */ 53 I, /* days from 21 March to Paschal full moon */ 54 J, /* weekday of full moon */ 55 L; /* days from 21 March to Sunday on of before full moon */ 56 57 G = year % 19; 58 C = year / 100; 59 H = (C - C / 4 - (8 * C + 13) / 25 + 19 * G + 15) % 30; 60 I = H - (H / 28) * (1 - (H / 28) * (29 / (H + 1)) * ((21 - G) / 11)); 61 J = (year + year / 4 + I + 2 - C + C / 4) % 7; 62 63 L = I - J; 64 65 if (isleap(year)) 66 return 31 + 29 + 21 + L + 7; 67 else 68 return 31 + 28 + 21 + L + 7; 69 } 70