1 /*- 2 * Copyright (c) 1997 Wolfgang Helbig 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * 1. Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * 2. Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in the 12 * documentation and/or other materials provided with the distribution. 13 * 14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24 * SUCH DAMAGE. 25 * 26 * $FreeBSD: src/lib/libcalendar/easter.c,v 1.4 1999/08/28 00:04:04 peter Exp $ 27 * $DragonFly: src/lib/libcalendar/easter.c,v 1.2 2003/06/17 04:26:48 dillon Exp $ 28 */ 29 30 #include "calendar.h" 31 32 typedef struct date date; 33 34 static int easterodn(int y); 35 36 /* Compute Easter Sunday in Gregorian Calendar */ 37 date * 38 easterg(int y, date *dt) 39 { 40 int c, i, j, k, l, n; 41 42 n = y % 19; 43 c = y / 100; 44 k = (c - 17) / 25; 45 i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30; 46 i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11)); 47 j = (y + y/4 + i + 2 - c + c/4) % 7; 48 l = i - j; 49 dt->m = 3 + (l + 40) / 44; 50 dt->d = l + 28 - 31*(dt->m / 4); 51 dt->y = y; 52 return (dt); 53 } 54 55 /* Compute the Gregorian date of Easter Sunday in Julian Calendar */ 56 date * 57 easterog(int y, date *dt) 58 { 59 60 return (gdate(easterodn(y), dt)); 61 } 62 63 /* Compute the Julian date of Easter Sunday in Julian Calendar */ 64 date * 65 easteroj(int y, date * dt) 66 { 67 68 return (jdate(easterodn(y), dt)); 69 } 70 71 /* Compute the day number of Easter Sunday in Julian Calendar */ 72 static int 73 easterodn(int y) 74 { 75 /* 76 * Table for the easter limits in one metonic (19-year) cycle. 21 77 * to 31 is in March, 1 through 18 in April. Easter is the first 78 * sunday after the easter limit. 79 */ 80 int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4, 81 24, 12, 1, 21, 9, 29, 17}; 82 83 /* Offset from a weekday to next sunday */ 84 int ns[] = {6, 5, 4, 3, 2, 1, 7}; 85 date dt; 86 int dn; 87 88 /* Assign the easter limit of y to dt */ 89 dt.d = mc[y % 19]; 90 91 if (dt.d < 21) 92 dt.m = 4; 93 else 94 dt.m = 3; 95 96 dt.y = y; 97 98 /* Return the next sunday after the easter limit */ 99 dn = ndaysj(&dt); 100 return (dn + ns[weekday(dn)]); 101 } 102