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 * $Id: easter.c,v 1.2 1997/12/07 19:04:14 helbig Exp $ 27 */ 28 29 #include "calendar.h" 30 31 typedef struct date date; 32 33 static int easterodn(int y); 34 35 /* Compute Easter Sunday in Gregorian Calendar */ 36 date * 37 easterg(int y, date *dt) 38 { 39 int c, i, j, k, l, n; 40 41 n = y % 19; 42 c = y / 100; 43 k = (c - 17) / 25; 44 i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30; 45 i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11)); 46 j = (y + y/4 + i + 2 - c + c/4) % 7; 47 l = i - j; 48 dt->m = 3 + (l + 40) / 44; 49 dt->d = l + 28 - 31*(dt->m / 4); 50 dt->y = y; 51 return (dt); 52 } 53 54 /* Compute the Gregorian date of Easter Sunday in Julian Calendar */ 55 date * 56 easterog(int y, date *dt) 57 { 58 59 return (gdate(easterodn(y), dt)); 60 } 61 62 /* Compute the Julian date of Easter Sunday in Julian Calendar */ 63 date * 64 easteroj(int y, date * dt) 65 { 66 67 return (jdate(easterodn(y), dt)); 68 } 69 70 /* Compute the day number of Easter Sunday in Julian Calendar */ 71 static int 72 easterodn(int y) 73 { 74 /* 75 * Table for the easter limits in one metonic (19-year) cycle. 21 76 * to 31 is in March, 1 through 18 in April. Easter is the first 77 * sunday after the easter limit. 78 */ 79 int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4, 80 24, 12, 1, 21, 9, 29, 17}; 81 82 /* Offset from a weekday to next sunday */ 83 int ns[] = {6, 5, 4, 3, 2, 1, 7}; 84 date dt; 85 int dn; 86 87 /* Assign the easter limit of y to dt */ 88 dt.d = mc[y % 19]; 89 90 if (dt.d < 21) 91 dt.m = 4; 92 else 93 dt.m = 3; 94 95 dt.y = y; 96 97 /* Return the next sunday after the easter limit */ 98 dn = ndaysj(&dt); 99 return (dn + ns[weekday(dn)]); 100 } 101