1 /*- 2 * SPDX-License-Identifier: BSD-2-Clause 3 * 4 * Copyright (c) 1997 Wolfgang Helbig 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 "calendar.h" 33 34 typedef struct date date; 35 36 static int easterodn(int y); 37 38 /* Compute Easter Sunday in Gregorian Calendar */ 39 date * 40 easterg(int y, date *dt) 41 { 42 int c, i, j, k, l, n; 43 44 n = y % 19; 45 c = y / 100; 46 k = (c - 17) / 25; 47 i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30; 48 i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11)); 49 j = (y + y/4 + i + 2 - c + c/4) % 7; 50 l = i - j; 51 dt->m = 3 + (l + 40) / 44; 52 dt->d = l + 28 - 31*(dt->m / 4); 53 dt->y = y; 54 return (dt); 55 } 56 57 /* Compute the Gregorian date of Easter Sunday in Julian Calendar */ 58 date * 59 easterog(int y, date *dt) 60 { 61 62 return (gdate(easterodn(y), dt)); 63 } 64 65 /* Compute the Julian date of Easter Sunday in Julian Calendar */ 66 date * 67 easteroj(int y, date * dt) 68 { 69 70 return (jdate(easterodn(y), dt)); 71 } 72 73 /* Compute the day number of Easter Sunday in Julian Calendar */ 74 static int 75 easterodn(int y) 76 { 77 /* 78 * Table for the easter limits in one metonic (19-year) cycle. 21 79 * to 31 is in March, 1 through 18 in April. Easter is the first 80 * sunday after the easter limit. 81 */ 82 int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4, 83 24, 12, 1, 21, 9, 29, 17}; 84 85 /* Offset from a weekday to next sunday */ 86 int ns[] = {6, 5, 4, 3, 2, 1, 7}; 87 date dt; 88 int dn; 89 90 /* Assign the easter limit of y to dt */ 91 dt.d = mc[y % 19]; 92 93 if (dt.d < 21) 94 dt.m = 4; 95 else 96 dt.m = 3; 97 98 dt.y = y; 99 100 /* Return the next sunday after the easter limit */ 101 dn = ndaysj(&dt); 102 return (dn + ns[weekday(dn)]); 103 } 104