131529Shelbig/*- 231529Shelbig * Copyright (c) 1997 Wolfgang Helbig 331529Shelbig * All rights reserved. 431529Shelbig * 531529Shelbig * Redistribution and use in source and binary forms, with or without 631529Shelbig * modification, are permitted provided that the following conditions 731529Shelbig * are met: 831529Shelbig * 1. Redistributions of source code must retain the above copyright 931529Shelbig * notice, this list of conditions and the following disclaimer. 1031529Shelbig * 2. Redistributions in binary form must reproduce the above copyright 1131529Shelbig * notice, this list of conditions and the following disclaimer in the 1231529Shelbig * documentation and/or other materials provided with the distribution. 1331529Shelbig * 1431529Shelbig * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 1531529Shelbig * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 1631529Shelbig * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 1731529Shelbig * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 1831529Shelbig * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 1931529Shelbig * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 2031529Shelbig * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 2131529Shelbig * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 2231529Shelbig * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 2331529Shelbig * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 2431529Shelbig * SUCH DAMAGE. 2531529Shelbig */ 2631529Shelbig 2784197Sdillon#include <sys/cdefs.h> 2884197Sdillon__FBSDID("$FreeBSD$"); 2984197Sdillon 3031529Shelbig#include "calendar.h" 3131529Shelbig 3231698Shelbigtypedef struct date date; 3331698Shelbig 3431611Shelbigstatic int easterodn(int y); 3531611Shelbig 3631529Shelbig/* Compute Easter Sunday in Gregorian Calendar */ 3731529Shelbigdate * 3831529Shelbigeasterg(int y, date *dt) 3931529Shelbig{ 4031529Shelbig int c, i, j, k, l, n; 4131529Shelbig 4231529Shelbig n = y % 19; 4331529Shelbig c = y / 100; 4431529Shelbig k = (c - 17) / 25; 4531529Shelbig i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30; 4631529Shelbig i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11)); 4731529Shelbig j = (y + y/4 + i + 2 - c + c/4) % 7; 4831529Shelbig l = i - j; 4931529Shelbig dt->m = 3 + (l + 40) / 44; 5031529Shelbig dt->d = l + 28 - 31*(dt->m / 4); 5131529Shelbig dt->y = y; 5231529Shelbig return (dt); 5331529Shelbig} 5431529Shelbig 5531611Shelbig/* Compute the Gregorian date of Easter Sunday in Julian Calendar */ 5631611Shelbigdate * 5731611Shelbigeasterog(int y, date *dt) 5831611Shelbig{ 5931611Shelbig 6031611Shelbig return (gdate(easterodn(y), dt)); 6131611Shelbig} 6231611Shelbig 6331611Shelbig/* Compute the Julian date of Easter Sunday in Julian Calendar */ 6431529Shelbigdate * 6531611Shelbigeasteroj(int y, date * dt) 6631529Shelbig{ 6731529Shelbig 6831611Shelbig return (jdate(easterodn(y), dt)); 6931611Shelbig} 7031611Shelbig 7131611Shelbig/* Compute the day number of Easter Sunday in Julian Calendar */ 7231611Shelbigstatic int 7331611Shelbigeasterodn(int y) 7431611Shelbig{ 7531529Shelbig /* 7631529Shelbig * Table for the easter limits in one metonic (19-year) cycle. 21 7731529Shelbig * to 31 is in March, 1 through 18 in April. Easter is the first 7831529Shelbig * sunday after the easter limit. 7931529Shelbig */ 8031529Shelbig int mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4, 8131529Shelbig 24, 12, 1, 21, 9, 29, 17}; 8231529Shelbig 8331529Shelbig /* Offset from a weekday to next sunday */ 8431529Shelbig int ns[] = {6, 5, 4, 3, 2, 1, 7}; 8531611Shelbig date dt; 8631529Shelbig int dn; 8731529Shelbig 8831611Shelbig /* Assign the easter limit of y to dt */ 8931611Shelbig dt.d = mc[y % 19]; 9031529Shelbig 9131611Shelbig if (dt.d < 21) 9231611Shelbig dt.m = 4; 9331529Shelbig else 9431611Shelbig dt.m = 3; 9531529Shelbig 9631611Shelbig dt.y = y; 9731529Shelbig 9831611Shelbig /* Return the next sunday after the easter limit */ 9931611Shelbig dn = ndaysj(&dt); 10031611Shelbig return (dn + ns[weekday(dn)]); 10131529Shelbig} 102