lib/libcalendar/easter.c

   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 }