usr.bin/calendar/ostern.c

   1 /*
   2  * Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
   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/usr.bin/calendar/ostern.c,v 1.12 2007/05/07 11:18:30 dwmalone Exp $
  27  * $DragonFly: src/usr.bin/calendar/ostern.c,v 1.6 2007/09/24 20:49:09 pavalos Exp $
  28  */
  29
  30 #include <stdio.h>
  31 #include <stdlib.h>
  32 #include <string.h>
  33 #include <time.h>
  34
  35 #include "calendar.h"
  36
  37 static int      easter(int);
  38
  39 /* return year day for Easter */
  40
  41 /*
  42  * This code is based on the Calendar FAQ's code for how to calculate
  43  * easter is. This is the Gregorian calendar version. They refer to
  44  * the Algorithm of Oudin in the "Explanatory Supplement to the
  45  * Astronomical Almanac".
  46  */
  47
  48 static int
  49 easter(int year) /* 0 ... abcd, NOT since 1900 */
  50 {
  51     int G,      /* Golden number - 1 */
  52         C,      /* Century */
  53         H,      /* 23 - epact % 30 */
  54         I,      /* days from 21 March to Paschal full moon */
  55         J,      /* weekday of full moon */
  56         L;      /* days from 21 March to Sunday on of before full moon */
  57
  58     G = year % 19;
  59     C = year / 100;
  60     H = (C - C/4 - (8*C+13)/25 + 19*G + 15) % 30;
  61     I = H - (H/28)*(1 - (H/28)*(29/(H + 1))*((21 - G)/11));
  62     J = (year + year/4 + I + 2 - C + C/4) % 7;
  63
  64     L = I - J;
  65
  66     if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
  67         return(31 + 29 + 21 + L + 7);
  68     else
  69         return(31 + 28 + 21 + L + 7);
  70 }
  71
  72 /* return year day for  Easter or easter depending days
  73  * Match: Easter([+-][0-9]+)?
  74  * e.g: Easter-2 is  Good Friday (2 days before Easter)
  75  */
  76
  77 int
  78 geteaster(char *s, int year)
  79 {
  80         int offset = 0;
  81
  82 #define EASTER "easter"
  83 #define EASTERNAMELEN (sizeof(EASTER) - 1)
  84
  85         if (strncasecmp(s, EASTER, EASTERNAMELEN) == 0)
  86             s += EASTERNAMELEN;
  87         else if (neaster.name != NULL
  88                  && strncasecmp(s, neaster.name, neaster.len) == 0
  89                 )
  90             s += neaster.len;
  91         else
  92             return(0);
  93
  94 #if DEBUG
  95         printf("%s %d %d\n", s, year, EASTERNAMELEN);
  96 #endif
  97
  98         /* Easter+1  or Easter-2
  99          *       ^            ^   */
 100
 101         switch(*s) {
 102         case '-':
 103         case '+':
 104             offset = atoi(s);
 105             break;
 106
 107         default:
 108             offset = 0;
 109         }
 110
 111         return(easter(year) + offset);
 112 }