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