1 /* $OpenBSD: pesach.c,v 1.4 2015/03/15 00:41:28 millert Exp $ */
2
3 /*
4 * Copyright (c) 2004 Michael Shalayeff
5 * All rights reserved.
6 *
7 * Permission to use, copy, modify, and distribute this software for any
8 * purpose with or without fee is hereby granted, provided that the above
9 * copyright notice and this permission notice appear in all copies.
10 *
11 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
12 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
13 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
14 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
15 * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER IN
16 * AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT
17 * OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
18 */
19
20 #include <stdio.h>
21
22 #include "calendar.h"
23
24 /* Calculate the Julian date of Pesach using the Gauss formula */
25
26 #define T (33. + 14. / 24.)
27 #define L ((1. + 485. / 1080.) / 24. / 19.)
28 #define K ((29. + (12. + 793. / 1080.) / 24. ) / 19.)
29
30 int
pesach(int R)31 pesach(int R)
32 {
33 int a, b, y, cumdays;
34 double d;
35
36 y = R + 3760;
37
38 a = (12 * y + 17) % 19;
39 b = y % 4;
40 d = (T - 10 * K + L + 14) + K * a + b / 4. - L * y;
41 cumdays = d;
42
43 /* the postponement */
44 switch ((int)(cumdays + 3 * y + 5 * b + 5) % 7) {
45 case 1:
46 if (a > 6 && d - cumdays >= (15. + 204. / 1080.) / 24.)
47 cumdays += 2;
48 break;
49
50 case 0:
51 if (a <= 11 || d - cumdays < (21. + 589. / 1080.) / 24.)
52 break;
53 /* FALLTHROUGH */
54 case 2:
55 case 4:
56 case 6:
57 cumdays++;
58 break;
59 }
60
61 if (R > 1582)
62 cumdays += R / 100 - R /400 - 2;
63
64 return (31 + 28 + cumdays + (isleap(R)? 1 : 0));
65 }
66