Provides date of easter and other calendar related arithmetic.

1 files changed, 82 insertions, 0 deletions

diff --git a/lib/libcalendar/easter.c b/lib/libcalendar/easter.c
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+++ b/lib/libcalendar/easter.c
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+/*-
+ * Copyright (c) 1997 Wolfgang Helbig
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ *	$Id$
+ */
+
+#include "calendar.h"
+
+/* Compute Easter Sunday in Gregorian Calendar */
+date *
+easterg(int y, date *dt)
+{
+	int c, i, j, k, l, n;
+
+	n = y % 19;
+	c = y / 100;
+	k = (c - 17) / 25;
+	i = (c - c/4 -(c-k)/3 + 19 * n + 15) % 30;
+	i = i -(i/28) * (1 - (i/28) * (29/(i + 1)) * ((21 - n)/11));
+	j = (y + y/4 + i + 2 - c + c/4) % 7;
+	l = i - j;
+	dt->m = 3 + (l + 40) / 44;
+	dt->d = l + 28 - 31*(dt->m / 4);
+	dt->y = y;
+	return (dt);
+}
+
+/* Compute Easter Sunday in Julian Calendar */
+date   *
+easterj(int y, date * dt)
+{
+
+	/*
+	 * Table for the easter limits in one metonic (19-year) cycle. 21
+	 * to 31 is in March, 1 through 18 in April. Easter is the first
+	 * sunday after the easter limit.
+	 */
+	int     mc[] = {5, 25, 13, 2, 22, 10, 30, 18, 7, 27, 15, 4,
+		    24, 12, 1, 21, 9, 29, 17};
+
+	/* Offset from a weekday to next sunday */
+	int     ns[] = {6, 5, 4, 3, 2, 1, 7};
+	int     dn;
+
+	/* Assign the easter limit of y to *dt */
+	dt->d = mc[y % 19];
+
+	if (dt->d < 21)
+		dt->m = 4;
+	else
+		dt->m = 3;
+
+	dt->y = y;
+
+	/* Compute the next sunday after the easter limit */
+	dn = ndaysj(dt);
+	dn += ns[weekday(dn)];
+
+	return (jdate(dn, dt));
+}