1 files changed, 67 insertions, 0 deletions
diff --git a/usr.bin/calendar/ostern.c b/usr.bin/calendar/ostern.c
new file mode 100644
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--- /dev/null
+++ b/usr.bin/calendar/ostern.c
@@ -0,0 +1,67 @@
+/*-
+ * Copyright (c) 1996 Wolfram Schneider <wosch@FreeBSD.org>. Berlin.
+ * 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.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "calendar.h"
+
+/* return year day for Easter */
+
+/*
+ * This code is based on the Calendar FAQ's code for how to calculate
+ * easter is. This is the Gregorian calendar version. They refer to
+ * the Algorithm of Oudin in the "Explanatory Supplement to the
+ * Astronomical Almanac".
+ */
+
+int
+easter(int year) /* 0 ... abcd, NOT since 1900 */
+{
+	int G,	/* Golden number - 1 */
+	    C,	/* Century */
+	    H,	/* 23 - epact % 30 */
+	    I,	/* days from 21 March to Paschal full moon */
+	    J,	/* weekday of full moon */
+	    L;	/* days from 21 March to Sunday on of before full moon */
+
+	G = year % 19;
+	C = year / 100;
+	H = (C - C / 4 - (8 * C + 13) / 25 + 19 * G + 15) % 30;
+	I = H - (H / 28) * (1 - (H / 28) * (29 / (H + 1)) * ((21 - G) / 11));
+	J = (year + year / 4 + I + 2 - C + C / 4) % 7;
+
+	L = I - J;
+
+	if (isleap(year))
+		return 31 + 29 + 21 + L + 7;
+	else
+		return 31 + 28 + 21 + L + 7;
+}