Provides date of easter and other calendar related arithmetic.

5 files changed, 650 insertions, 0 deletions

diff --git a/lib/libcalendar/Makefile b/lib/libcalendar/Makefile
new file mode 100644
index 0000000..fe41148
--- /dev/null
+++ b/lib/libcalendar/Makefile
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+#	$Id$
+
+LIB=	calendar
+
+SRCS=	calendar.c easter.c
+
+MAN3=   calendar.3
+
+MLINKS=	calendar.3 easterg.3 calendar.3 easterj.3 \
+	calendar.3 gdate.3 calendar.3 jdate.3 \
+	calendar.3 ndaysg.3 calendar.3 ndaysj.3 \
+	calendar.3 week.3 calendar.3 weekday.3
+
+CFLAGS+=-I. -I${.CURDIR} -Wall
+
+beforeinstall:
+	${INSTALL} -C -m 444 -o $(BINOWN) -g $(BINGRP) ${.CURDIR}/calendar.h \
+		${DESTDIR}/usr/include
+
+.include <bsd.lib.mk>
diff --git a/lib/libcalendar/calendar.3 b/lib/libcalendar/calendar.3
new file mode 100644
index 0000000..4f51a02
--- /dev/null
+++ b/lib/libcalendar/calendar.3
@@ -0,0 +1,180 @@
+.\" 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.
+.\"
+.\"	$Id$
+.\"
+.Dd November 29, 1997
+.Dt CALENDAR 3
+.Os
+.Sh NAME
+.Nm easterg ,
+.Nm easterj ,
+.Nm gdate ,
+.Nm jdate ,
+.Nm ndaysg ,
+.Nm ndaysj ,
+.Nm week ,
+.Nm weekday
+.Nd Calendar arithmetic for the Christian era.
+.Sh SYNOPSIS
+.Fd #include <calendar.h>
+.Ft date *
+.Fn easterg "int year" "date *dt"
+.Ft date *
+.Fn easterj "int year" "date *dt"
+.Ft date *
+.Fn gdate "int nd" "date *dt"
+.Ft date *
+.Fn jdate "int nd" "date *dt"
+.Ft int
+.Fn ndaysg "date *dt"
+.Ft int
+.Fn ndaysj "date *dt"
+.Ft int
+.Fn week "int nd" "int *year"
+.Ft int
+.Fn weekday "int nd"
+.Sh DESCRIPTION
+These functions provide calendar arithmetic for a large range of years,
+starting at March 1st, year zero (i. e. 1 B.C.) and ending way beyond
+year 100000.
+
+Programs should be linked with
+.Fl lcalendar .
+
+The functions
+.Fn easterg
+and
+.Fn easterj
+store the date of Easter Sunday into the structure pointed at by
+.Fa dt
+and return a pointer to this structure.
+The function
+.Fn easterg
+assumes Gregorian Calendar (adopted by most western churches after 1582) and
+.Fn easterj
+assumes Julian Calendar (Western churches before 1582 and Greek Church
+until today).
+
+The functions 
+.Fn gdate ,
+.Fn jdate ,
+.Fn ndaysg
+and
+.Fn ndaysj
+provide conversions between the common "year, month, day" notation
+of a date and the "number of days" representation, which is better suited
+for calculations. The days are numbered from March 1st year 1 B.C., starting
+with zero, so the number of a day gives the number of days since March 1st,
+year 1 B.C. The conversions work for nonnegative day numbers only.
+
+The
+.Fn gdate
+and
+.Fn jdate
+functions
+store the date corresponding to the day number
+.Fa nd
+into the structure pointed at by
+.Fa dt
+and return a pointer to this structure.
+
+The
+.Fn ndaysg
+and
+.Fn ndaysj
+functions
+return the day number of the date pointed at by
+.Fa dt .
+
+The
+.Fn gdate
+and
+.Fn ndaysg
+functions
+assume Gregorian Calendar after October 4th 1582 and Julian Calendar before,
+whereas
+.Fn jdate
+and
+.Fn ndaysj
+assume Julian Calendar throughout.
+
+The two calendars differ by the definition of the leap year. The
+Julian Calendar says every year that is a multiple of four is a
+leap year. The Gregorian Calendar excludes years that are multiples of
+100 and not multiples of 400.
+This means the years 1700, 1800, 1900, 2100 are not leap years
+and the year 2000 is
+a leap year.
+The new rules were inaugurated on October 4th 1582 by deleting ten
+days following this date.
+
+The function
+.Fn week
+returns the number of the week which contains the day numbered
+.Fa nd .
+The argument
+.Fa *year
+is set with the year that contains (the greater part of) the week.
+The weeks are numbered per year starting with week 1, which is the
+first week in a year that includes more than three days of the year.
+Weeks start on Monday.
+This function is defined for Gregorian Calendar only.
+
+The function
+.Fn weekday
+returns the weekday (Mo = 0 .. Su = 6) of the day numbered 
+.Fa nd .
+
+The type
+.Fa date
+is a structure defined in
+.Aq Pa calendar.h .
+It contains these fields:
+.Bd -literal -offset indent
+int y;          /\(** year (0000 - ????) \(**/
+int m;          /\(** month (1 - 12) \(**/
+int d;          /\(** day of month (1 - 31) \(**/
+.Ed
+
+The year zero is written as "1 B.C." by historians and "0" by astronomers
+and in this library.
+.Sh SEE ALSO
+.Xr ncal 1 ,
+.Xr strftime 3
+.Rs
+.%A A. B. Author
+.%D November 1997
+.Sh STANDARDS
+The week number conforms to ISO 8601: 1988. 
+.Sh HISTORY
+The
+.Nm calendar
+library first appeared in
+.Fx 3.0 .
+.Sh AUTHOR
+This manual page and the library was written by
+.An Wolfgang Helbig Aq helbig@FreeBSD.ORG .
+.Sh BUGS
+The library was coded with great care so there are no bugs left.
diff --git a/lib/libcalendar/calendar.c b/lib/libcalendar/calendar.c
new file mode 100644
index 0000000..41e79b2
--- /dev/null
+++ b/lib/libcalendar/calendar.c
@@ -0,0 +1,327 @@
+/*-
+ * 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"
+
+#ifndef NULL
+#define NULL 0
+#endif
+
+/*
+ * For each month tabulate the number of days elapsed in a year before the
+ * month. This assumes the internal date representation, where a year
+ * starts on March 1st. So we don't need a special table for leap years.
+ * But we do need a special table for the year 1582, since 10 days are
+ * deleted in October. This is month1s for the switch from Julian to
+ * Gregorian calendar.
+ */
+static int const month1[] =
+    {0, 31, 61, 92, 122, 153, 184, 214, 245, 275, 306, 337}; 
+   /*  M   A   M   J    J    A    S    O    N    D    J */
+static int const month1s[]=
+    {0, 31, 61, 92, 122, 153, 184, 214, 235, 265, 296, 327}; 
+
+/* The last day of Julian calendar, in internal and ndays representation */
+static int nswitch;	/* The last day of Julian calendar */
+static date jiswitch = {1582, 7, 3};
+
+static date	*date2idt(date *idt, date *dt);
+static date	*idt2date(date *dt, date *idt);
+static int	 ndaysji(date *idt);
+static int	 ndaysgi(date *idt);
+static int	 firstweek(int year);
+
+/*
+ * Compute the Julian date from the number of days elapsed since
+ * March 1st of year zero.
+ */
+date *
+jdate(int ndays, date *dt)
+{
+	date    idt;		/* Internal date representation */
+	int     r;		/* hold the rest of days */
+
+	/*
+	 * Compute the year by starting with an approximation not smaller
+	 * than the answer and using linear search for the greatest
+	 * year which does not begin after ndays.
+	 */
+	idt.y = ndays / 365;
+	idt.m = 0;
+	idt.d = 0;
+	while ((r = ndaysji(&idt)) > ndays)
+		idt.y--;
+	
+	/*
+	 * Set r to the days left in the year and compute the month by
+	 * linear search as the largest month that does not begin after r
+	 * days.
+	 */
+	r = ndays - r;
+	for (idt.m = 11; month1[idt.m] > r; idt.m--)
+		;
+
+	/* Compute the days left in the month */
+	idt.d = r - month1[idt.m];
+
+	/* return external representation of the date */
+	return (idt2date(dt, &idt));
+}
+
+/*
+ * Return the number of days since March 1st of the year zero.
+ * The date is given according to Julian calendar.
+ */
+int
+ndaysj(date *dt)
+{
+	date    idt;		/* Internal date representation */
+
+	if (date2idt(&idt, dt) == NULL)
+		return (-1);
+	else
+		return (ndaysji(&idt));
+}
+
+/*
+ * Same as above, where the Julian date is given in internal notation.
+ * This formula shows the beauty of this notation.
+ */
+static int
+ndaysji(date * idt)
+{
+
+	return (idt->d + month1[idt->m] + idt->y * 365 + idt->y / 4);
+}
+
+/*
+ * Compute the date according to the Gregorian calendar from the number of
+ * days since March 1st, year zero. The date computed will be Julian if it
+ * is older than 1582-10-05. This is the reverse of the function ndaysg().
+ */
+date   *
+gdate(int ndays, date *dt)
+{
+	int const *montht;	/* month-table */
+	date    idt;		/* for internal date representation */
+	int     r;		/* holds the rest of days */
+
+	/*
+	 * Compute the year by starting with an approximation not smaller
+	 * than the answer and search linearly for the greatest year not
+	 * starting after ndays.
+	 */
+	idt.y = ndays / 365;
+	idt.m = 0;
+	idt.d = 0;
+	while ((r = ndaysgi(&idt)) > ndays)
+		idt.y--;
+
+	/*
+	 * Set ndays to the number of days left and compute by linear
+	 * search the greatest month which does not start after ndays. We
+	 * use the table month1 which provides for each month the number
+	 * of days that elapsed in the year before that month. Here the
+	 * year 1582 is special, as 10 days are left out in October to
+	 * resynchronize the calendar with the earth's orbit. October 4th
+	 * 1582 is followed by October 15th 1582. We use the "switch"
+	 * table month1s for this year.
+	 */
+	ndays = ndays - r;
+	if (idt.y == 1582)
+		montht = month1s;
+	else
+		montht = month1;
+
+	for (idt.m = 11; montht[idt.m] > ndays; idt.m--)
+		;
+
+	idt.d = ndays - montht[idt.m]; /* the rest is the day in month */
+
+	/* Advance ten days deleted from October if after switch in Oct 1582 */
+	if (idt.y == jiswitch.y && idt.m == jiswitch.m && jiswitch.d < idt.d)
+		idt.d += 10;
+
+	/* return external representation of found date */
+	return (idt2date(dt, &idt));
+}
+
+/*
+ * Return the number of days since March 1st of the year zero. The date is
+ * assumed Gregorian if younger than 1582-10-04 and Julian otherwise. This
+ * is the reverse of gdate.
+ */
+int
+ndaysg(date *dt)
+{
+	date    idt;		/* Internal date representation */
+
+	if (date2idt(&idt, dt) == NULL)
+		return (-1);
+	return (ndaysgi(&idt));
+}
+
+/*
+ * Same as above, but with the Gregorian date given in internal
+ * representation.
+ */
+static int
+ndaysgi(date *idt)
+{
+	int     nd;		/* Number of days--return value */
+
+	/* Cache nswitch if not already done */
+	if (nswitch == 0)
+		nswitch = ndaysji(&jiswitch);
+
+	/*
+	 * Assume Julian calendar and adapt to Gregorian if necessary, i. e.
+	 * younger than nswitch. Gregori deleted
+	 * the ten days from Oct 5th to Oct 14th 1582.
+	 * Thereafter years which are multiples of 100 and not multiples
+	 * of 400 were not leap years anymore.
+	 * This makes the average length of a year
+	 * 365d +.25d - .01d + .0025d = 365.2425d. But the tropical
+	 * year measures 365.2422d. So in 10000/3 years we are
+	 * again one day ahead of the earth. Sigh :-)
+	 * (d is the average length of a day and tropical year is the
+	 * time from one spring point to the next.)
+	 */
+	if ((nd = ndaysji(idt)) == -1)
+		return (-1);
+	if (idt->y >= 1600)
+		nd = (nd - 10 - (idt->y - 1600) / 100 + (idt->y - 1600) / 400);
+	else if (nd > nswitch)
+		nd -= 10;
+	return (nd);
+}
+
+/*
+ * Compute the week number from the number of days since March 1st year 0.
+ * The weeks are numbered per year starting with 1. If the first
+ * week of a year includes at least four days of that year it is week 1,
+ * otherwise it gets the number of the last week of the previous year.
+ * The variable y will be filled with the year that contains the greater
+ * part of the week.
+ */
+int
+week(int nd, int *y)
+{
+	date    dt;
+	int     fw;		/* 1st day of week 1 of previous, this and
+				 * next year */
+	gdate(nd, &dt);
+	for (*y = dt.y + 1; nd < (fw = firstweek(*y)); (*y)--)
+		;
+	return ((nd - fw) / 7 + 1);
+}
+		
+/* return the first day of week 1 of year y */
+static int
+firstweek(int y)
+{
+	date idt;
+	int nd, wd;
+
+	idt.y = y - 1;   /* internal representation of y-1-1 */
+	idt.m = 10;
+	idt.d = 0;
+
+	nd = ndaysgi(&idt);
+	/*
+	 * If more than 3 days of this week are in the preceding year, the
+	 * next week is week 1 (and the next monday is the answer),
+	 * otherwise this week is week 1 and the last monday is the
+	 * answer.
+	 */
+	if ((wd = weekday(nd)) > 3)
+		return (nd - wd + 7);
+	else
+		return (nd - wd);
+}
+
+/* return the weekday (Mo = 0 .. Su = 6) */
+int
+weekday(int nd)
+{
+	date dmondaygi = {1997, 8, 16}; /* Internal repr. of 1997-11-17 */
+	static int nmonday;             /* ... which is a monday        */ 
+
+	/* Cache the daynumber of one monday */
+	if (nmonday == 0)
+		nmonday = ndaysgi(&dmondaygi);
+
+	/* return (nd - nmonday) modulo 7 which is the weekday */
+	nd = (nd - nmonday) % 7;
+	if (nd < 0)
+		return (nd + 7);
+	else
+		return (nd);
+}
+
+/*
+ * Convert a date to internal date representation: The year starts on
+ * March 1st, month and day numbering start at zero. E. g. March 1st of
+ * year zero is written as y=0, m=0, d=0.
+ */
+static date *
+date2idt(date *idt, date *dt)
+{
+
+	idt->d = dt->d - 1;
+	if (dt->m > 2) {
+		idt->m = dt->m - 3;
+		idt->y = dt->y;
+	} else {
+		idt->m = dt->m + 9;
+		idt->y = dt->y - 1;
+	}
+	if (idt->m < 0 || idt->m > 11 || idt->y < 0)
+		return (NULL);
+	else
+		return idt;
+}
+
+/* Reverse of date2idt */
+static date *
+idt2date(date *dt, date *idt)
+{
+
+	dt->d = idt->d + 1;
+	if (idt->m < 10) {
+		dt->m = idt->m + 3;
+		dt->y = idt->y;
+	} else {
+		dt->m = idt->m - 9;
+		dt->y = idt->y + 1;
+	}
+	if (dt->m < 1)
+		return (NULL);
+	else
+		return (dt);
+}
diff --git a/lib/libcalendar/calendar.h b/lib/libcalendar/calendar.h
new file mode 100644
index 0000000..6c8d8f9
--- /dev/null
+++ b/lib/libcalendar/calendar.h
@@ -0,0 +1,41 @@
+/*-
+ * 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.
+ *
+ *	$Id$
+ */
+typedef struct date {
+	int y;	/* year */
+	int m;	/* month */
+	int d;	/* day */
+} date;
+
+date    *easterg(int _year, date *_dt);
+date    *easterj(int _year, date *_dt);
+date	*gdate(int _nd,  date *_dt);
+date	*jdate(int _nd, date *_dt);
+int	 ndaysg(date *_dt);
+int	 ndaysj(date *_dt);
+int	 week(int _nd, int *_year);
+int	 weekday(int _nd);
diff --git a/lib/libcalendar/easter.c b/lib/libcalendar/easter.c
new file mode 100644
index 0000000..53021f7
--- /dev/null
+++ b/lib/libcalendar/easter.c
@@ -0,0 +1,82 @@
+/*-
+ * 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));
+}