baker@csl.dl.nec.com (Larry Baker) (05/30/91)
Here's a little `date' class I wrote, using some Julian routines I got
out of the Simtel-20 archives. I make no claims as to the validity
or stability of the two julian date routines - I haven't even looked up
the reference to the algorithm - but they got the dates and days right
in the simple test I ran (also attached).
Note that it doesn't parse date formats, and it doesn't deal with time.
Dealing with time is a relatively simple extention: extend the precision
of the julian integer to include however many units of precision - time
quanta - you need. Time is actually a lot easier to deal with than dates,
if you are willing to grant that it is a continuous function within the scope
of a day. This isn't really true, as any astronomer will tell you, but it works
for the rest of us.
There is one potential gotcha in the code, and it has to do with C++'s
handling of "temporary" objects. On pages 267+, 300+ of
Ellis & Strourstrap, you'll find that the scope of "temporary objects"
per se is implementation dependent. The use of complicated combinations
of the "+" and "-" operators, e.g. a = (d1 - d2) + d3, will generate
temporary values whose existence must be *at least* for the scope
of the expression. This may not be true for some implementations
(it worked on all the versions of CFRONT I had access to, and Borland's
compiler, though I couldn't test Borland's very extensively). E&S imply
that the temporary must exist for as long as it is bound to a reference, but
there are places where they seem to contradict themselves.
Note that the "+" and "-" operators each pass a Date object by *value*,
thus causing a call to the copy constructor and the creation of a temporary
value, which is returned by the operator. This is expensive. A better way
to do this is described on P. 300 of Ellis & Strourstrap:
class Date {
// ...
friend Date operator + (int, Date&);
friend Date operator + (Date&, int);
// etc.
};
But I didn't do that here.
Cheers,
Larry Baker
File follows
////////////////
//
// class Date - perform simple date calculations and conversions.
//
// Copyright (c) 1991, Larry Baker. All Rights Reserved. Free reproduction
// and use of this code for any purpose is hereby granted, under the condition
// that this copyright notice remain intact in this and all subsequent versions,
// or any works derived from the computer source code provided herin.
//
////////////////
class Date {
public:
Date(long jdays);
Date(int month, int day, int year);
Date(Date&);
~Date();
long _month;
long _day;
long _year;
long _dow; /* day of week */
long _jdays; /* julian date day count */
};
Date& operator + (Date, int); /* add days to a date */
Date& operator + (int, Date); /* add days to a date */
Date& operator - (Date, int); /* subtract days from a date */
Date& operator - (int, Date); /* subtract days from a date */
int operator - (Date&, Date&); /* days between dates */
static long jday(int mon, int day, int year);
static long * jdate(long j);
Date& operator + (Date dt, int i)
{
long * ret;
dt._jdays += i;
ret = jdate(dt._jdays);
dt._month = ret[0];
dt._day = ret[1];
dt._year = ret[2];
dt._dow = ret[3];
return dt;
}
Date& operator + (int i, Date dt)
{
long * ret;
dt._jdays += i;
ret = jdate(dt._jdays);
dt._month = ret[0];
dt._day = ret[1];
dt._year = ret[2];
dt._dow = ret[3];
return dt;
}
Date& operator - (Date dt, int i)
{
long * ret;
dt._jdays -= i;
ret = jdate(dt._jdays);
dt._month = ret[0];
dt._day = ret[1];
dt._year = ret[2];
dt._dow = ret[3];
return dt;
}
Date& operator - (int i, Date dt)
{
long * ret;
dt._jdays -= i;
ret = jdate(dt._jdays);
dt._month = ret[0];
dt._day = ret[1];
dt._year = ret[2];
dt._dow = ret[3];
return dt;
}
int operator - (Date& d1, Date& d2)
{
return d1._jdays - d2._jdays;
}
Date::Date(int month, int day, int year)
{
long * ret;
_jdays = jday(month, day, year);
_month = month;
_day = day;
_year = year;
ret = jdate(_jdays);
_dow = ret[3];
}
Date::Date(long jdays)
{
long * ret;
_jdays = jdays;
ret = jdate(_jdays);
_month = ret[0];
_day = ret[1];
_year = ret[2];
_dow = ret[3];
}
Date::Date(Date& dt)
{
_jdays = dt._jdays;
_month = dt._month;
_day = dt._day;
_year = dt._year;
_dow = dt._dow;
}
Date::~Date()
{
}
/*
** PD Programs retrieved from simtel-20 archives.
*/
/*
** Takes a date, and returns a Julian day. A Julian day is the number of
** days since some base date (in the very distant past).
** Author: Robert G. Tantzen, translator: Nat Howard
** Translated from the algol original in Collected Algorithms of CACM
** (This and jdate are algorithm 199).
**
*/
static long jday(int mon, int day, int year)
{
long m = mon, d = day, y = year;
long c, ya, j;
if(m > 2) m -= 3;
else {
m += 9;
--y;
}
c = y/100L;
ya = y - (100L * c);
j = (146097L*c)/4L + (1461L*ya)/4L + (153L*m + 2L)/5L + d + 1721119L;
return(j);
}
/* Julian date converter. Takes a julian date (the number of days since
** some distant epoch or other), and returns an int pointer to static space.
** ip[0] = month;
** ip[1] = day of month;
** ip[2] = year (actual year, like 1977, not 77 unless it was 77 a.d.);
** ip[3] = day of week (0->Sunday to 6->Saturday)
** These are Gregorian.
** Copied from Algorithm 199 in Collected algorithms of the CACM
** Author: Robert G. Tantzen, Translator: Nat Howard
*/
static long * jdate(long j)
{
static long ret[4];
long d, m, y;
ret[3] = (j + 1L)%7L;
j -= 1721119L;
y = (4L * j - 1L)/146097L;
j = 4L * j - 1L - 146097L * y;
d = j/4L;
j = (4L * d + 3L)/1461L;
d = 4L * d + 3L - 1461L * j;
d = (d + 4L)/4L;
m = (5L * d - 3L)/153L;
d = 5L * d - 3 - 153L * m;
d = (d + 5L) / 5L;
y = 100L * y + j;
if(m < 10)
m += 3;
else {
m -= 9;
++y;
}
ret[0] = m;
ret[1] = d;
ret[2] = y;
return(ret);
}
#include <stdio.h>
/* print a simple calendar for 1991 */
main()
{
int i;
int t;
Date today(1,1,1991);
Date day(1,1,1991);
printf("%d\n\n", Date(1,1,1992) - Date(1,1,1991));
for (i = 0; day._year != 1992; i++) {
if (day._day == 1) {
printf("\n\n");
for (t = 0; t <= day._dow - 1; t++)
printf(" ");
}
printf("%3d ", day._day);
if (day._dow == 6)
printf("\n");
day = 1 + day;
}
}
--
Larry Baker
NEC America C&C Software Laboratories, Irving (near Dallas), TX
baker@csl.dl.nec.com cs.utexas.edu!necssd!baker