Category : Files from Magazines
Archive   : V12N09.ZIP
Filename : DOWGJ.C

/*********************************************************************
DOWGJ - Day_Of_the_Week Gregorian or Julian calendar
**********************************************************************

Author: Roberto R. Viscogliosi
Date Written : 10/92
Release: 1.1

This program computes the Day_Of_the_Week for a given date using
the Gregorian calendar as default, or the Julian calendar when an
extra argument is given at the command line.

Input Arguments:
mm dd yyyy [c]
where:
mm = month, dd = day, yyyy = year
[c] = any value forces the use of the Julian calendar;
no value uses the Gregorian calendar.

Example: DOWGJ 10 12 1492 j
Last change: 14 Jan 93 2:30 pm
***********************************************************************/

#include
#include

#define OK 0
#define GRG 1

long dow (int, int, int, int);
void help (char *);

//====================================================================
// MAIN process
//====================================================================
int main (ac, av)
int ac;
char *av[];
{
int day, month, year, calendar = GRG, ret = OK;

static char *dowstr[7] = {
"Sunday", "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday"
};

for (;;) {
// check arguments passed
if (ac < 4) { // not enough args.
ret = !OK;
break;
}
// get/verify the input values
if (((month = atoi (av[1])) < 1) ||
((day = atoi (av[2])) < 1) ||
((year = atoi (av[3])) < 1)
) { // invalid values
ret = !OK;
break;
}
// check if Julian calendar requested
if (ac > 4) { // reset the calendar indicator
calendar = !GRG;
}
// prevent overflow by check year size
if (year > 9999) { // year too large
ret = !OK;
break;
}
// compute and show the day_of_the_week
printf ("%02d/%02d/%04d is on %s %s\n",
month, day, year,
dowstr [dow (month, day, year, calendar)],
(calendar == GRG) ? "" : "(Julian calendar)"
);
break;
}
// check for errors
if (ret) {
help (av[0]);
}
exit (ret);
}
//====================================================================
// DOW - Return the day of the week value 0-6 (0 = Sunday)
// based on the date (month, day, year) and calendar
//====================================================================
long dow (month, day, year, calendar)
int month, day, year, calendar;
{
static char month_val[12] = {
// J F M A M J J A S O N D
6,2,2,5,0,3,5,1,4,6,2,4
};

return (( day \
+ (month_val [month - 1]) \

- ((month < 3) \
? ((calendar == GRG) \
? ((!(year % 4)) && \
((year % 100) || (!(year % 400)))) \
: (1*(!(year % 4)))) \
: 0 \
) \
+ year \
+ (year / 4) \
+ ((calendar == GRG) \
? ((year / 400) - (year / 100)) \
: 5 \
) \
) % 7 \
);
}

//====================================================================
// HELP - Print the help info
//====================================================================
static void help (fn)
char *fn;
{
printf ("\nWelcome to DOWGJ (DOW Gregorian or Julian) Version 1.1\n");
printf (" by R. Viscogliosi\n");
printf ("\nTo use it, please enter:\n");
printf ("\n %s mm dd yyyy [c]\n", fn);
printf ("\n (If [c] is specified then the date is in Julian,\n");
printf (" else the date defaults to the Gregorian calendar)\n");
printf ("\n (e.g. 'DOWGJ 10 12 1492 j' was on Friday (Julian))\n");
}