Date to Day-of-Week Conversion (Question)

[Tom Reingold]

This will work for this century.  I'll leave fixing it for any century
as an exercise for the reader.

#include <stdio.h>
#include <sys/types.h>
#include <time.h>

static int monlens[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
static char *daytab[] = { "Sat", "Sun", "Mon", "Tue", "Wed", "Thu", "Fri" };
static int keys[12];

void initkeys();

main(argc, argv)
int argc;
char *argv[];
{
    int day;
    time_t t;
    struct tm *tp;

    if (argc != 3 && argc != 4) {
yuck:
	fprintf(stderr, "usage: %s month date [year]\n", argv[0]);
	exit(1);
    }
    t = time(&t);
    tp = localtime(&t);
    tp->tm_mon = atoi(argv[1]) - 1;
    tp->tm_mday = atoi(argv[2]);
    if (argc == 4)
	tp->tm_year = atoi(argv[3]);
    initkeys(tp->tm_year);
    if (tp->tm_mon == -1 || tp->tm_mday == 0 || tp->tm_year == 0)
	goto yuck;
    if (tp->tm_mday > monlens[tp->tm_mon]) {
	fprintf(stderr, "%s: invalid date\n", argv[0]);
	exit(2);
    }
    day = dateof(tp);
    printf("%02d/%02d/%02d is a %s\n", tp->tm_mon + 1, tp->tm_mday,
	tp->tm_year, daytab[day]);
    exit(0);
}

dateof(tp) /* return day of week: Saturday=0, ..., Friday=6 */
struct tm *tp;
{
    int crap;

    crap = tp->tm_year + tp->tm_mday + keys[tp->tm_mon] +
     ((int) tp->tm_year / 4);
    return(crap % 7);
}

isleap(year)
int year;
{
    if (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0))
	return(1);
    return(0);
}

void initkeys(n)
int n;
{
    int i;

    /* This generates a table.  I could hard code it, but I wanted
    to show what the table means. */
    if (isleap(n))
	keys[0] = 0, monlens[1] = 29;
    else
	keys[0] = 1;
    for (i = 1; i < 12; i++)
	keys[i] = (monlens[i-1] + keys[i-1]) % 7;
}
--
        Tom Reingold
        tr at samadams.princeton.edu  OR  ...!princeton!samadams!tr
        "Warning: Do not drive with Auto-Shade in place.  Remove
        from windshield before starting ignition."