date.c

// The code for dayofweek was obtained at:
// https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week

#ifdef CS333_P1
#include "types.h"
#include "user.h"
#include "date.h"

#define PAD(x) ((x) < 10 ? "0" : "")

static char *months[] = {"NULL", "Jan", "Feb", "Mar", "Apr",
  "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
static char *days[] = {"Sun", "Mon", "Tue", "Wed",
  "Thu", "Fri", "Sat"};

static int
dayofweek(int y, int m, int d)
{
  return (d+=m<3?y--:y-2,23*m/9+d+4+y/4-y/100+y/400)%7;
}

int
main(int argc, char *argv[])
{
  int day;
  char *s;
  struct rtcdate r;

  if (date(&r)) {
    printf(2,"Error: date call failed. %s at line %d\n",
        __FILE__, __LINE__);
    exit();
  }

  day = dayofweek(r.year, r.month, r.day);
  s = r.hour < 12 ? "AM" : "PM";

  r.hour %= 12;
  if (r.hour == 0) r.hour = 12;

  printf(1, "%s %s%d %s %d %s%d:%s%d:%s%d %s UTC\n", days[day], PAD(r.day), r.day,
      months[r.month], r.year, PAD(r.hour), r.hour, PAD(r.minute), r.minute,
      PAD(r.second), r.second, s);

  exit();
}
#endif // CS333_P1