doomsday.c

int dayOfWeek (int doomsday, int leapYear, int month, int day) {
   int dayOfWeek=0;
   int anchor=0;
   int moddifference=0;

   //Determine the anchor day based on the month
   if ((month==1 && leapYear==FALSE)||month==10){
      anchor = doomsday - JANOCTDOOM;
   } else if ((month==1 && leapYear==TRUE)|| month==7) {
      anchor = doomsday - JANJULDOOML;
   } else if ((month==2 && leapYear==TRUE) || month==8){
      anchor = doomsday - AUGFEBLDOOM;
   } else if (month==3||month==11||(month==2 && leapYear==FALSE)){
      anchor = doomsday - MARNOVFEBDOOM;
   } else if (month==9||month==12){
      anchor = doomsday - SEPDECDOOM;
   } else if (month==4 || month==6){
      anchor = doomsday - month;
   } else if (month==5){
      anchor = doomsday - MAYDOOM;
   }
   // "wrap" the anchor around if it goes below 0 so that it is still valid
   if (anchor<THURSDAY){
      anchor=anchor+DAYS_PER_WEEK;
   }
   day=day%DAYS_PER_WEEK; //finds the day difference, that is how far the day is from the anchor day (ie tuesday is 1 from monday)
   moddifference=DAYS_PER_WEEK-day; //inverts the difference
   dayOfWeek=anchor-moddifference; // uses the inverted difference to find the dayofweek by subracting it from anchor
   if (dayOfWeek<THURSDAY){ // Corrects the dayOfWeek if it is a number above or below the weeks values and "wraps it around"
      dayOfWeek+=DAYS_PER_WEEK;
   } else if (dayOfWeek>WEDNESDAY){
      dayOfWeek-=DAYS_PER_WEEK;
   }
   return (dayOfWeek); // returns dayofweek to main function for verification
}