Schwerdtfeger's Variation of Kraitchik's Algorithm Without Using Lookup Tables

gistfile1.cpp

enum day_of_week_t { SUN, MON, TUES, WED, THURS, FRI, SAT };

day_of_week_t calcDayOfWeek(unsigned int day, unsigned int month, unsigned int year)
{
	int Y = 0, y = 0, c = 0, m = 0, d = 0, w = 0;

	// Check for vaild arguments
	year = year % 10000;

	if (month < 1 || month > 12)
	{
		std::string error = "Invalid Month.";
		//throw InvalidDateException(error); // TODO: Implement InvalidDateException
		throw error;
	}	

	if (day > daysInMonth(month))
	{
		std::string error = "Day exceeds days in Month.";
		//throw InvalidDateException(error); // TODO: Implement InvalidDateException
		throw error;
	}

	// Determine Y, which is the year minus 1 for  January or February, and the year for  the rest of the year
	Y = year;
	if (month < 3)
	{
		Y--;
	}

	// Determine y, which is the last 2 digits of Y
	y = Y % 100;

	// Determine c, which is the first 2 digits of Y
	c = ((int)(Y / 100)) % 100;

	// Determine d, which is the day of the month
	d = day;

	// Determine m, which is the month shifted left by 2 (March = 1,... February = 12)	
	m = ( (month - 2) < 1 ? month + 10 : month - 2 );

	int temp = d + ((2.6f*m) - .2f) + y + (y / 4) + (c / 4) - (2*c);
	w =  temp % 7;

	return (day_of_week_t)w;
}