gistfile1.java

package findDay;

public class MainClass {
	
	private final static int YEAR = 2100;
	private final static int MONTH = 12;
	private final static int DATE = 31;
	
	private final static Date INPUT = new Date(YEAR, MONTH, DATE);
	private final static Date STANDARD = new Date(2012, 1, 1);
	private final static int STD_DAY = 0;
	
	public static void main (String args[]) {
		int differenceY = INPUT.getYear() - STANDARD.getYear();
		int leapDays = countLeapDays(differenceY, INPUT.getMonth());
		int normalDifferenceDays = countNormalDaysDifference(INPUT.getMonth(), INPUT.getDate());
		int monthExtraDays = countMonthExtraDays(INPUT.getMonth());
		
		int differenceDays = (differenceY * 365) + normalDifferenceDays + monthExtraDays + leapDays;
		
		int result = STD_DAY + differenceDays % 7;
		System.out.println(result);
	}
	
	private static int countLeapDays(int differenceY, int month) {
		int counter = 0;
		for (int i = 0; i < differenceY; i++) {
			if (isLeapYear(STANDARD.getYear() + i)) {
				counter++;
			}
		}
		if (isLeapYear(STANDARD.getYear() + differenceY) && month > 2) {
			counter++;
		}
		return counter;
	}
	
	private static int countNormalDaysDifference(int month, int date) {
		return (month - STANDARD.getMonth()) * 30 + (date - STANDARD.getDate());
	}
	
	private static int countMonthExtraDays(int month) {
		int counter = 0;
		for (int i = STANDARD.getMonth(); i < month; i++) {
			if (i <= 7 && i % 2 == 1 || i >= 8 && i % 2 == 0) {
				counter++;
			}else if (i == 2) {
				counter -= 2;
			}
		}
		return counter;
	}
	
	private static boolean isLeapYear(int year) {
		return year % 400 == 0 || year % 4 == 0 && year % 100 != 0;
	}

}