Calendar.java

package calendar;

public class Calendar {
	public static final int[] DAY_NUMS = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
	
	public static final String[] Days = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

	public static final int BASIS_YEAR = 2014;
	public static final int BASIS_JAN_1ST_DAY_INDEX = 3; // Wed
	
	public void print(int year, int month) {
		if (month <= 0 || 13 <= month) {
			throw new IllegalArgumentException();
		}
		final int firstDayIndex = this.getFirstDayIndex(year, month);
		final int dayNum = this.getDayNum(year, month);
		String line1 = "", line2 = "";
		int dayIndex = firstDayIndex;
		for (int day = 1; day <= dayNum; day++) {
			line1 += Days[dayIndex];
			if (day <= 9) {
				line2 += "  " + day;
			} else {
				line2 += " " + day;
			}
			dayIndex = (dayIndex + 1) % Days.length;
			
			if (day % 7 == 0 || day == dayNum) {
				System.out.println(line1);
				System.out.println(line2);
				line1 = "";
				line2 = "";
			} else {
				line1 += " ";
				line2 += " ";
			}
		}
		
	}

	private boolean isUruudoshi(int year) {
		if (year % 4 != 0) {
			return false;
		}
		if (year % 100 == 0 && year % 400 != 0) {
			return false;
		}
		return true;
	}
	
	private int getDayNum(int year, int month) {
		if (month == 2) {
			return isUruudoshi(year) ? 29 : 28;
		}
		return DAY_NUMS[month - 1];
	}
	
	private int getFirstDayIndex(final int year, final int month) {
		// 2800年周期
		int diffYear = (year - BASIS_YEAR) % 2800;
		if (diffYear < 0) {
			diffYear += 2800;
		}
		// クソ効率悪い
		int dayIndex = BASIS_JAN_1ST_DAY_INDEX;
		for (int i = 0; i < diffYear; i++) {
			if (isUruudoshi(BASIS_YEAR + i)) {
				dayIndex += 2;
			} else {
				dayIndex += 1;
			}
			dayIndex %= 7;
		}
		for (int i = 1; i <= month - 1; i++) {
			dayIndex += this.getDayNum(year, i);
		}
		dayIndex %= 7;
		
		return dayIndex;
	}
	
	public static void main(String[] args) {
		Calendar c = new Calendar();
		c.print(2017, 2);
	}
}