13일의 금요일 with C++ Template Meta Programming

find_13_friday.cpp

/**
* https://twitter.com/Code_60/status/577062012250796032
* 오늘의 주제는 "13일의 금요일" 입니다.
* Input : 년 수
* Output : 해당 년도의 13일의 금요일이 포함된 월
* ex) input : 2015 output : 3
*/
 
#include <cstdio>
#include <vector>
#include <cassert>

enum {
	DAY_MON = 0,
	DAY_TUE,
	DAY_WED,
	DAY_THU,
	DAY_FRI,
	DAY_SAT,
	DAY_SUN
};

template<int Y>
struct LeapYear {
	enum {
		value = ((Y % 4 == 0) && (Y % 100 != 0)) || (Y % 400 == 0),
		total = value ? 366 : 365
	};
};

// 서력 기원 연수가 4로 나누어 떨어지는 해는 윤년으로 한다.(2004년, 2008년, 2012년, 2016년…)
static_assert(LeapYear<2004>::value == true, "");
static_assert(LeapYear<2008>::value == true, "");
static_assert(LeapYear<2012>::value == true, "");
static_assert(LeapYear<2016>::value == true, "");
// 이 중에서 100으로 나누어 떨어지는 해는 평년으로 한다.(1900년, 2100년, 2200년, 2300년…)
static_assert(LeapYear<1900>::value == false, "");
static_assert(LeapYear<2100>::value == false, "");
static_assert(LeapYear<2200>::value == false, "");
static_assert(LeapYear<2300>::value == false, "");
// 그중에 400으로 나누어 떨어지는 해는 윤년으로 둔다.(1600년, 2000년, 2400년 …)
static_assert(LeapYear<1600>::value == true, "");
static_assert(LeapYear<2000>::value == true, "");
static_assert(LeapYear<2400>::value == true, "");

template<int idx, int I, int... Remainder>
struct ElementAt {
	enum {
		value = ElementAt<idx - 1, Remainder...>::value
	};
};

template<int I, int... Remainder>
struct ElementAt <0, I, Remainder...> {
	enum {
		value = I
	};
};

static_assert(ElementAt<0, 1, 2, 4>::value == 1, "");   //[1, 2, 4][0]
static_assert(ElementAt<1, 1, 2, 4>::value == 2, "");   //[1, 2, 4][1]
static_assert(ElementAt<2, 1, 2, 4>::value == 4, "");   //[1, 2, 4][2]

template<int Month>
struct NormalYearDayCount {
	enum {
		value = ElementAt<Month, 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31>::value
	};
};

template<int Month>
struct LeapYearDayCount {
	enum {
		value = ElementAt<Month, 0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31>::value
	};
};

static_assert(NormalYearDayCount<2>::value == 28, "");
static_assert(LeapYearDayCount<2>::value == 29, "");

template<int Year, int Month>
struct YearDayCount {
	enum {
		value = LeapYear<Year>::value ? LeapYearDayCount<Month>::value : NormalYearDayCount<Month>::value
	};
};

static_assert(YearDayCount<2015, 2>::value == 28, "");
static_assert(YearDayCount<2016, 2>::value == 29, "");

template<int Year, int Month, int Day>
struct DayCount {
	enum {
		value = DayCount<Year, Month, 1>::value + Day - 1
	};
};

template<int Year, int Month>
struct DayCount < Year, Month, 1 > {
	enum {
		value = DayCount<Year, Month - 1, 1>::value + YearDayCount<Year, Month - 1>::value
	};
};

template<int Year>
struct DayCount <Year, 1, 1> {
	enum {
		value = 0
	};
};

static_assert(DayCount<2015, 1, 1>::value == 0, "");
static_assert(DayCount<2015, 1, 11>::value == 10, "");

static_assert(DayCount<2015, 2, 1>::value == 31, "");
static_assert(DayCount<2015, 3, 1>::value == 31 + 28, "");
static_assert(DayCount<2015, 4, 1>::value == 31 + 28 + 31, "");

template<int Year>
struct YearTotalDay {
	enum {
		value = YearTotalDay<Year - 1>::value + LeapYear<Year - 1>::total
	};
};

template<>
struct YearTotalDay <2000> {
	// 2000/01/01 = SAT
	enum {
		value = DAY_SAT
	};
};

static_assert(YearTotalDay<2000>::value == DAY_SAT, "");
static_assert(YearTotalDay<2001>::value % 7 == DAY_MON, "");
static_assert(YearTotalDay<2002>::value % 7 == DAY_TUE, "");

template<int Y, int M, int D>
struct Weekday {
	enum {
		start = YearTotalDay<Y>::value % 7,
		month_start = (DayCount<Y, M, 1>::value + start) % 7,
		value = (DayCount<Y, M, D>::value + start) % 7
	};
};

static_assert(Weekday<2001, 1, 1>::value == DAY_MON, "");
static_assert(Weekday<2002, 1, 1>::value == DAY_TUE, "");
static_assert(Weekday<2003, 1, 1>::value == DAY_WED, "");
static_assert(Weekday<2015, 1, 1>::value == DAY_THU, "");

static_assert(Weekday<2015, 1, 1>::month_start == DAY_THU, "");
static_assert(Weekday<2015, 2, 1>::month_start == DAY_SUN, "");
static_assert(Weekday<2015, 3, 1>::month_start == DAY_SUN, "");

static_assert(Weekday<2015, 1, 13>::value == DAY_TUE, "");
static_assert(Weekday<2015, 2, 13>::value == DAY_FRI, "");
static_assert(Weekday<2015, 3, 13>::value == DAY_FRI, "");

template<int v>
struct Int2Type {
	enum { value = v };
};

template<int Year, int Month>
struct EvilYear {
	typedef EvilYear<Year, Month + 1> Next;

	enum {
		Weekday = Weekday<2015, Month, 13>::value,
		value = Weekday == DAY_FRI
	};

	static void run(std::vector<int> &result) {
		run(result, Int2Type<value>());
	}

	static void run(std::vector<int> &result, Int2Type<true>) {
		result.push_back(Month);
		Next::run(result);
	}

	static void run(std::vector<int> &result, Int2Type<false>) {
		Next::run(result);
	}
};

template<int Year>
struct EvilYear<Year, 12> {
	enum {
		Month = 12,
		Weekday = Weekday<2015, Month, 13>::value,
		value = Weekday == DAY_FRI
	};

	static void run(std::vector<int> &result) {
		run(result, Int2Type<value>());
	}

	static void run(std::vector<int> &result, Int2Type<true>) {
		result.push_back(Month);
	}
	static void run(std::vector<int> &result, Int2Type<false>) {
	}
};


template<int Year>
struct EvilYearRunner {
	typedef EvilYear<Year, 1> Start;

	static void run(std::vector<int> &result) {
		Start::run(result);
	}
};

int main()
{
	const int Year = 2015;

	std::vector<int> retval;
	EvilYearRunner<Year>::run(retval);

	if (Year == 2015) {
		assert(retval.size() == 3);
		assert(retval[0] == 2); // 2015/02/13
		assert(retval[1] == 3); // 2015/03/13
		assert(retval[2] == 11);	// 2015/11/13
	}

	for (int month : retval) {
		printf("%d ", month);
	}

	getchar();
	return 0;
}

해설
http://libsora.so/posts/friday-the-13th-tmp/