How to know what day January first is.

january_first_is.cpp

#include <ctime>
#include <stdio.h>
#include <iostream>

using namespace std;

bool is_leap_year(int year);
int get_wday_january_first(int yday, int wday);

int main()
{
	string week_names[7] = {"Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"};
	int weeks[30] = { 
					  6, 1, 2, 3, 4		//2000 Sat, 2001 Mon, 2002 Tue 2003 Wed, 2004 Thu
					, 6, 0, 1, 2, 4
					, 5, 6, 0, 2, 3
					, 4, 5, 0, 1, 2
					, 3, 5, 6, 0, 1		// 2020 Wed, 2021 Fri, 2022 Sat, 2023 Sun, 2024 Mon
					, 3, 4, 5, 6, 1		// 2025
					};

	
	time_t rawtime;
	struct tm * timeinfo;

	time(&rawtime);
	timeinfo = localtime(&rawtime);

	int startYear = 2000;
	int endYear = startYear + (sizeof(weeks) / sizeof(weeks[0]));

	for (int y = startYear; y < endYear; y++) {
		int m = 5;
		int d = 28;

		timeinfo->tm_year = y - 1900;
		timeinfo->tm_mon = m - 1;
		timeinfo->tm_mday = d;
		mktime(timeinfo);

		int wday = timeinfo->tm_wday; // weekday
		int yday = timeinfo->tm_yday; // days of year
		int year = timeinfo->tm_year + 1900;

		int wday_jf = get_wday_january_first(yday, wday);

		if (wday_jf != weeks[year - startYear]) {
			cout << "[X] ";
		}
		else{
			cout << "[O] ";
		}
		cout << year << " (" << (is_leap_year(year) ? "leap" : "comm") << ") January, 1st is " << "Expected:" << week_names[weeks[year - startYear]].c_str() << "(" << weeks[year - startYear] << ")" << " Result:" << week_names[wday_jf].c_str() << "(" << wday_jf << ")" << endl;
	}
	return 0;
}

int get_wday_january_first(int yday, int wday) {
	int wday_jf = wday - (yday % 7);
	if (wday_jf < 0) wday_jf = 7 + wday_jf;
	return wday_jf;
}

bool is_leap_year(int year) {
	if ((year % 4) == 0){
		if ((year % 100) == 0) {
			if ((year % 400) == 0) {
				return true;
			}
			else{
				return false;
			}
		}
		else{
			return true;
		}		
	}
	else{
		return false;
	}
}