date.h

#pragma once

#include <iostream>
#include <cstdint>

// 1/1/1900 -> Monday

uint64_t days_in_year(uint64_t y) {
	bool div4 = (y & 0x3) == 0;
	bool div16 = (y & 0xf) == 0;
	bool div100 = (y % 100) == 0;
	bool leap_year =
		(div4 && !div100) || (div16 && div100);
	return 365 + leap_year;
}

enum months : uint8_t {
	jan,
	feb,
	mar,
	apr,
	may,
	jun,
	jul,
	aug,
	sep,
	oct,
	nov,
	dec
};

enum days : uint8_t {
	mon,
	tue,
	wed,
	thu,
	fri,
	sat,
	sun
};

uint64_t days_in_month(uint64_t y, uint64_t m) {
	uint64_t extra_years = m / 12;
	uint64_t real_month = m % 12;
	uint64_t days = days_in_year(y+extra_years);
	const uint8_t mxb[12] = { 31,28 | 0x80,31,30,31,30,31,31,30,31,30,31 };

	uint8_t v = mxb[real_month];
	return (v & 0x7f) + ((v >> 7) * (days == 366));
}