#include <time.h>
#include "timeutil.h"

/* TODO: mktime: DST not supported */
time_t mktime(struct tm *time)
{
	/* Normalize time */
	time->tm_min   += (time->tm_sec / 60);
	time->tm_hour  += (time->tm_min / 60);
	time->tm_mday  += (time->tm_hour / 24);
	time->tm_sec   %= 60;
	time->tm_min   %= 60;
	time->tm_hour  %= 24;

	/* Normalize date */
	int days = daysin(time->tm_mon, time->tm_year + 1900);
	while(time->tm_mday > days) {
		time->tm_mday -= days;
		if(++time->tm_mon == 12) {
			time->tm_mon = 0;
			time->tm_year++;
		}
		days = daysin(time->tm_mon, time->tm_year + 1900);
	}

	/* Determine day in year */
	time->tm_yday = time->tm_mday - 1;
	for(int i = 0; i < time->tm_mon; i++)
		time->tm_yday += daysin(i, time->tm_year + 1900);

	/* Determine day in week. The calendar has a period of 400 years and
	   1601-01-01 was a Monday. */

	/* Years elapsed since last 400n+1 year (1601-2001-etc). */
	int yr = (time->tm_year + 1900 - 1) % 400;
	/* Leap years during this period */
	int leaps = (yr / 4) - (yr >= 100) - (yr >= 200) - (yr >= 300);
	/* Days elapsed since 01-01 on the last 400n+1 year */
	days = 365 * yr + leaps + time->tm_yday;
	/* Current day of week (1 is Monday on 01-01 of last 400n+1 year) */
	time->tm_wday = (1 + days) % 7;

	/* We don't determine DST at the targeted time */
	time->tm_isdst = 0;

	/* Number of periods elapsed since 1601-01-01 (may be negative) */
	int periods = (time->tm_year + 1900 - 1601) / 400;
	periods -= (time->tm_year + 1900 < 1601);
	/* Days elapsed since 1970-01-01; the calendar period is 146097 days
	   and there are 134774 days between 1601-01-01 and 1970-01-01 */
	days += 146097 * periods - 134774;

	time_t t = days;
	t = 24 * t + time->tm_hour;
	t = 60 * t + time->tm_min;
	t = 60 * t + time->tm_sec;
	return t;
}