#include <time.h>
#include <sys/ksys.h>
#include <ctype.h>
#include <errno.h>
#include <limits.h>

#define LEAPOCH (946684800LL + 86400*(31+29))

#define DAYS_PER_400Y (365*400 + 97)
#define DAYS_PER_100Y (365*100 + 24)
#define DAYS_PER_4Y   (365*4   + 1)

static int __secs_to_tm(long long t, struct tm *tm)
{
	long long days, secs, years;
	int remdays, remsecs, remyears;
	int qc_cycles, c_cycles, q_cycles;
	int months;
	int wday, yday, leap;
	static const char days_in_month[] = {31,30,31,30,31,31,30,31,30,31,31,29};

	// Reject time_t values whose year would overflow int
	if (t < INT_MIN * 31622400LL || t > INT_MAX * 31622400LL)
		return -1;
	
	secs = t - LEAPOCH;
	days = secs / 86400;
	remsecs = secs % 86400;
	
	if (remsecs < 0) {
		remsecs += 86400;
		days--;
	}

	wday = (3+days)%7;
	if (wday < 0) wday += 7;

	qc_cycles = days / DAYS_PER_400Y;
	remdays = days % DAYS_PER_400Y;
	if (remdays < 0) {
		remdays += DAYS_PER_400Y;
		qc_cycles--;
	}

	c_cycles = remdays / DAYS_PER_100Y;
	if (c_cycles == 4) c_cycles--;
	remdays -= c_cycles * DAYS_PER_100Y;

	q_cycles = remdays / DAYS_PER_4Y;
	if (q_cycles == 25) q_cycles--;
	remdays -= q_cycles * DAYS_PER_4Y;

	remyears = remdays / 365;
	if (remyears == 4) remyears--;
	remdays -= remyears * 365;

	leap = !remyears && (q_cycles || !c_cycles);
	yday = remdays + 31 + 28 + leap;
	if (yday >= 365+leap) yday -= 365+leap;

	years = remyears + 4*q_cycles + 100*c_cycles + 400LL*qc_cycles;

	for (months=0; days_in_month[months] <= remdays; months++)
		remdays -= days_in_month[months];

	if (months >= 10) {
		months -= 12;
		years++;
	}

	if (years+100 > INT_MAX || years+100 < INT_MIN)
		return -1;

	tm->tm_year = years + 100;
	tm->tm_mon = months + 2;
	tm->tm_mday = remdays + 1;
	tm->tm_wday = wday;
	tm->tm_yday = yday;

	tm->tm_hour = remsecs / 3600;
	tm->tm_min = remsecs / 60 % 60;
	tm->tm_sec = remsecs % 60;

	return 0;
}

struct tm *__localtime_r(const time_t *restrict t, struct tm *restrict tm)
{
	// Reject time_t values whose year would overflow int because
	// __secs_to_zone cannot safely handle them. 
	if (*t < INT_MIN * 31622400LL || *t > INT_MAX * 31622400LL) {
		errno = EOVERFLOW;
		return NULL;
	}
	//__secs_to_zone(*t, 0, &tm->tm_isdst, &tm->__tm_gmtoff, 0, &tm->__tm_zone);
	if (__secs_to_tm((long long)*t,tm) < 0) {
		errno = EOVERFLOW;
		return NULL;
	}
	return tm;
}

struct tm * localtime (const time_t * timer)
{
	static struct tm tm;
	return __localtime_r(timer, &tm);
}