/*
 * ----------------------------------------------------------------------
 * Copyright © 2005-2014 Rich Felker, et al.
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 * ----------------------------------------------------------------------
 */

#include <errno.h>
#include <limits.h>
#include <time.h>

/* 2000-03-01 (mod 400 year, immediately after feb29 */
#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 __month_to_secs(int month, int is_leap)
{
	static const int secs_through_month[] = {
		0, 31*86400, 59*86400, 90*86400,
		120*86400, 151*86400, 181*86400, 212*86400,
		243*86400, 273*86400, 304*86400, 334*86400 };
	int t = secs_through_month[month];
	if (is_leap && month >= 2) t+=86400;
	return t;
}

static long long __year_to_secs(long long year, int *is_leap)
{
	if (year-2ULL <= 136) {
		int y = year;
		int leaps = (y-68)>>2;
		if (!((y-68)&3)) {
			leaps--;
			if (is_leap) *is_leap = 1;
		} else if (is_leap) *is_leap = 0;
		return 31536000*(y-70) + 86400*leaps;
	}

	int cycles, centuries, leaps, rem;

	if (!is_leap) is_leap = &(int){0};
	cycles = (year-100) / 400;
	rem = (year-100) % 400;
	if (rem < 0) {
		cycles--;
		rem += 400;
	}
	if (!rem) {
		*is_leap = 1;
		centuries = 0;
		leaps = 0;
	} else {
		if (rem >= 200) {
			if (rem >= 300) centuries = 3, rem -= 300;
			else centuries = 2, rem -= 200;
		} else {
			if (rem >= 100) centuries = 1, rem -= 100;
			else centuries = 0;
		}
		if (!rem) {
			*is_leap = 0;
			leaps = 0;
		} else {
			leaps = rem / 4U;
			rem %= 4U;
			*is_leap = !rem;
		}
	}

	leaps += 97*cycles + 24*centuries - *is_leap;

	return (year-100) * 31536000LL + leaps * 86400LL + 946684800 + 86400;
}

static long long __tm_to_secs(const struct tm *tm)
{
	int is_leap;
	long long year = tm->tm_year;
	int month = tm->tm_mon;
	if (month >= 12 || month < 0) {
		int adj = month / 12;
		month %= 12;
		if (month < 0) {
			adj--;
			month += 12;
		}
		year += adj;
	}
	long long t = __year_to_secs(year, &is_leap);
	t += __month_to_secs(month, is_leap);
	t += 86400LL * (tm->tm_mday-1);
	t += 3600LL * tm->tm_hour;
	t += 60LL * tm->tm_min;
	t += tm->tm_sec;
	return t;
}

static int __secs_to_tm(long long t, struct tm *tm)
{
	long long days, secs;
	int remdays, remsecs, remyears;
	int qc_cycles, c_cycles, q_cycles;
	int years, 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 + 400*qc_cycles;

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

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

	tm->tm_year = years + 100;
	tm->tm_mon = months + 2;
	if (tm->tm_mon >= 12) {
		tm->tm_mon -=12;
		tm->tm_year++;
	}
	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;
}

#ifdef _WIN32
struct tm *__gmtime_r(const time_t *t, struct tm *tm)
{
	if (__secs_to_tm(*t, tm) < 0) {
		errno = EOVERFLOW;
		return 0;
	}
	tm->tm_isdst = 0;
	return tm;
}
#endif

time_t timegm(struct tm *tm)
{
	struct tm new;
	long long t = __tm_to_secs(tm);
	if (__secs_to_tm(t, &new) < 0) {
		errno = EOVERFLOW;
		return -1;
	}
#if SIZEOF_TIME_T != 8
	if (t > (long long)INT_MAX || t < (long long)INT_MIN) {
		errno = EOVERFLOW;
		return -1;
	}
#endif
	*tm = new;
	tm->tm_isdst = 0;
	return t;
}