nativelibc/time/gmtime.c

/*	A part of the Native C Library for Windows NT
	Copyright 2007-2015 PC GO Ld.

	This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

	This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
*/

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

#define SECS_PER_HOUR (60 * 60)
#define SECS_PER_DAY (SECS_PER_HOUR * 24)

static const unsigned short int __mon_yday[2][13] = {
	/* Normal years.  */
	{ 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 },
	/* Leap years.  */
	{ 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 }
};

struct tm *gmtime_r(const time_t *timep, struct tm *tp) {
	if(!timep) {
		errno = EINVAL;
		return NULL;
	}

	long int days, rem, y;
	const unsigned short int *ip;

	days = *timep / SECS_PER_DAY;
	rem = *timep % SECS_PER_DAY;

	while(rem < 0) {
		rem += SECS_PER_DAY;
		days--;
	}
	while(rem >= SECS_PER_DAY) {
		rem -= SECS_PER_DAY;
		days++;
	}
	tp->tm_hour = rem / SECS_PER_HOUR;
	rem %= SECS_PER_HOUR;
	tp->tm_min = rem / 60;
	tp->tm_sec = rem % 60;
	/* January 1, 1970 was a Thursday.  */
	tp->tm_wday = (4 + days) % 7;
	if(tp->tm_wday < 0) tp->tm_wday += 7;
	y = 1970;

#define DIV(a, b) ((a) / (b) - ((a) % (b) < 0))
#define LEAPS_THRU_END_OF(y) (DIV (y, 4) - DIV (y, 100) + DIV (y, 400))

	while(days < 0 || days >= (__isleap(y) ? 366 : 365)) {
		/* Guess a corrected year, assuming 365 days per year.  */
		long int yg = y + days / 365 - (days % 365 < 0);

		/* Adjust DAYS and Y to match the guessed year.  */
		days -= ((yg - y) * 365 + LEAPS_THRU_END_OF(yg - 1) - LEAPS_THRU_END_OF(y - 1));
		y = yg;
	}
	tp->tm_year = y - 1900;
	if(tp->tm_year != y - 1900) {
		/* The year cannot be represented due to overflow.  */
		errno = EOVERFLOW;
		return NULL;
	}
	tp->tm_yday = days;
	ip = __mon_yday[__isleap(y)];
	for(y = 11; days < (long int)ip[y]; y--);
	days -= ip[y];
	tp->tm_mon = y;
	tp->tm_mday = days + 1;
	return tp;
}

struct tm *gmtime(const time_t *timep) {
	static struct tm buffer;
	return gmtime_r(timep, &buffer);
}