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#include <time.h>
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/*
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 * BUG: Doesn't do leap years in full glory,
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 * or calendar changes. In 2038 the sign bit
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 * will be needed in time_t, but we say it
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 * can't be represented.
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 */
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static int
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dysize(int y)
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{
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	y += 1900; /* arg is a tm_year, number of years since 1900 */
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	if((y%4) == 0 && ((y%100) !=0 || (y%400) == 0))
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		return 366;
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	return 365;
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}
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static int
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dmsize(int m, int y)
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{
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	static	char	sizes[12] =
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		{ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
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	if(m == 1)
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		return (dysize(y)==366)? 29 : 28;
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	else
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		return sizes[m];
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}
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/* Reduce *v to [0, mult), adding 1 to *next for every mult
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 * subtracted from *v, and return 1 if reduction worked (no overflow)
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 */
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static int
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reduce(int *v, int *next, int mult)
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{
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	int oldnext;
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	while(*v < 0){
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		*v += mult;
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		oldnext = *next;
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		--*next;
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		if(!(*next < oldnext))
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			return 0;
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	}
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	while(*v >= mult){
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		*v -= mult;
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		oldnext = *next;
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		++*next;
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		if(!(*next > oldnext))
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			return 0;
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	}
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	return 1;
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}
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static int
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jan1(int yr)
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{
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	int y, d;
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	y = yr+1900;
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	d = (4+y+(y+3)/4-(y-1701)/100+(y-1601)/400+3)%7;
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	return d;
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}
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time_t
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mktime(struct tm *t)
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{
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	time_t a;
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	int i, d;
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	struct tm *ptm;
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	if(!(reduce(&t->tm_sec, &t->tm_min, 60) &&
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	     reduce(&t->tm_min, &t->tm_hour, 60) &&
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	     reduce(&t->tm_hour, &t->tm_mday, 24) &&
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	     reduce(&t->tm_mon, &t->tm_year, 12)))
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		return -1;
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	while(t->tm_mday < 1){
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		if(--t->tm_mon == -1){
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			t->tm_mon = 11;
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			t->tm_year--;
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		}
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		t->tm_mday += dmsize(t->tm_mon, t->tm_year);
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	}
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	while(t->tm_mday > dmsize(t->tm_mon, t->tm_year)){
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		t->tm_mday -= dmsize(t->tm_mon, t->tm_year);
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		if(++t->tm_mon == 12){
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			t->tm_mon = 0;
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			t->tm_year++;
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		}
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	}
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	a = t->tm_sec + 60*t->tm_min + 3600*t->tm_hour;
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	t->tm_yday = t->tm_mday-1;
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	for(i=0; i<t->tm_mon; i++)
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		t->tm_yday += dmsize(i, t->tm_year);
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	a += t->tm_yday*86400L;
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	if(t->tm_year < 70){
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		for(i=t->tm_year; i<70; i++)
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			if((a -= dysize(i)*86400L) < 0)
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				return -1;
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	}else if(t->tm_year > 70){
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		for(i=70; i<t->tm_year; i++)
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			if((a += dysize(i)*86400L) < 0)
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				return -1;
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	}
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	/*
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	 * Now a is number of seconds past Jan 1 1970.
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	 * Convert to GMT.
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	 */
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	ptm = gmtime(&a);
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	d = ptm->tm_hour;
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	ptm = localtime(&a);
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	d -= ptm->tm_hour;
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	if(d < 0)
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		d += 24;
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	if(t->tm_isdst == 0 && ptm->tm_isdst)
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		d--;
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	if(t->tm_isdst > 0 && !ptm->tm_isdst)
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		d++;
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	a += d*3600;
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	t->tm_wday = (jan1(t->tm_year)+t->tm_yday)%7;
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	return a;
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}