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/* Fallback timegm() for systems that lack one.
* Algorithm: http://howardhinnant.github.io/date_algorithms.html
* https://stackoverflow.com/a/58037981/4715872
*/
#ifdef TIME_WITH_SYS_TIME
# include <sys/time.h>
# include <time.h>
#else
# ifdef HAVE_SYS_TIME_H
# include <sys/time.h>
# else
# include <time.h>
# endif
#endif
static int days_from_epoch_1970(int y, int m, int d)
{
y -= m <= 2;
int era = y / 400;
int yoe = y - era * 400; // [0, 399]
int doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1; // [0, 365]
int doe = yoe * 365 + yoe / 4 - yoe / 100 + doy; // [0, 146096]
return era * 146097 + doe - 719468;
}
/* It does not modify broken-down time */
time_t timegm_fallback(struct tm const* t)
{
int year = t->tm_year + 1900;
int month = t->tm_mon; // 0-11
int days_since_epoch_1970;
if (month > 11)
{
year += month / 12;
month %= 12;
}
else if (month < 0)
{
int years_diff = (11 - month) / 12;
year -= years_diff;
month += 12 * years_diff;
}
days_since_epoch_1970 = days_from_epoch_1970(year, month + 1, t->tm_mday);
return 60 * (60 * (24L * days_since_epoch_1970 + t->tm_hour) + t->tm_min) + t->tm_sec;
}
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