mblaze/mytimegm.c

#include <time.h>

// from musl@1cc81f5cb, slightly tweaked

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;

        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 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;
}

time_t 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;
}