#include <ctype.h>
#include <errno.h>
#include <limits.h>
#include <sys/ksys.h>
#include <time.h>

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

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

    if (months >= 10) {
        months -= 12;
        years++;
    }

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

    tm->tm_year = years + 100;
    tm->tm_mon = months + 2;
    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;
}

struct tm* __localtime_r(const time_t* restrict t, struct tm* restrict tm)
{
    // Reject time_t values whose year would overflow int because
    // __secs_to_zone cannot safely handle them.
    if (*t < INT_MIN * 31622400LL || *t > INT_MAX * 31622400LL) {
        errno = EOVERFLOW;
        return NULL;
    }
    //__secs_to_zone(*t, 0, &tm->tm_isdst, &tm->__tm_gmtoff, 0, &tm->__tm_zone);
    if (__secs_to_tm((long long)*t, tm) < 0) {
        errno = EOVERFLOW;
        return NULL;
    }
    return tm;
}

struct tm* localtime(const time_t* timer)
{
    static struct tm tm;
    return __localtime_r(timer, &tm);
}