/* Originally released by the musl project (http://www.musl-libc.org/) under the

 * MIT license. Taken from the file /src/time/__secs_to_tm.c */

#include <limits.h>

#include "libc_time.h"

/* 2000-03-01 (mod 400 year, immediately after feb29 */

#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)

int

musl_secs_to_tm(long long t, struct musl_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 / 86400LL;

    remsecs = (int)(secs % 86400);

    if (remsecs < 0) {

        remsecs += 86400;

        --days;

    }

    wday = (3+days)%7;

    if (wday < 0) wday += 7;

    qc_cycles = (int)(days / DAYS_PER_400Y);

    remdays = (int)(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 = (int)(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;

}

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 };

static int

musl_month_to_secs(int month, int is_leap) {

    int t = secs_through_month[month];

    if (is_leap && month >= 2)

        t+=86400;

    return t;

}

static long long

musl_year_to_secs(const long long year, int *is_leap) {

    if (year-2ULL <= 136) {

        int y = (int)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, dummy;

    if (!is_leap) is_leap = &dummy;

    cycles = (int)((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 / 4;

            rem %= 4;

            *is_leap = !rem;

        }

    }

    leaps += 97*cycles + 24*centuries - *is_leap;

    return (year-100) * 31536000LL + leaps * 86400LL + 946684800 + 86400;

}

long long

musl_tm_to_secs(const struct musl_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 = musl_year_to_secs(year, &is_leap);

    t += musl_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;

}