/*

 * Copyright (C) Igor Sysoev

 * Copyright (C) NGINX, Inc.

 */

#include <nxt_main.h>

/* The function is valid for positive nxt_time_t only. */

void

nxt_gmtime(nxt_time_t s, struct tm *tm)

{

    nxt_int_t   yday;

    nxt_uint_t  daytime, mday, mon, year, days, leap;

    days = (nxt_uint_t) (s / 86400);

    daytime = (nxt_uint_t) (s % 86400);

    /* January 1, 1970 was Thursday. */

    tm->tm_wday = (4 + days) % 7;

    /* The algorithm based on Gauss' formula. */

    /* Days since March 1, 1 BCE. */

    days = days - (31 + 28) + 719527;

    /*

     * The "days" should be adjusted by 1 only, however some March 1st's

     * go to previous year, so "days" are adjusted by 2.  This also shifts

     * the last February days to the next year, but this is catched by

     * negative "yday".

     */

    year = (days + 2) * 400 / (365 * 400 + 100 - 4 + 1);

    yday = days - (365 * year + year / 4 - year / 100 + year / 400);

    leap = (year % 4 == 0) && (year % 100 || (year % 400 == 0));

    if (yday < 0) {

        yday = 365 + leap + yday;

        year--;

    }

    /*

     * An empirical formula that maps "yday" to month.

     * There are at least 10 variants, some of them are:

     *     mon = (yday + 31) * 15 / 459

     *     mon = (yday + 31) * 17 / 520

     *     mon = (yday + 31) * 20 / 612

     */

    mon = (yday + 31) * 10 / 306;

    /* The Gauss' formula that evaluates days before month. */

    mday = yday - (367 * mon / 12 - 30) + 1;

    if (yday >= 306) {

        year++;

        mon -= 11;

        yday -= 306;

    } else {

        mon++;

        yday += 31 + 28 + leap;

    }

    tm->tm_mday = mday;

    tm->tm_mon = mon;

    tm->tm_year = year - 1900;

    tm->tm_yday = yday;

    tm->tm_hour = daytime / 3600;

    daytime %= 3600;

    tm->tm_min = daytime / 60;

    tm->tm_sec = daytime % 60;

}