compat/gmtime_r.c

   1 /*****************************************************************************
   2  * gmtime_r.c: POSIX gmtime_r() replacement
   3  *****************************************************************************
   4  * Copyright © 2011-2012 Rémi Denis-Courmont
   5  *
   6  * This program is free software; you can redistribute it and/or modify it
   7  * under the terms of the GNU Lesser General Public License as published by
   8  * the Free Software Foundation; either version 2.1 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14  * GNU Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public License
  17  * along with this program; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
  19  *****************************************************************************/
  20
  21 #ifdef HAVE_CONFIG_H
  22 # include <config.h>
  23 #endif
  24
  25 #include <time.h>
  26 #include <stdio.h>
  27 #include <stdlib.h>
  28
  29 struct tm *gmtime_r (const time_t *timep, struct tm *tm)
  30 {
  31     static const unsigned short normal[12] =
  32         { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
  33     static const unsigned short leap[12] =
  34         { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
  35     lldiv_t d;
  36     const unsigned short *months;
  37
  38     /* Rebase from 1970 Unix epoch to fictitious year 0 (i.e. would-be
  39      * year 1 BC had the Gregorian calendar already existed then) */
  40     d.quot = 60LL * 60 * 24 * (4 * 146097 + 135140) + *timep;
  41
  42     d = lldiv (d.quot, 60); /* 60 seconds per minute */
  43     if (*timep < 0)
  44     {
  45         d.rem += 60;
  46         d.quot--;
  47     }
  48     tm->tm_sec = d.rem;
  49
  50     d = lldiv (d.quot, 60); /* 60 minutes per hour */
  51     tm->tm_min = d.rem;
  52
  53     d = lldiv (d.quot, 24); /* 24 hours per day */
  54     tm->tm_hour = d.rem;
  55
  56     tm->tm_wday = (d.quot + 6) % 7; /* 7 days per week */
  57
  58     d = lldiv (d.quot, 146097); /* 146097 days per 4 centuries */
  59     tm->tm_year = 400 * d.quot;
  60
  61     if (d.rem >= 36525)
  62     {    /* Century with 24 leap years */
  63         d.rem -= 36525;
  64         tm->tm_year += 100;
  65
  66         d = lldiv (d.rem, 36524); /* 36524 days per normal century */
  67         tm->tm_year += d.quot * 100;
  68         if (d.rem >= 59) /* not in January or February of year 00 */
  69             d.rem++; /* insert missing February 29th 00*/
  70     }
  71     tm->tm_year -= 1900; /* Rebase to 1900 for struct tm */
  72
  73     d = lldiv (d.rem, 1461); /* 1461 days per 4 years */
  74     tm->tm_year += 4 * d.quot;
  75
  76     if (d.rem >= 366)
  77     {    /* Non-leap year */
  78         d.rem -= 366;
  79         tm->tm_year++;
  80
  81         d = lldiv (d.rem, 365); /* 365 days per normal year */
  82         tm->tm_year += d.quot;
  83         months = normal;
  84     }
  85     else
  86         months = leap;
  87     tm->tm_yday = d.rem;
  88
  89     for (tm->tm_mon = 0; d.rem >= months[tm->tm_mon]; tm->tm_mon++)
  90         d.rem -= months[tm->tm_mon]; /* 28 to 31 days in a month */
  91     tm->tm_mday = d.rem + 1;
  92
  93     tm->tm_isdst = 0; /* UTC time */
  94     return tm;
  95 }