compat/timegm.c

   1 /*****************************************************************************
   2  * timegm.c: BSD/GNU timegm() replacement
   3  *****************************************************************************
   4  * Copyright © 2007 Laurent Aimar
   5  * Copyright © 2015 Rémi Denis-Courmont
   6  *
   7  * This program is free software; you can redistribute it and/or modify it
   8  * under the terms of the GNU Lesser General Public License as published by
   9  * the Free Software Foundation; either version 2.1 of the License, or
  10  * (at your option) any later version.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  15  * GNU Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public License
  18  * along with this program; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
  20  *****************************************************************************/
  21
  22 #ifdef HAVE_CONFIG_H
  23 # include <config.h>
  24 #endif
  25
  26 #include <stdbool.h>
  27 #include <time.h>
  28
  29 static bool is_leap_year(unsigned y)
  30 {
  31     if (y % 4)
  32         return false;
  33     if (y % 100)
  34         return true;
  35     if (y % 400)
  36         return false;
  37     return true;
  38 }
  39
  40 time_t timegm(struct tm *tm)
  41 {
  42     static const unsigned ydays[12 + 1] = {
  43         0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
  44     };
  45
  46     if (tm->tm_year < 70 /* FIXME: (negative) dates before Epoch */
  47      || tm->tm_mon < 0 || tm->tm_mon > 11
  48      || tm->tm_mday < 1 || tm->tm_mday > 31
  49      || tm->tm_hour < 0 || tm->tm_hour > 23
  50      || tm->tm_min < 0 || tm->tm_min > 59
  51      || tm->tm_sec < 0 || tm->tm_sec > 60 /* mind the leap second */)
  52         return -1;
  53
  54     /* Count the number of days */
  55     unsigned t = 365 * (tm->tm_year - 70)
  56                  + ydays[tm->tm_mon] + (tm->tm_mday - 1);
  57
  58     /* TODO: unroll */
  59     for (int i = 70; i < tm->tm_year; i++)
  60         t += is_leap_year(1900 + i);
  61
  62     if (tm->tm_mon > 1)
  63         t += is_leap_year(1900 + tm->tm_year);
  64
  65     t *= 24;
  66     t += tm->tm_hour;
  67     t *= 60;
  68     t += tm->tm_min;
  69     t *= 60;
  70     t += tm->tm_sec;
  71     return t;
  72 }