time/offtime.c

   1 /* Copyright (C) 1991, 1993 Free Software Foundation, Inc.
   2 This file is part of the GNU C Library.
   3
   4 The GNU C Library is free software; you can redistribute it and/or
   5 modify it under the terms of the GNU Library General Public License as
   6 published by the Free Software Foundation; either version 2 of the
   7 License, or (at your option) any later version.
   8
   9 The GNU C Library is distributed in the hope that it will be useful,
  10 but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12 Library General Public License for more details.
  13
  14 You should have received a copy of the GNU Library General Public
  15 License along with the GNU C Library; see the file COPYING.LIB.  If
  16 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  17 Cambridge, MA 02139, USA.  */
  18
  19 #include <ansidecl.h>
  20 #include <time.h>
  21
  22
  23 /* Defined in mktime.c.  */
  24 extern CONST unsigned short int __mon_lengths[2][12];
  25
  26 #define SECS_PER_HOUR   (60 * 60)
  27 #define SECS_PER_DAY    (SECS_PER_HOUR * 24)
  28
  29 /* Returns the `struct tm' representation of *T,
  30    offset OFFSET seconds east of UCT.   */
  31 struct tm *
  32 DEFUN(__offtime, (t, offset), CONST time_t *t AND long int offset)
  33 {
  34   static struct tm tbuf;
  35   register long int days, rem;
  36   register int y;
  37   register CONST unsigned short int *ip;
  38
  39   if (t == NULL)
  40     return NULL;
  41
  42   days = *t / SECS_PER_DAY;
  43   rem = *t % SECS_PER_DAY;
  44   rem += offset;
  45   while (rem < 0)
  46     {
  47       rem += SECS_PER_DAY;
  48       --days;
  49     }
  50   while (rem >= SECS_PER_DAY)
  51     {
  52       rem -= SECS_PER_DAY;
  53       ++days;
  54     }
  55   tbuf.tm_hour = rem / SECS_PER_HOUR;
  56   rem %= SECS_PER_HOUR;
  57   tbuf.tm_min = rem / 60;
  58   tbuf.tm_sec = rem % 60;
  59   /* January 1, 1970 was a Thursday.  */
  60   tbuf.tm_wday = (4 + days) % 7;
  61   if (tbuf.tm_wday < 0)
  62     tbuf.tm_wday += 7;
  63   y = 1970;
  64   while (days >= (rem = __isleap(y) ? 366 : 365))
  65     {
  66       ++y;
  67       days -= rem;
  68     }
  69   while (days < 0)
  70     {
  71       --y;
  72       days += __isleap(y) ? 366 : 365;
  73     }
  74   tbuf.tm_year = y - 1900;
  75   tbuf.tm_yday = days;
  76   ip = __mon_lengths[__isleap(y)];
  77   for (y = 0; days >= ip[y]; ++y)
  78     days -= ip[y];
  79   tbuf.tm_mon = y;
  80   tbuf.tm_mday = days + 1;
  81   tbuf.tm_isdst = -1;
  82
  83   return &tbuf;
  84 }