time/offtime.c

   1 /* Copyright (C) 1991, 1993, 1997 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 not,
  16    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17    Boston, MA 02111-1307, USA.  */
  18
  19 #include <time.h>
  20
  21 /* Defined in mktime.c.  */
  22 extern const unsigned short int __mon_yday[2][13];
  23
  24 #define SECS_PER_HOUR   (60 * 60)
  25 #define SECS_PER_DAY    (SECS_PER_HOUR * 24)
  26
  27 /* Compute the `struct tm' representation of *T,
  28    offset OFFSET seconds east of UTC,
  29    and store year, yday, mon, mday, wday, hour, min, sec into *TP.  */
  30 void
  31 __offtime (t, offset, tp)
  32      const time_t *t;
  33      long int offset;
  34      struct tm *tp;
  35 {
  36   register long int days, rem, y;
  37   register const unsigned short int *ip;
  38
  39   days = *t / SECS_PER_DAY;
  40   rem = *t % SECS_PER_DAY;
  41   rem += offset;
  42   while (rem < 0)
  43     {
  44       rem += SECS_PER_DAY;
  45       --days;
  46     }
  47   while (rem >= SECS_PER_DAY)
  48     {
  49       rem -= SECS_PER_DAY;
  50       ++days;
  51     }
  52   tp->tm_hour = rem / SECS_PER_HOUR;
  53   rem %= SECS_PER_HOUR;
  54   tp->tm_min = rem / 60;
  55   tp->tm_sec = rem % 60;
  56   /* January 1, 1970 was a Thursday.  */
  57   tp->tm_wday = (4 + days) % 7;
  58   if (tp->tm_wday < 0)
  59     tp->tm_wday += 7;
  60   y = 1970;
  61
  62 #define LEAPS_THRU_END_OF(y) ((y) / 4 - (y) / 100 + (y) / 400)
  63
  64   while (days < 0 || days >= (__isleap (y) ? 366 : 365))
  65     {
  66       /* Guess a corrected year, assuming 365 days per year.  */
  67       long int yg = y + days / 365 - (days % 365 < 0);
  68
  69       /* Adjust DAYS and Y to match the guessed year.  */
  70       days -= ((yg - y) * 365
  71                + LEAPS_THRU_END_OF (yg - 1)
  72                - LEAPS_THRU_END_OF (y - 1));
  73       y = yg;
  74     }
  75   tp->tm_year = y - 1900;
  76   tp->tm_yday = days;
  77   ip = __mon_yday[__isleap(y)];
  78   for (y = 11; days < ip[y]; --y)
  79     continue;
  80   days -= ip[y];
  81   tp->tm_mon = y;
  82   tp->tm_mday = days + 1;
  83 }