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    Return nonzero if successful.  */
  31 int
  32 __offtime (t, offset, tp)
  33      const time_t *t;
  34      long int offset;
  35      struct tm *tp;
  36 {
  37   long int days, rem, y;
  38   const unsigned short int *ip;
  39
  40   days = *t / SECS_PER_DAY;
  41   rem = *t % SECS_PER_DAY;
  42   rem += offset;
  43   while (rem < 0)
  44     {
  45       rem += SECS_PER_DAY;
  46       --days;
  47     }
  48   while (rem >= SECS_PER_DAY)
  49     {
  50       rem -= SECS_PER_DAY;
  51       ++days;
  52     }
  53   tp->tm_hour = rem / SECS_PER_HOUR;
  54   rem %= SECS_PER_HOUR;
  55   tp->tm_min = rem / 60;
  56   tp->tm_sec = rem % 60;
  57   /* January 1, 1970 was a Thursday.  */
  58   tp->tm_wday = (4 + days) % 7;
  59   if (tp->tm_wday < 0)
  60     tp->tm_wday += 7;
  61   y = 1970;
  62
  63 #define DIV(a, b) ((a) / (b) - ((a) % (b) < 0))
  64 #define LEAPS_THRU_END_OF(y) (DIV (y, 4) - DIV (y, 100) + DIV (y, 400))
  65
  66   while (days < 0 || days >= (__isleap (y) ? 366 : 365))
  67     {
  68       /* Guess a corrected year, assuming 365 days per year.  */
  69       long int yg = y + days / 365 - (days % 365 < 0);
  70
  71       /* Adjust DAYS and Y to match the guessed year.  */
  72       days -= ((yg - y) * 365
  73                + LEAPS_THRU_END_OF (yg - 1)
  74                - LEAPS_THRU_END_OF (y - 1));
  75       y = yg;
  76     }
  77   tp->tm_year = y - 1900;
  78   if (tp->tm_year != y - 1900)
  79     return 0;
  80   tp->tm_yday = days;
  81   ip = __mon_yday[__isleap(y)];
  82   for (y = 11; days < (long int) ip[y]; --y)
  83     continue;
  84   days -= ip[y];
  85   tp->tm_mon = y;
  86   tp->tm_mday = days + 1;
  87   return 1;
  88 }