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