time/offtime.c

   1 /* Copyright (C) 1991, 1993, 1997, 1998, 2002 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 Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the 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    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <time.h>
  20
  21 #define SECS_PER_HOUR   (60 * 60)
  22 #define SECS_PER_DAY    (SECS_PER_HOUR * 24)
  23
  24 /* Compute the `struct tm' representation of *T,
  25    offset OFFSET seconds east of UTC,
  26    and store year, yday, mon, mday, wday, hour, min, sec into *TP.
  27    Return nonzero if successful.  */
  28 int
  29 __offtime (t, offset, tp)
  30      const time_t *t;
  31      long int offset;
  32      struct tm *tp;
  33 {
  34   time_t days, rem, y;
  35   const unsigned short int *ip;
  36
  37   days = *t / SECS_PER_DAY;
  38   rem = *t % SECS_PER_DAY;
  39   rem += offset;
  40   while (rem < 0)
  41     {
  42       rem += SECS_PER_DAY;
  43       --days;
  44     }
  45   while (rem >= SECS_PER_DAY)
  46     {
  47       rem -= SECS_PER_DAY;
  48       ++days;
  49     }
  50   tp->tm_hour = rem / SECS_PER_HOUR;
  51   rem %= SECS_PER_HOUR;
  52   tp->tm_min = rem / 60;
  53   tp->tm_sec = rem % 60;
  54   /* January 1, 1970 was a Thursday.  */
  55   tp->tm_wday = (4 + days) % 7;
  56   if (tp->tm_wday < 0)
  57     tp->tm_wday += 7;
  58   y = 1970;
  59
  60 #define DIV(a, b) ((a) / (b) - ((a) % (b) < 0))
  61 #define LEAPS_THRU_END_OF(y) (DIV (y, 4) - DIV (y, 100) + DIV (y, 400))
  62
  63   while (days < 0 || days >= (__isleap (y) ? 366 : 365))
  64     {
  65       /* Guess a corrected year, assuming 365 days per year.  */
  66       time_t 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   if (tp->tm_year != y - 1900)
  76     {
  77       /* The year cannot be represented due to overflow.  */
  78       __set_errno (EOVERFLOW);
  79       return 0;
  80     }
  81   tp->tm_yday = days;
  82   ip = __mon_yday[__isleap(y)];
  83   for (y = 11; days < (long int) ip[y]; --y)
  84     continue;
  85   days -= ip[y];
  86   tp->tm_mon = y;
  87   tp->tm_mday = days + 1;
  88   return 1;
  89 }