time/offtime.c

   1 /* Copyright (C) 1991-2024 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    <https://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 (__time64_t t, long int offset, struct tm *tp)
  30 {
  31   __time64_t days, rem, y;
  32   const unsigned short int *ip;
  33
  34   days = t / SECS_PER_DAY;
  35   rem = t % SECS_PER_DAY;
  36   rem += offset;
  37   while (rem < 0)
  38     {
  39       rem += SECS_PER_DAY;
  40       --days;
  41     }
  42   while (rem >= SECS_PER_DAY)
  43     {
  44       rem -= SECS_PER_DAY;
  45       ++days;
  46     }
  47   tp->tm_hour = rem / SECS_PER_HOUR;
  48   rem %= SECS_PER_HOUR;
  49   tp->tm_min = rem / 60;
  50   tp->tm_sec = rem % 60;
  51   /* January 1, 1970 was a Thursday.  */
  52   tp->tm_wday = (4 + days) % 7;
  53   if (tp->tm_wday < 0)
  54     tp->tm_wday += 7;
  55   y = 1970;
  56
  57 #define DIV(a, b) ((a) / (b) - ((a) % (b) < 0))
  58 #define LEAPS_THRU_END_OF(y) (DIV (y, 4) - DIV (y, 100) + DIV (y, 400))
  59
  60   while (days < 0 || days >= (__isleap (y) ? 366 : 365))
  61     {
  62       /* Guess a corrected year, assuming 365 days per year.  */
  63       __time64_t yg = y + days / 365 - (days % 365 < 0);
  64
  65       /* Adjust DAYS and Y to match the guessed year.  */
  66       days -= ((yg - y) * 365
  67                + LEAPS_THRU_END_OF (yg - 1)
  68                - LEAPS_THRU_END_OF (y - 1));
  69       y = yg;
  70     }
  71   tp->tm_year = y - 1900;
  72   if (tp->tm_year != y - 1900)
  73     {
  74       /* The year cannot be represented due to overflow.  */
  75       __set_errno (EOVERFLOW);
  76       return 0;
  77     }
  78   tp->tm_yday = days;
  79   ip = __mon_yday[__isleap(y)];
  80   for (y = 11; days < (long int) ip[y]; --y)
  81     continue;
  82   days -= ip[y];
  83   tp->tm_mon = y;
  84   tp->tm_mday = days + 1;
  85   return 1;
  86 }