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
  23 /* Defined in mktime.c.  */
  24 extern CONST unsigned short int __mon_yday[2][13];
  25
  26 #define SECS_PER_HOUR   (60 * 60)
  27 #define SECS_PER_DAY    (SECS_PER_HOUR * 24)
  28
  29 /* Compute the `struct tm' representation of *T,
  30    offset OFFSET seconds east of UTC,
  31    and store year, yday, mon, mday, wday, hour, min, sec into *TP.  */
  32 void
  33 DEFUN(__offtime, (t, offset, tp),
  34       CONST time_t *t AND long int offset AND struct tm *tp)
  35 {
  36   register long int days, rem;
  37   register int y;
  38   register 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   while (days >= (rem = __isleap(y) ? 366 : 365))
  63     {
  64       ++y;
  65       days -= rem;
  66     }
  67   while (days < 0)
  68     {
  69       --y;
  70       days += __isleap(y) ? 366 : 365;
  71     }
  72   tp->tm_year = y - 1900;
  73   tp->tm_yday = days;
  74   ip = __mon_yday[__isleap(y)];
  75   for (y = 11; days < ip[y]; --y)
  76     continue;
  77   days -= ip[y];
  78   tp->tm_mon = y;
  79   tp->tm_mday = days + 1;
  80 }