/* Copyright (C) 1991-2018 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#include <errno.h>

#include <time.h>

#define	SECS_PER_HOUR	(60 * 60)

#define	SECS_PER_DAY	(SECS_PER_HOUR * 24)

/* Compute the `struct tm' representation of *T,

   offset OFFSET seconds east of UTC,

   and store year, yday, mon, mday, wday, hour, min, sec into *TP.

   Return nonzero if successful.  */

int

__offtime (const time_t *t, long int offset, struct tm *tp)

{

  time_t days, rem, y;

  const unsigned short int *ip;

  days = *t / SECS_PER_DAY;

  rem = *t % SECS_PER_DAY;

  rem += offset;

  while (rem < 0)

    {

      rem += SECS_PER_DAY;

      --days;

    }

  while (rem >= SECS_PER_DAY)

    {

      rem -= SECS_PER_DAY;

      ++days;

    }

  tp->tm_hour = rem / SECS_PER_HOUR;

  rem %= SECS_PER_HOUR;

  tp->tm_min = rem / 60;

  tp->tm_sec = rem % 60;

  /* January 1, 1970 was a Thursday.  */

  tp->tm_wday = (4 + days) % 7;

  if (tp->tm_wday < 0)

    tp->tm_wday += 7;

  y = 1970;

#define DIV(a, b) ((a) / (b) - ((a) % (b) < 0))

#define LEAPS_THRU_END_OF(y) (DIV (y, 4) - DIV (y, 100) + DIV (y, 400))

  while (days < 0 || days >= (__isleap (y) ? 366 : 365))

    {

      /* Guess a corrected year, assuming 365 days per year.  */

      time_t yg = y + days / 365 - (days % 365 < 0);

      /* Adjust DAYS and Y to match the guessed year.  */

      days -= ((yg - y) * 365

	       + LEAPS_THRU_END_OF (yg - 1)

	       - LEAPS_THRU_END_OF (y - 1));

      y = yg;

    }

  tp->tm_year = y - 1900;

  if (tp->tm_year != y - 1900)

    {

      /* The year cannot be represented due to overflow.  */

      __set_errno (EOVERFLOW);

      return 0;

    }

  tp->tm_yday = days;

  ip = __mon_yday[__isleap(y)];

  for (y = 11; days < (long int) ip[y]; --y)

    continue;

  days -= ip[y];

  tp->tm_mon = y;

  tp->tm_mday = days + 1;

  return 1;

}