/*
 * gmtime_r.c
 * Original Author: Adapted from tzcode maintained by Arthur David Olson.
 * Modifications:
 * - Changed to mktm_r and added __tzcalc_limits - 04/10/02, Jeff Johnston
 * - Fixed bug in mday computations - 08/12/04, Alex Mogilnikov <alx@intellectronika.ru>
 * - Fixed bug in __tzcalc_limits - 08/12/04, Alex Mogilnikov <alx@intellectronika.ru>
 * - Move code from _mktm_r() to gmtime_r() - 05/09/14, Freddie Chopin <freddie_chopin@op.pl>
 * - Fixed bug in calculations for dates after year 2069 or before year 1901. Ideas for
 *   solution taken from musl's __secs_to_tm() - 07/12/2014, Freddie Chopin
 *   <freddie_chopin@op.pl>
 *
 * Converts the calendar time pointed to by tim_p into a broken-down time
 * expressed as local time. Returns a pointer to a structure containing the
 * broken-down time.
 */

#include "local.h"

/* move epoch from 01.01.1970 to 01.03.2000 - this is the first day of new
 * 400-year long cycle, right after additional day of leap year. This adjustment
 * is required only for date calculation, so instead of modifying time_t value
 * (which would require 64-bit operations to work correctly) it's enough to
 * adjust the calculated number of days since epoch. */
#define EPOCH_ADJUSTMENT_DAYS	11017
/* year to which the adjustment was made */
#define ADJUSTED_EPOCH_YEAR	2000
/* 1st March of 2000 is Wednesday */
#define ADJUSTED_EPOCH_WDAY	3
/* there are 97 leap years in 400-year periods. ((400 - 97) * 365 + 97 * 366) */
#define DAYS_PER_400_YEARS	146097L
/* there are 24 leap years in 100-year periods. ((100 - 24) * 365 + 24 * 366) */
#define DAYS_PER_100_YEARS	36524L
/* there is one leap year every 4 years */
#define DAYS_PER_4_YEARS	(3 * 365 + 366)
/* number of days in a non-leap year */
#define DAYS_PER_YEAR		365
/* number of days in January */
#define DAYS_IN_JANUARY		31
/* number of days in non-leap February */
#define DAYS_IN_FEBRUARY	28

struct tm *
_DEFUN (gmtime_r, (tim_p, res),
	_CONST time_t *__restrict tim_p _AND
	struct tm *__restrict res)
{
  long days, rem;
  _CONST time_t lcltime = *tim_p;
  int year, month, yearday, weekday;
  int years400, years100, years4, remainingyears;
  int yearleap;
  _CONST int *ip;

  days = ((long)lcltime) / SECSPERDAY - EPOCH_ADJUSTMENT_DAYS;
  rem = ((long)lcltime) % SECSPERDAY;
  if (rem < 0)
    {
      rem += SECSPERDAY;
      --days;
    }

  /* compute hour, min, and sec */
  res->tm_hour = (int) (rem / SECSPERHOUR);
  rem %= SECSPERHOUR;
  res->tm_min = (int) (rem / SECSPERMIN);
  res->tm_sec = (int) (rem % SECSPERMIN);

  /* compute day of week */
  if ((weekday = ((ADJUSTED_EPOCH_WDAY + days) % DAYSPERWEEK)) < 0)
    weekday += DAYSPERWEEK;
  res->tm_wday = weekday;

  /* compute year & day of year */
  years400 = days / DAYS_PER_400_YEARS;
  days -= years400 * DAYS_PER_400_YEARS;
  /* simplify by making the values positive */
  if (days < 0)
    {
      days += DAYS_PER_400_YEARS;
      --years400;
    }

  years100 = days / DAYS_PER_100_YEARS;
  if (years100 == 4) /* required for proper day of year calculation */
    --years100;
  days -= years100 * DAYS_PER_100_YEARS;
  years4 = days / DAYS_PER_4_YEARS;
  days -= years4 * DAYS_PER_4_YEARS;
  remainingyears = days / DAYS_PER_YEAR;
  if (remainingyears == 4) /* required for proper day of year calculation */
    --remainingyears;
  days -= remainingyears * DAYS_PER_YEAR;

  year = ADJUSTED_EPOCH_YEAR + years400 * 400 + years100 * 100 + years4 * 4 +
      remainingyears;

  /* If remainingyears is zero, it means that the years were completely
   * "consumed" by modulo calculations by 400, 100 and 4, so the year is:
   * 1. a multiple of 4, but not a multiple of 100 or 400 - it's a leap year,
   * 2. a multiple of 4 and 100, but not a multiple of 400 - it's not a leap
   * year,
   * 3. a multiple of 4, 100 and 400 - it's a leap year.
   * If years4 is non-zero, it means that the year is not a multiple of 100 or
   * 400 (case 1), so it's a leap year. If years100 is zero (and years4 is zero
   * - due to short-circuiting), it means that the year is a multiple of 400
   * (case 3), so it's also a leap year. */
  yearleap = remainingyears == 0 && (years4 != 0 || years100 == 0);

  /* adjust back to 1st January */
  yearday = days + DAYS_IN_JANUARY + DAYS_IN_FEBRUARY + yearleap;
  if (yearday >= DAYS_PER_YEAR + yearleap)
    {
      yearday -= DAYS_PER_YEAR + yearleap;
      ++year;
    }
  res->tm_yday = yearday;
  res->tm_year = year - YEAR_BASE;

  /* Because "days" is the number of days since 1st March, the additional leap
   * day (29th of February) is the last possible day, so it doesn't matter much
   * whether the year is actually leap or not. */
  ip = __month_lengths[1];
  month = 2;
  while (days >= ip[month])
    {
      days -= ip[month];
      if (++month >= MONSPERYEAR)
        month = 0;
    }
  res->tm_mon = month;
  res->tm_mday = days + 1;

  res->tm_isdst = 0;

  return (res);
}