/*
 * 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>
 * - Use faster algorithm from civil_from_days() by Howard Hinnant - 12/06/2014,
 * 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 01.03.0000 (yes, Year 0) - this is the first
 * day of a 400-year long "era", 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	719468L
/* year to which the adjustment was made */
#define ADJUSTED_EPOCH_YEAR	0
/* 1st March of year 0 is Wednesday */
#define ADJUSTED_EPOCH_WDAY	3
/* there are 97 leap years in 400-year periods. ((400 - 97) * 365 + 97 * 366) */
#define DAYS_PER_ERA		146097L
/* there are 24 leap years in 100-year periods. ((100 - 24) * 365 + 24 * 366) */
#define DAYS_PER_CENTURY	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
/* number of years per era */
#define YEARS_PER_ERA		400

struct tm *
_DEFUN (gmtime_r, (tim_p, res),
	_CONST time_t *__restrict tim_p,
	struct tm *__restrict res)
{
  long days, rem;
  _CONST time_t lcltime = *tim_p;
  int era, weekday, year;
  unsigned erayear, yearday, month, day;
  unsigned long eraday;

  days = lcltime / SECSPERDAY + EPOCH_ADJUSTMENT_DAYS;
  rem = 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, month, day & day of year */
  /* for description of this algorithm see
   * http://howardhinnant.github.io/date_algorithms.html#civil_from_days */
  era = (days >= 0 ? days : days - (DAYS_PER_ERA - 1)) / DAYS_PER_ERA;
  eraday = days - era * DAYS_PER_ERA;	/* [0, 146096] */
  erayear = (eraday - eraday / (DAYS_PER_4_YEARS - 1) + eraday / DAYS_PER_CENTURY -
      eraday / (DAYS_PER_ERA - 1)) / 365;	/* [0, 399] */
  yearday = eraday - (DAYS_PER_YEAR * erayear + erayear / 4 - erayear / 100);	/* [0, 365] */
  month = (5 * yearday + 2) / 153;	/* [0, 11] */
  day = yearday - (153 * month + 2) / 5 + 1;	/* [1, 31] */
  month += month < 10 ? 2 : -10;
  year = ADJUSTED_EPOCH_YEAR + erayear + era * YEARS_PER_ERA + (month <= 1);

  res->tm_yday = yearday >= DAYS_PER_YEAR - DAYS_IN_JANUARY - DAYS_IN_FEBRUARY ?
      yearday - (DAYS_PER_YEAR - DAYS_IN_JANUARY - DAYS_IN_FEBRUARY) :
      yearday + DAYS_IN_JANUARY + DAYS_IN_FEBRUARY + isleap(erayear);
  res->tm_year = year - YEAR_BASE;
  res->tm_mon = month;
  res->tm_mday = day;

  res->tm_isdst = 0;

  return (res);
}