352 lines
8.4 KiB
C
352 lines
8.4 KiB
C
/*
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* Copyright (c) 2013, 2023 Jonas 'Sortie' Termansen.
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*
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* time/gmtime_r.c
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* Convert a timestamp into a date and time according to UTC.
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*/
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#include <stdbool.h>
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#include <time.h>
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#include <stdint.h>
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#include <time.h>
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#define DAYS_JANUARY 31
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#define DAYS_FEBRUARY 28
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#define DAYS_MARCH 31
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#define DAYS_APRIL 30
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#define DAYS_MAY 31
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#define DAYS_JUNE 30
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#define DAYS_JULY 31
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#define DAYS_AUGUST 31
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#define DAYS_SEPTEMBER 30
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#define DAYS_OCTOBER 31
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#define DAYS_NOVEMBER 30
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#define DAYS_DECEMBER 31
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#define UNKNOWN 127
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#define DECL_LEAP_SECOND(year, jun, dec) \
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{0, 0, 0, 0, 0, jun, 0, 0, 0, 0, 0, dec}
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static const char leap_seconds[][12] =
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{
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DECL_LEAP_SECOND(1970, 0, 0),
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DECL_LEAP_SECOND(1971, 0, 0),
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DECL_LEAP_SECOND(1972, 1, 1),
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DECL_LEAP_SECOND(1973, 0, 1),
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DECL_LEAP_SECOND(1974, 0, 1),
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DECL_LEAP_SECOND(1975, 0, 1),
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DECL_LEAP_SECOND(1976, 0, 1),
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DECL_LEAP_SECOND(1977, 0, 1),
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DECL_LEAP_SECOND(1978, 0, 1),
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DECL_LEAP_SECOND(1979, 0, 1),
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DECL_LEAP_SECOND(1980, 0, 0),
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DECL_LEAP_SECOND(1981, 1, 0),
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DECL_LEAP_SECOND(1982, 1, 0),
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DECL_LEAP_SECOND(1983, 1, 0),
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DECL_LEAP_SECOND(1984, 0, 0),
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DECL_LEAP_SECOND(1985, 1, 0),
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DECL_LEAP_SECOND(1986, 0, 0),
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DECL_LEAP_SECOND(1987, 0, 1),
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DECL_LEAP_SECOND(1988, 0, 0),
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DECL_LEAP_SECOND(1989, 0, 1),
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DECL_LEAP_SECOND(1990, 0, 1),
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DECL_LEAP_SECOND(1991, 0, 0),
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DECL_LEAP_SECOND(1992, 1, 0),
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DECL_LEAP_SECOND(1993, 1, 0),
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DECL_LEAP_SECOND(1994, 1, 0),
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DECL_LEAP_SECOND(1995, 0, 1),
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DECL_LEAP_SECOND(1996, 0, 0),
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DECL_LEAP_SECOND(1997, 1, 0),
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DECL_LEAP_SECOND(1998, 0, 1),
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DECL_LEAP_SECOND(1999, 0, 0),
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DECL_LEAP_SECOND(2000, 0, 0),
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DECL_LEAP_SECOND(2001, 0, 0),
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DECL_LEAP_SECOND(2002, 0, 0),
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DECL_LEAP_SECOND(2003, 0, 0),
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DECL_LEAP_SECOND(2004, 0, 0),
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DECL_LEAP_SECOND(2005, 0, 1),
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DECL_LEAP_SECOND(2006, 0, 0),
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DECL_LEAP_SECOND(2007, 0, 0),
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DECL_LEAP_SECOND(2008, 0, 1),
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DECL_LEAP_SECOND(2009, 0, 0),
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DECL_LEAP_SECOND(2010, 0, 0),
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DECL_LEAP_SECOND(2011, 0, 0),
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DECL_LEAP_SECOND(2012, 1, 0),
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DECL_LEAP_SECOND(2013, 0, 0),
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DECL_LEAP_SECOND(2014, 0, 0),
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DECL_LEAP_SECOND(2015, 1, 0),
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DECL_LEAP_SECOND(2016, 0, 1),
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DECL_LEAP_SECOND(2017, 0, 0),
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DECL_LEAP_SECOND(2018, 0, 0),
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DECL_LEAP_SECOND(2019, 0, 0),
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DECL_LEAP_SECOND(2020, 0, 0),
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DECL_LEAP_SECOND(2021, 0, 0),
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DECL_LEAP_SECOND(2022, 0, 0),
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DECL_LEAP_SECOND(2023, 0, UNKNOWN),
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};
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static const char month_days_list[12] =
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{
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DAYS_JANUARY,
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DAYS_FEBRUARY,
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DAYS_MARCH,
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DAYS_APRIL,
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DAYS_MAY,
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DAYS_JUNE,
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DAYS_JULY,
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DAYS_AUGUST,
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DAYS_SEPTEMBER,
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DAYS_OCTOBER,
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DAYS_NOVEMBER,
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DAYS_DECEMBER,
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};
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static time_t get_leap_second_maybe(int year, int month)
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{
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const time_t num_years = sizeof(leap_seconds) / sizeof(leap_seconds[0]);
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if ( year < 1970 )
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return 0;
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if ( num_years <= year-1970 )
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return UNKNOWN;
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return leap_seconds[year-1970][month];
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}
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static time_t get_leap_second(int year, int month)
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{
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time_t result = get_leap_second_maybe(year, month);
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return result == UNKNOWN ? 0 : result;
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}
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static time_t leap_seconds_in_year(int year)
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{
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time_t ret = 0;
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for ( int i = 0; i < 12; i++ )
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ret += get_leap_second(year, i);
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return ret;
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}
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static bool is_leap_year(int year)
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{
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return (year % 4 == 0 && year % 100 != 0) || year % 400 == 0;
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}
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static time_t days_in_year(int year)
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{
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return DAYS_JANUARY +
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DAYS_FEBRUARY + (is_leap_year(year) ? 1 : 0) +
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DAYS_MARCH +
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DAYS_APRIL +
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DAYS_MAY +
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DAYS_JUNE +
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DAYS_JULY +
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DAYS_AUGUST +
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DAYS_SEPTEMBER +
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DAYS_OCTOBER +
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DAYS_NOVEMBER +
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DAYS_DECEMBER;
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}
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static int days_in_month(int year, int month)
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{
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return month_days_list[month] + (month == 1 && is_leap_year(year));
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}
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struct tm* gmtime_r(const time_t* time_ptr, struct tm* ret)
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{
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time_t left = *time_ptr;
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ret->tm_year = 1970;
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ret->tm_wday = 4 /* Supposedly, the world began on a Thursday. */;
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// If the timestamp is after the epoch.
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while ( 0 < left )
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{
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time_t year_leaps = leap_seconds_in_year(ret->tm_year);
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time_t year_days = days_in_year(ret->tm_year);
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time_t year_seconds = year_days * 24 * 60 * 60 + year_leaps;
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if ( year_seconds <= left )
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{
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left -= year_seconds;
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ret->tm_wday = (ret->tm_wday + year_days) % 7;
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ret->tm_year++;
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continue;
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}
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break;
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}
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// If the timestamp was before the epoch.
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while ( left < 0 )
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{
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ret->tm_year--;
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time_t year_leaps = leap_seconds_in_year(ret->tm_year);
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time_t year_days = days_in_year(ret->tm_year);
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time_t year_seconds = year_days * 24 * 60 * 60 + year_leaps;
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left += year_seconds;
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// We need to avoid taking the modulo of a negative value or the
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// (broken) C modulo operator gives the wrong result.
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ret->tm_wday = (ret->tm_wday - year_days + 7*7*7*7) % 7;
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}
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// Figure out the correct month.
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ret->tm_mon = 0;
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ret->tm_yday = 0;
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while ( true )
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{
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int month_leaps = get_leap_second(ret->tm_year, ret->tm_mon);
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int month_days = days_in_month(ret->tm_year, ret->tm_mon);
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int month_seconds = month_days * 24 * 60 * 60 + month_leaps;
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if ( month_seconds <= left )
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{
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left -= month_seconds;
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ret->tm_mon++;
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ret->tm_yday += month_days;
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ret->tm_wday = (ret->tm_wday + month_days) % 7;
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continue;
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}
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break;
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}
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ret->tm_mday = left / (24 * 60 * 60);
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left = left % (24 * 60 * 60);
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// If this is a regular timestamp.
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if ( ret->tm_mday < days_in_month(ret->tm_year, ret->tm_mon) )
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{
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ret->tm_yday += ret->tm_mday;
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ret->tm_hour = left / (60 * 60);
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left = left % (60 * 60);
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ret->tm_min = left / 60;
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left = left % 60;
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ret->tm_sec = left;
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}
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// If we got the timestamp for an added leap second.
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else
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{
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ret->tm_mday--; // Seemingly additional day.
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ret->tm_yday += ret->tm_mday;
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ret->tm_hour = 23;
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ret->tm_min = 59;
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ret->tm_sec = 60;
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}
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ret->tm_wday = (ret->tm_wday + ret->tm_mday) % 7;
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// TODO: Support daylight savings and timezones.
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ret->tm_isdst = -1;
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// Fix the ranges of some of the variables.
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ret->tm_mday += 1;
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ret->tm_year -= 1900;
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return ret;
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}
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time_t timegm(struct tm* tm)
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{
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time_t year = tm->tm_year + 1900;
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time_t month = tm->tm_mon;
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time_t day = tm->tm_mday - 1;
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time_t hour = tm->tm_hour;
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time_t minute = tm->tm_min;
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time_t second = tm->tm_sec;
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time_t ret = 0;
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for ( time_t y = 1970; y < year; y++ )
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{
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time_t year_leaps = leap_seconds_in_year(y);
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time_t year_days = days_in_year(y);
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time_t year_seconds = year_days * 24 * 60 * 60 + year_leaps;
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ret += year_seconds;
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}
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for ( int m = 0; m < month; m++ )
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{
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int month_leaps = get_leap_second(year, m);
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int month_days = days_in_month(year, m);
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int month_seconds = month_days * 24 * 60 * 60 + month_leaps;
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ret += month_seconds;
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}
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ret += (time_t) day * 24 * 60 * 60;
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ret += (time_t) hour * 60 * 60;
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ret += (time_t) minute * 60;
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ret += (time_t) second * 1;
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gmtime_r(&ret, tm);
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return ret;
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}
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int sub_leap_seconds(time_t* ptr)
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{
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time_t t = *ptr;
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time_t next = 0;
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time_t offset = 0;
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for ( int year = 1970; true; year++ )
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{
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for ( int month = 0; month < 12; month++ )
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{
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time_t seconds = days_in_month(year, month) * 24 * 60 * 60;
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time_t leap = get_leap_second_maybe(year, month);
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next += seconds;
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if ( leap == UNKNOWN )
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{
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*ptr = t - offset;
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return t < next - 1 ? 1 : -1;
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}
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next += leap;
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if ( t < next )
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{
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*ptr = t - offset;
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return 0 < leap && t + 1 == next ? 0 : 1;
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}
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offset += leap;
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}
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}
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}
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int add_leap_seconds(time_t* ptr)
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{
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time_t t = *ptr;
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time_t next = 0;
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time_t offset = 0;
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for ( int year = 1970; true; year++ )
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{
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for ( int month = 0; month < 12; month++ )
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{
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time_t seconds = days_in_month(year, month) * 24 * 60 * 60;
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time_t leap = get_leap_second_maybe(year, month);
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next += seconds;
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if ( leap == UNKNOWN )
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{
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*ptr = t + offset;
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return t < next - 1 ? 1 : -1;
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}
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if ( t < next )
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{
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*ptr = t + offset;
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return leap < 0 && t + 1 == next ? 0 : 1;
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}
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offset += leap;
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}
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}
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}
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