142 lines
4.5 KiB
C
142 lines
4.5 KiB
C
// SPDX-License-Identifier: LGPL-2.0+
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/*
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* Copyright (C) 1993, 1994, 1995, 1996, 1997 Free Software Foundation, Inc.
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* This file is part of the GNU C Library.
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* Contributed by Paul Eggert (eggert@twinsun.com).
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*
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* The GNU C Library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Library General Public License as
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* published by the Free Software Foundation; either version 2 of the
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* License, or (at your option) any later version.
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*
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* The GNU C Library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Library General Public License for more details.
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*
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* You should have received a copy of the GNU Library General Public
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* License along with the GNU C Library; see the file COPYING.LIB. If not,
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* write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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* Boston, MA 02111-1307, USA.
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*/
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/*
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* Converts the calendar time to broken-down time representation
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*
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* 2009-7-14:
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* Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
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* 2021-06-02:
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* Reimplemented by Cassio Neri <cassio.neri@gmail.com>
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*/
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#include <linux/time.h>
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#include <linux/module.h>
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#include <linux/kernel.h>
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#define SECS_PER_HOUR (60 * 60)
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#define SECS_PER_DAY (SECS_PER_HOUR * 24)
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/**
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* time64_to_tm - converts the calendar time to local broken-down time
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*
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* @totalsecs: the number of seconds elapsed since 00:00:00 on January 1, 1970,
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* Coordinated Universal Time (UTC).
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* @offset: offset seconds adding to totalsecs.
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* @result: pointer to struct tm variable to receive broken-down time
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*/
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void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
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{
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u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
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u64 u64tmp, udays, century, year;
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bool is_Jan_or_Feb, is_leap_year;
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long days, rem;
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int remainder;
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days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
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rem = remainder;
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rem += offset;
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while (rem < 0) {
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rem += SECS_PER_DAY;
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--days;
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}
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while (rem >= SECS_PER_DAY) {
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rem -= SECS_PER_DAY;
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++days;
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}
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result->tm_hour = rem / SECS_PER_HOUR;
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rem %= SECS_PER_HOUR;
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result->tm_min = rem / 60;
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result->tm_sec = rem % 60;
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/* January 1, 1970 was a Thursday. */
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result->tm_wday = (4 + days) % 7;
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if (result->tm_wday < 0)
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result->tm_wday += 7;
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/*
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* The following algorithm is, basically, Proposition 6.3 of Neri
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* and Schneider [1]. In a few words: it works on the computational
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* (fictitious) calendar where the year starts in March, month = 2
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* (*), and finishes in February, month = 13. This calendar is
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* mathematically convenient because the day of the year does not
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* depend on whether the year is leap or not. For instance:
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*
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* March 1st 0-th day of the year;
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* ...
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* April 1st 31-st day of the year;
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* ...
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* January 1st 306-th day of the year; (Important!)
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* ...
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* February 28th 364-th day of the year;
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* February 29th 365-th day of the year (if it exists).
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*
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* After having worked out the date in the computational calendar
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* (using just arithmetics) it's easy to convert it to the
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* corresponding date in the Gregorian calendar.
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*
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* [1] "Euclidean Affine Functions and Applications to Calendar
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* Algorithms". https://arxiv.org/abs/2102.06959
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*
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* (*) The numbering of months follows tm more closely and thus,
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* is slightly different from [1].
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*/
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udays = ((u64) days) + 2305843009213814918ULL;
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u64tmp = 4 * udays + 3;
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century = div64_u64_rem(u64tmp, 146097, &u64tmp);
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day_of_century = (u32) (u64tmp / 4);
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u32tmp = 4 * day_of_century + 3;
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u64tmp = 2939745ULL * u32tmp;
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year_of_century = upper_32_bits(u64tmp);
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day_of_year = lower_32_bits(u64tmp) / 2939745 / 4;
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year = 100 * century + year_of_century;
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is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4);
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u32tmp = 2141 * day_of_year + 132377;
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month = u32tmp >> 16;
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day = ((u16) u32tmp) / 2141;
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/*
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* Recall that January 1st is the 306-th day of the year in the
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* computational (not Gregorian) calendar.
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*/
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is_Jan_or_Feb = day_of_year >= 306;
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/* Convert to the Gregorian calendar and adjust to Unix time. */
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year = year + is_Jan_or_Feb - 6313183731940000ULL;
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month = is_Jan_or_Feb ? month - 12 : month;
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day = day + 1;
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day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
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/* Convert to tm's format. */
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result->tm_year = (long) (year - 1900);
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result->tm_mon = (int) month;
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result->tm_mday = (int) day;
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result->tm_yday = (int) day_of_year;
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}
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EXPORT_SYMBOL(time64_to_tm);
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