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141 lines
4.5 KiB
141 lines
4.5 KiB
// 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 ([email protected]). |
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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<[email protected]> |
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* 2021-06-02: |
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* Reimplemented by Cassio Neri <[email protected]> |
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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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