mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 10:49:33 +00:00
![Cassio Neri](/assets/img/avatar_default.png)
The current implementation of time64_to_tm() contains unnecessary loops, branches and look-up tables. The new one uses an arithmetic-based algorithm appeared in [1] and is approximately 3x faster (YMMV). The drawback is that the new code isn't intuitive and contains many 'magic numbers' (not unusual for this type of algorithm). However, [1] justifies all those numbers and, given this function's history, the code is unlikely to need much maintenance, if any at all. Add a KUnit test for it which checks every day in a 160,000 years interval centered at 1970-01-01 against the expected result. [1] Neri, Schneider, "Euclidean Affine Functions and Applications to Calendar Algorithms". https://arxiv.org/abs/2102.06959 Signed-off-by: Cassio Neri <cassio.neri@gmail.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20210622213616.313046-1-cassio.neri@gmail.com
141 lines
4.5 KiB
C
141 lines
4.5 KiB
C
// SPDX-License-Identifier: LGPL-2.0+
|
|
/*
|
|
* Copyright (C) 1993, 1994, 1995, 1996, 1997 Free Software Foundation, Inc.
|
|
* This file is part of the GNU C Library.
|
|
* Contributed by Paul Eggert (eggert@twinsun.com).
|
|
*
|
|
* The GNU C Library is free software; you can redistribute it and/or
|
|
* modify it under the terms of the GNU Library General Public License as
|
|
* published by the Free Software Foundation; either version 2 of the
|
|
* License, or (at your option) any later version.
|
|
*
|
|
* The GNU C Library is distributed in the hope that it will be useful,
|
|
* but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
* Library General Public License for more details.
|
|
*
|
|
* You should have received a copy of the GNU Library General Public
|
|
* License along with the GNU C Library; see the file COPYING.LIB. If not,
|
|
* write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
|
|
* Boston, MA 02111-1307, USA.
|
|
*/
|
|
|
|
/*
|
|
* Converts the calendar time to broken-down time representation
|
|
*
|
|
* 2009-7-14:
|
|
* Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
|
|
* 2021-06-02:
|
|
* Reimplemented by Cassio Neri <cassio.neri@gmail.com>
|
|
*/
|
|
|
|
#include <linux/time.h>
|
|
#include <linux/module.h>
|
|
#include <linux/kernel.h>
|
|
|
|
#define SECS_PER_HOUR (60 * 60)
|
|
#define SECS_PER_DAY (SECS_PER_HOUR * 24)
|
|
|
|
/**
|
|
* time64_to_tm - converts the calendar time to local broken-down time
|
|
*
|
|
* @totalsecs: the number of seconds elapsed since 00:00:00 on January 1, 1970,
|
|
* Coordinated Universal Time (UTC).
|
|
* @offset: offset seconds adding to totalsecs.
|
|
* @result: pointer to struct tm variable to receive broken-down time
|
|
*/
|
|
void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
|
|
{
|
|
u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
|
|
u64 u64tmp, udays, century, year;
|
|
bool is_Jan_or_Feb, is_leap_year;
|
|
long days, rem;
|
|
int remainder;
|
|
|
|
days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
|
|
rem = remainder;
|
|
rem += offset;
|
|
while (rem < 0) {
|
|
rem += SECS_PER_DAY;
|
|
--days;
|
|
}
|
|
while (rem >= SECS_PER_DAY) {
|
|
rem -= SECS_PER_DAY;
|
|
++days;
|
|
}
|
|
|
|
result->tm_hour = rem / SECS_PER_HOUR;
|
|
rem %= SECS_PER_HOUR;
|
|
result->tm_min = rem / 60;
|
|
result->tm_sec = rem % 60;
|
|
|
|
/* January 1, 1970 was a Thursday. */
|
|
result->tm_wday = (4 + days) % 7;
|
|
if (result->tm_wday < 0)
|
|
result->tm_wday += 7;
|
|
|
|
/*
|
|
* The following algorithm is, basically, Proposition 6.3 of Neri
|
|
* and Schneider [1]. In a few words: it works on the computational
|
|
* (fictitious) calendar where the year starts in March, month = 2
|
|
* (*), and finishes in February, month = 13. This calendar is
|
|
* mathematically convenient because the day of the year does not
|
|
* depend on whether the year is leap or not. For instance:
|
|
*
|
|
* March 1st 0-th day of the year;
|
|
* ...
|
|
* April 1st 31-st day of the year;
|
|
* ...
|
|
* January 1st 306-th day of the year; (Important!)
|
|
* ...
|
|
* February 28th 364-th day of the year;
|
|
* February 29th 365-th day of the year (if it exists).
|
|
*
|
|
* After having worked out the date in the computational calendar
|
|
* (using just arithmetics) it's easy to convert it to the
|
|
* corresponding date in the Gregorian calendar.
|
|
*
|
|
* [1] "Euclidean Affine Functions and Applications to Calendar
|
|
* Algorithms". https://arxiv.org/abs/2102.06959
|
|
*
|
|
* (*) The numbering of months follows tm more closely and thus,
|
|
* is slightly different from [1].
|
|
*/
|
|
|
|
udays = ((u64) days) + 2305843009213814918ULL;
|
|
|
|
u64tmp = 4 * udays + 3;
|
|
century = div64_u64_rem(u64tmp, 146097, &u64tmp);
|
|
day_of_century = (u32) (u64tmp / 4);
|
|
|
|
u32tmp = 4 * day_of_century + 3;
|
|
u64tmp = 2939745ULL * u32tmp;
|
|
year_of_century = upper_32_bits(u64tmp);
|
|
day_of_year = lower_32_bits(u64tmp) / 2939745 / 4;
|
|
|
|
year = 100 * century + year_of_century;
|
|
is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4);
|
|
|
|
u32tmp = 2141 * day_of_year + 132377;
|
|
month = u32tmp >> 16;
|
|
day = ((u16) u32tmp) / 2141;
|
|
|
|
/*
|
|
* Recall that January 1st is the 306-th day of the year in the
|
|
* computational (not Gregorian) calendar.
|
|
*/
|
|
is_Jan_or_Feb = day_of_year >= 306;
|
|
|
|
/* Convert to the Gregorian calendar and adjust to Unix time. */
|
|
year = year + is_Jan_or_Feb - 6313183731940000ULL;
|
|
month = is_Jan_or_Feb ? month - 12 : month;
|
|
day = day + 1;
|
|
day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
|
|
|
|
/* Convert to tm's format. */
|
|
result->tm_year = (long) (year - 1900);
|
|
result->tm_mon = (int) month;
|
|
result->tm_mday = (int) day;
|
|
result->tm_yday = (int) day_of_year;
|
|
}
|
|
EXPORT_SYMBOL(time64_to_tm);
|