kernel/time/timeconv.c

   1 // SPDX-License-Identifier: LGPL-2.0+
   2 /*
   3  * Copyright (C) 1993, 1994, 1995, 1996, 1997 Free Software Foundation, Inc.
   4  * This file is part of the GNU C Library.
   5  * Contributed by Paul Eggert (eggert@twinsun.com).
   6  *
   7  * The GNU C Library is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU Library General Public License as
   9  * published by the Free Software Foundation; either version 2 of the
  10  * License, or (at your option) any later version.
  11  *
  12  * The GNU C Library is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15  * Library General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Library General Public
  18  * License along with the GNU C Library; see the file COPYING.LIB.  If not,
  19  * write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  20  * Boston, MA 02111-1307, USA.
  21  */
  22
  23 /*
  24  * Converts the calendar time to broken-down time representation
  25  *
  26  * 2009-7-14:
  27  *   Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
  28  * 2021-06-02:
  29  *   Reimplemented by Cassio Neri <cassio.neri@gmail.com>
  30  */
  31
  32 #include <linux/time.h>
  33 #include <linux/module.h>
  34 #include <linux/kernel.h>
  35
  36 #define SECS_PER_HOUR   (60 * 60)
  37 #define SECS_PER_DAY    (SECS_PER_HOUR * 24)
  38
  39 /**
  40  * time64_to_tm - converts the calendar time to local broken-down time
  41  *
  42  * @totalsecs:  the number of seconds elapsed since 00:00:00 on January 1, 1970,
  43  *              Coordinated Universal Time (UTC).
  44  * @offset:     offset seconds adding to totalsecs.
  45  * @result:     pointer to struct tm variable to receive broken-down time
  46  */
  47 void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
  48 {
  49         u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
  50         u64 u64tmp, udays, century, year;
  51         bool is_Jan_or_Feb, is_leap_year;
  52         long days, rem;
  53         int remainder;
  54
  55         days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
  56         rem = remainder;
  57         rem += offset;
  58         while (rem < 0) {
  59                 rem += SECS_PER_DAY;
  60                 --days;
  61         }
  62         while (rem >= SECS_PER_DAY) {
  63                 rem -= SECS_PER_DAY;
  64                 ++days;
  65         }
  66
  67         result->tm_hour = rem / SECS_PER_HOUR;
  68         rem %= SECS_PER_HOUR;
  69         result->tm_min = rem / 60;
  70         result->tm_sec = rem % 60;
  71
  72         /* January 1, 1970 was a Thursday. */
  73         result->tm_wday = (4 + days) % 7;
  74         if (result->tm_wday < 0)
  75                 result->tm_wday += 7;
  76
  77         /*
  78          * The following algorithm is, basically, Proposition 6.3 of Neri
  79          * and Schneider [1]. In a few words: it works on the computational
  80          * (fictitious) calendar where the year starts in March, month = 2
  81          * (*), and finishes in February, month = 13. This calendar is
  82          * mathematically convenient because the day of the year does not
  83          * depend on whether the year is leap or not. For instance:
  84          *
  85          * March 1st            0-th day of the year;
  86          * ...
  87          * April 1st            31-st day of the year;
  88          * ...
  89          * January 1st          306-th day of the year; (Important!)
  90          * ...
  91          * February 28th        364-th day of the year;
  92          * February 29th        365-th day of the year (if it exists).
  93          *
  94          * After having worked out the date in the computational calendar
  95          * (using just arithmetics) it's easy to convert it to the
  96          * corresponding date in the Gregorian calendar.
  97          *
  98          * [1] "Euclidean Affine Functions and Applications to Calendar
  99          * Algorithms". https://arxiv.org/abs/2102.06959
 100          *
 101          * (*) The numbering of months follows tm more closely and thus,
 102          * is slightly different from [1].
 103          */
 104
 105         udays   = ((u64) days) + 2305843009213814918ULL;
 106
 107         u64tmp          = 4 * udays + 3;
 108         century         = div64_u64_rem(u64tmp, 146097, &u64tmp);
 109         day_of_century  = (u32) (u64tmp / 4);
 110
 111         u32tmp          = 4 * day_of_century + 3;
 112         u64tmp          = 2939745ULL * u32tmp;
 113         year_of_century = upper_32_bits(u64tmp);
 114         day_of_year     = lower_32_bits(u64tmp) / 2939745 / 4;
 115
 116         year            = 100 * century + year_of_century;
 117         is_leap_year    = year_of_century ? !(year_of_century % 4) : !(century % 4);
 118
 119         u32tmp          = 2141 * day_of_year + 132377;
 120         month           = u32tmp >> 16;
 121         day             = ((u16) u32tmp) / 2141;
 122
 123         /*
 124          * Recall that January 1st is the 306-th day of the year in the
 125          * computational (not Gregorian) calendar.
 126          */
 127         is_Jan_or_Feb   = day_of_year >= 306;
 128
 129         /* Convert to the Gregorian calendar and adjust to Unix time. */
 130         year            = year + is_Jan_or_Feb - 6313183731940000ULL;
 131         month           = is_Jan_or_Feb ? month - 12 : month;
 132         day             = day + 1;
 133         day_of_year     += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
 134
 135         /* Convert to tm's format. */
 136         result->tm_year = (long) (year - 1900);
 137         result->tm_mon  = (int) month;
 138         result->tm_mday = (int) day;
 139         result->tm_yday = (int) day_of_year;
 140 }
 141 EXPORT_SYMBOL(time64_to_tm);