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 */
47void 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(dividend: totalsecs, SECS_PER_DAY, remainder: &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(dividend: u64tmp, divisor: 146097, remainder: &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}
141EXPORT_SYMBOL(time64_to_tm);
142