1 // SPDX-License-Identifier: LGPL-2.0+ 1
2 /*
3 * Copyright (C) 1993, 1994, 1995, 1996, 1997
4 * This file is part of the GNU C Library.
5 * Contributed by Paul Eggert (eggert@twinsun.
6 *
7 * The GNU C Library is free software; you can
8 * modify it under the terms of the GNU Librar
9 * published by the Free Software Foundation;
10 * License, or (at your option) any later vers
11 *
12 * The GNU C Library is distributed in the hop
13 * but WITHOUT ANY WARRANTY; without even the
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR
15 * Library General Public License for more det
16 *
17 * You should have received a copy of the GNU
18 * License along with the GNU C Library; see t
19 * write to the Free Software Foundation, Inc.
20 * Boston, MA 02111-1307, USA.
21 */
22
23 /*
24 * Converts the calendar time to broken-down t
25 *
26 * 2009-7-14:
27 * Moved from glibc-2.6 to kernel by Zhaolei
28 * 2021-06-02:
29 * Reimplemented by Cassio Neri <cassio.neri
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 t
41 *
42 * @totalsecs: the number of seconds elapsed
43 * Coordinated Universal Time (UT
44 * @offset: offset seconds adding to total
45 * @result: pointer to struct tm variable
46 */
47 void time64_to_tm(time64_t totalsecs, int offs
48 {
49 u32 u32tmp, day_of_century, year_of_ce
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
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, basical
79 * and Schneider [1]. In a few words:
80 * (fictitious) calendar where the yea
81 * (*), and finishes in February, mont
82 * mathematically convenient because t
83 * depend on whether the year is leap
84 *
85 * March 1st 0-th day of th
86 * ...
87 * April 1st 31-st day of t
88 * ...
89 * January 1st 306-th day of
90 * ...
91 * February 28th 364-th day of
92 * February 29th 365-th day of
93 *
94 * After having worked out the date in
95 * (using just arithmetics) it's easy
96 * corresponding date in the Gregorian
97 *
98 * [1] "Euclidean Affine Functions and
99 * Algorithms". https://arxiv.org/abs/
100 *
101 * (*) The numbering of months follows
102 * is slightly different from [1].
103 */
104
105 udays = ((u64) days) + 2305843009213
106
107 u64tmp = 4 * udays + 3;
108 century = div64_u64_rem(u64tmp
109 day_of_century = (u32) (u64tmp / 4);
110
111 u32tmp = 4 * day_of_century +
112 u64tmp = 2939745ULL * u32tmp;
113 year_of_century = upper_32_bits(u64tmp
114 day_of_year = lower_32_bits(u64tmp
115
116 year = 100 * century + year
117 is_leap_year = year_of_century ? !(
118
119 u32tmp = 2141 * day_of_year +
120 month = u32tmp >> 16;
121 day = ((u16) u32tmp) / 214
122
123 /*
124 * Recall that January 1st is the 306-
125 * computational (not Gregorian) calen
126 */
127 is_Jan_or_Feb = day_of_year >= 306;
128
129 /* Convert to the Gregorian calendar a
130 year = year + is_Jan_or_Feb
131 month = is_Jan_or_Feb ? mont
132 day = day + 1;
133 day_of_year += is_Jan_or_Feb ? -30
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);
142
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