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(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 | } |
141 | EXPORT_SYMBOL(time64_to_tm); |
142 | |