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(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}
141EXPORT_SYMBOL(time64_to_tm);
142