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); 142