// SPDX-License-Identifier: LGPL-2.0+
/*
 * Copyright (C) 1993, 1994, 1995, 1996, 1997 Free Software Foundation, Inc.
 * This file is part of the GNU C Library.
 * Contributed by Paul Eggert (eggert@twinsun.com).
 *
 * The GNU C Library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * The GNU C Library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with the GNU C Library; see the file COPYING.LIB.  If not,
 * write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

/*
 * Converts the calendar time to broken-down time representation
 *
 * 2009-7-14:
 *   Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
 * 2021-06-02:
 *   Reimplemented by Cassio Neri <cassio.neri@gmail.com>
 */

#include <linux/time.h>
#include <linux/module.h>
#include <linux/kernel.h>

#define SECS_PER_HOUR   (60 * 60)
#define SECS_PER_DAY    (SECS_PER_HOUR * 24)

/**
 * time64_to_tm - converts the calendar time to local broken-down time
 *
 * @totalsecs:  the number of seconds elapsed since 00:00:00 on January 1, 1970,
 *              Coordinated Universal Time (UTC).
 * @offset:     offset seconds adding to totalsecs.
 * @result:     pointer to struct tm variable to receive broken-down time
 */
void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
{
        u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
        u64 u64tmp, udays, century, year;
        bool is_Jan_or_Feb, is_leap_year;
        long days, rem;
        int remainder;

        days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
        rem = remainder;
        rem += offset;
        while (rem < 0) {
                rem += SECS_PER_DAY;
                --days;
        }
        while (rem >= SECS_PER_DAY) {
                rem -= SECS_PER_DAY;
                ++days;
        }

        result->tm_hour = rem / SECS_PER_HOUR;
        rem %= SECS_PER_HOUR;
        result->tm_min = rem / 60;
        result->tm_sec = rem % 60;

        /* January 1, 1970 was a Thursday. */
        result->tm_wday = (4 + days) % 7;
        if (result->tm_wday < 0)
                result->tm_wday += 7;

        /*
         * The following algorithm is, basically, Proposition 6.3 of Neri
         * and Schneider [1]. In a few words: it works on the computational
         * (fictitious) calendar where the year starts in March, month = 2
         * (*), and finishes in February, month = 13. This calendar is
         * mathematically convenient because the day of the year does not
         * depend on whether the year is leap or not. For instance:
         *
         * March 1st            0-th day of the year;
         * ...
         * April 1st            31-st day of the year;
         * ...
         * January 1st          306-th day of the year; (Important!)
         * ...
         * February 28th        364-th day of the year;
         * February 29th        365-th day of the year (if it exists).
         *
         * After having worked out the date in the computational calendar
         * (using just arithmetics) it's easy to convert it to the
         * corresponding date in the Gregorian calendar.
         *
         * [1] "Euclidean Affine Functions and Applications to Calendar
         * Algorithms". https://arxiv.org/abs/2102.06959
         *
         * (*) The numbering of months follows tm more closely and thus,
         * is slightly different from [1].
         */

        udays   = ((u64) days) + 2305843009213814918ULL;

        u64tmp          = 4 * udays + 3;
        century         = div64_u64_rem(u64tmp, 146097, &u64tmp);
        day_of_century  = (u32) (u64tmp / 4);

        u32tmp          = 4 * day_of_century + 3;
        u64tmp          = 2939745ULL * u32tmp;
        year_of_century = upper_32_bits(u64tmp);
        day_of_year     = lower_32_bits(u64tmp) / 2939745 / 4;

        year            = 100 * century + year_of_century;
        is_leap_year    = year_of_century ? !(year_of_century % 4) : !(century % 4);

        u32tmp          = 2141 * day_of_year + 132377;
        month           = u32tmp >> 16;
        day             = ((u16) u32tmp) / 2141;

        /*
         * Recall that January 1st is the 306-th day of the year in the
         * computational (not Gregorian) calendar.
         */
        is_Jan_or_Feb   = day_of_year >= 306;

        /* Convert to the Gregorian calendar and adjust to Unix time. */
        year            = year + is_Jan_or_Feb - 6313183731940000ULL;
        month           = is_Jan_or_Feb ? month - 12 : month;
        day             = day + 1;
        day_of_year     += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;

        /* Convert to tm's format. */
        result->tm_year = (long) (year - 1900);
        result->tm_mon  = (int) month;
        result->tm_mday = (int) day;
        result->tm_yday = (int) day_of_year;
}
EXPORT_SYMBOL(time64_to_tm);