// SPDX-License-Identifier: GPL-2.0
/*
 * rtc and date/time utility functions
 *
 * Copyright (C) 2005-06 Tower Technologies
 * Author: Alessandro Zummo <a.zummo@towertech.it>
 *
 * based on arch/arm/common/rtctime.c and other bits
 *
 * Author: Cassio Neri <cassio.neri@gmail.com> (rtc_time64_to_tm)
 */

#include <linux/export.h>
#include <linux/rtc.h>

static const unsigned char rtc_days_in_month[] = {
        31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
};

static const unsigned short rtc_ydays[2][13] = {
        /* Normal years */
        { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 },
        /* Leap years */
        { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 }
};

/*
 * The number of days in the month.
 */
int rtc_month_days(unsigned int month, unsigned int year)
{
        return rtc_days_in_month[month] + (is_leap_year(year) && month == 1);
}
EXPORT_SYMBOL(rtc_month_days);

/*
 * The number of days since January 1. (0 to 365)
 */
int rtc_year_days(unsigned int day, unsigned int month, unsigned int year)
{
        return rtc_ydays[is_leap_year(year)][month] + day - 1;
}
EXPORT_SYMBOL(rtc_year_days);

/**
 * rtc_time64_to_tm - converts time64_t to rtc_time.
 *
 * @time:       The number of seconds since 01-01-1970 00:00:00.
 *              Works for values since at least 1900
 * @tm:         Pointer to the struct rtc_time.
 */
void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
{
        int secs;

        u64 u64tmp;
        u32 u32tmp, udays, century, day_of_century, year_of_century, year,
                day_of_year, month, day;
        bool is_Jan_or_Feb, is_leap_year;

        /*
         * The time represented by `time` is given in seconds since 1970-01-01
         * (UTC). As the division done below might misbehave for negative
         * values, we convert it to seconds since 0000-03-01 and then assume it
         * will be non-negative.
         * Below we do 4 * udays + 3 which should fit into a 32 bit unsigned
         * variable. So the latest date this algorithm works for is 1073741823
         * days after 0000-03-01 which is in the year 2939805.
         */
        time += (u64)719468 * 86400;

        udays = div_s64_rem(time, 86400, &secs);

        /*
         * day of the week, 0000-03-01 was a Wednesday (in the proleptic
         * Gregorian calendar)
         */
        tm->tm_wday = (udays + 3) % 7;

        /*
         * The following algorithm is, basically, Figure 12 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] Neri C, Schneider L. Euclidean affine functions and their
         *     application to calendar algorithms. Softw Pract Exper.
         *     2023;53(4):937-970. doi: 10.1002/spe.3172
         *     https://doi.org/10.1002/spe.3172
         *
         * (*) The numbering of months follows rtc_time more closely and
         * thus, is slightly different from [1].
         */

        u32tmp          = 4 * udays + 3;
        century         = u32tmp / 146097;
        day_of_century  = u32tmp % 146097 / 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 != 0 ?
                year_of_century % 4 == 0 : century % 4 == 0;

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

        /*
         * Recall that January 01 is the 306-th day of the year in the
         * computational (not Gregorian) calendar.
         */
        is_Jan_or_Feb   = day_of_year >= 306;

        /* Converts to the Gregorian calendar. */
        year            = year + is_Jan_or_Feb;
        month           = is_Jan_or_Feb ? month - 12 : month;
        day             = day + 1;

        day_of_year     = is_Jan_or_Feb ?
                day_of_year - 306 : day_of_year + 31 + 28 + is_leap_year;

        /* Converts to rtc_time's format. */
        tm->tm_year     = (int) (year - 1900);
        tm->tm_mon      = (int) month;
        tm->tm_mday     = (int) day;
        tm->tm_yday     = (int) day_of_year + 1;

        tm->tm_hour = secs / 3600;
        secs -= tm->tm_hour * 3600;
        tm->tm_min = secs / 60;
        tm->tm_sec = secs - tm->tm_min * 60;

        tm->tm_isdst = 0;
}
EXPORT_SYMBOL(rtc_time64_to_tm);

/*
 * Does the rtc_time represent a valid date/time?
 */
int rtc_valid_tm(struct rtc_time *tm)
{
        if (tm->tm_year < 70 ||
            tm->tm_year > (INT_MAX - 1900) ||
            ((unsigned int)tm->tm_mon) >= 12 ||
            tm->tm_mday < 1 ||
            tm->tm_mday > rtc_month_days(tm->tm_mon,
                                         ((unsigned int)tm->tm_year + 1900)) ||
            ((unsigned int)tm->tm_hour) >= 24 ||
            ((unsigned int)tm->tm_min) >= 60 ||
            ((unsigned int)tm->tm_sec) >= 60)
                return -EINVAL;

        return 0;
}
EXPORT_SYMBOL(rtc_valid_tm);

/*
 * rtc_tm_to_time64 - Converts rtc_time to time64_t.
 * Convert Gregorian date to seconds since 01-01-1970 00:00:00.
 */
time64_t rtc_tm_to_time64(struct rtc_time *tm)
{
        return mktime64(((unsigned int)tm->tm_year + 1900), tm->tm_mon + 1,
                        tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec);
}
EXPORT_SYMBOL(rtc_tm_to_time64);

/*
 * Convert rtc_time to ktime
 */
ktime_t rtc_tm_to_ktime(struct rtc_time tm)
{
        return ktime_set(rtc_tm_to_time64(&tm), 0);
}
EXPORT_SYMBOL_GPL(rtc_tm_to_ktime);

/*
 * Convert ktime to rtc_time
 */
struct rtc_time rtc_ktime_to_tm(ktime_t kt)
{
        struct timespec64 ts;
        struct rtc_time ret;

        ts = ktime_to_timespec64(kt);
        /* Round up any ns */
        if (ts.tv_nsec)
                ts.tv_sec++;
        rtc_time64_to_tm(ts.tv_sec, &ret);
        return ret;
}
EXPORT_SYMBOL_GPL(rtc_ktime_to_tm);