/* datetime.c - Module for common datetime function.  */
/*
 *  GRUB  --  GRand Unified Bootloader
 *  Copyright (C) 2008  Free Software Foundation, Inc.
 *
 *  GRUB is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  GRUB 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 General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <grub/types.h>
#include <grub/datetime.h>
#include <grub/misc.h>
#include <grub/mm.h>

static const char* const grub_weekday_names[] =
{
	"Sunday",
	"Monday",
	"Tuesday",
	"Wednesday",
	"Thursday",
	"Friday",
	"Saturday",
};

int
grub_get_weekday(struct grub_datetime* datetime)
{
	unsigned a, y, m;

	if (datetime->month <= 2)
		a = 1;
	else
		a = 0;
	y = datetime->year - a;
	m = datetime->month + 12 * a - 2;

	return (datetime->day + y + y / 4 - y / 100 + y / 400 + (31 * m / 12)) % 7;
}

const char*
grub_get_weekday_name(struct grub_datetime* datetime)
{
	return _(grub_weekday_names[grub_get_weekday(datetime)]);
}

#define SECPERMIN 60
#define SECPERHOUR (60*SECPERMIN)
#define SECPERDAY (24*SECPERHOUR)
#define DAYSPERYEAR 365
#define DAYSPER4YEARS (4*DAYSPERYEAR+1)


void
grub_unixtime2datetime(grub_int64_t nix, struct grub_datetime* datetime)
{
	int i;
	grub_uint8_t months[12] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
	/* In the period of validity of unixtime all years divisible by 4
	   are bissextile*/
	   /* Convenience: let's have 3 consecutive non-bissextile years
		  at the beginning of the counting date. So count from 1901. */
	int days_epoch;
	/* Number of days since 1st Januar, 1901.  */
	unsigned days;
	/* Seconds into current day.  */
	unsigned secs_in_day;

	/* Transform C divisions and modulos to mathematical ones */
	if (nix < 0)
		/*
		 * The result of division here shouldn't be larger than GRUB_INT_MAX.
		 * So, it's safe to store the result back in an int.
		 */
		days_epoch = -(grub_divmod64(((grub_int64_t)(SECPERDAY)-nix - 1), SECPERDAY, NULL));
	else
		days_epoch = grub_divmod64(nix, SECPERDAY, NULL);

	secs_in_day = nix - days_epoch * SECPERDAY;
	days = days_epoch + 69 * DAYSPERYEAR + 17;

	datetime->year = 1901 + 4 * (days / DAYSPER4YEARS);
	days %= DAYSPER4YEARS;
	/* On 31st December of bissextile years 365 days from the beginning
	   of the year elapsed but year isn't finished yet */
	if (days / DAYSPERYEAR == 4)
	{
		datetime->year += 3;
		days -= 3 * DAYSPERYEAR;
	}
	else
	{
		datetime->year += days / DAYSPERYEAR;
		days %= DAYSPERYEAR;
	}
	for (i = 0; i < 12
		&& days >= (i == 1 && datetime->year % 4 == 0
			? 29U : months[i]); i++)
		days -= (i == 1 && datetime->year % 4 == 0
			? 29U : months[i]);
	datetime->month = i + 1;
	datetime->day = 1 + days;
	datetime->hour = (secs_in_day / SECPERHOUR);
	secs_in_day %= SECPERHOUR;
	datetime->minute = secs_in_day / SECPERMIN;
	datetime->second = secs_in_day % SECPERMIN;
}
