/*
 * $Id: time_weekday.c,v 1.1 2005-01-26 18:41:39 obarthel Exp $
 *
 * :ts=4
 *
 * Portable ISO 'C' (1994) runtime library for the Amiga computer
 * Copyright (c) 2002-2005 by Olaf Barthel <olsen@sourcery.han.de>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Neither the name of Olaf Barthel nor the names of contributors
 *     may be used to endorse or promote products derived from this
 *     software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/****************************************************************************/

#ifndef _TIME_HEADERS_H
#include "time_headers.h"
#endif /* _TIME_HEADERS_H */

/****************************************************************************/

int
__calculate_weekday(int year,int month,int day)
{
	/* This algorithm comes from <http://klausler.com/new-dayofweek.html>. */
	static const char month_skew[12] =
	{
		0,3,3,6,
		1,4,6,2,
		5,0,3,5
	};

	int sum;

	/* Subtract 1900 from the year. */
	sum = year - 1900;

	/* To that number, add one fourth of itself, discarding
	   any remainder. This sum is the year's skew value. */
	sum += sum / 4;

	/* If the month in question is January or February in a leap
	   year, subtract 1 from the sum. */
	if(month == 1 || month == 2)
	{
		int leap_year_adjust;

		/* Figure out if the year is a leap year. */
		if((year % 4) != 0)
			leap_year_adjust = 0;
		else if ((year % 400) == 0)
			leap_year_adjust = 1;
		else if ((year % 100) == 0)
			leap_year_adjust = 0;
		else
			leap_year_adjust = 1;

		sum -= leap_year_adjust;
	}

	/* Add the month's skew value from the table. */
	sum += day + month_skew[month-1];

	/* The sum is the number of days after Sunday on which the date falls. */
	return(sum % 7);
}
