//-------------------------------------------------------------------
//	firstday.cpp
//
//	This program shows which day of the week is January first in
//	any year from 1582 to 9999 given by a command-line argument.
//
//	   compile-and-link with:  $ g++ firstday.cpp -o firstday
//
//	programmer: ALLAN CRUSE
//	written on: 02 APR 2006
//-------------------------------------------------------------------

#include <stdio.h>	// for printf(), fprintf() 
#include <stdlib.h>	// for exit(), atoi() 

int main( int argc, char **argv )
{
	if ( argc == 1 ) 
		{
		fprintf( stdout, "usage: $ firstday <year> \n" );
		exit(1);
		}

	int	year = atoi( argv[1] );
	if (( year < 1582 )||( year > 9999 ))
		{
		fprintf( stdout, "year must be from 1582 to 9999 \n" );
		exit(1);
		}

	//---------------------------------------------------------------
	// Here is our algorithm for computing which day is January first 
	int	years_since_1 = (year - 1)%400;
	int	leaps_since_1 = years_since_1 / 4  - years_since_1 / 100;
	int	january_first = (1 + years_since_1 + leaps_since_1) % 7;
	//---------------------------------------------------------------

	char	*day[] = { "Sunday", "Monday", "Tuesday", "Wednesday",
				"Thursday", "Friday", "Saturday" };
	printf( "\n\tYear %u begins %s \n\n", year, day[ january_first ] );
}