//-------------------------------------------------------------------
//	secs2day.cpp
//
//	This program implements an algorithm for displaying the date
//	in San Francisco after the prescribed number of seconds have 
//	elapsed since the epoch-date occurred in Greenwich, England.
//	(The number of seconds is entered as a command-line argument
//	in order to facilitate automated testing of this algorithm.)
//	The standard UTC Epoch began at midnight on January 1, 1970,
//	and San Francisco is 8 hours behind Greenwich Mean Time.
//
//	  compile-and-link using:  $ g++ secs2day.cpp -o secs2day
//
//	programmer: ALLAN CRUSE
//	written on: 23 FEB 2007
//-------------------------------------------------------------------

#include <stdlib.h>	// for atoi()
#include <string.h>	// for strncpy()
#include <stdio.h>	// for printf()

#define HOURS_PER_DAY	  	24
#define MINUTES_PER_HOUR  	60
#define SECONDS_PER_MINUTE	60
#define	MONTHS_PER_YEAR		12
#define DAYS_IN_USUAL_YEAR	365
#define	DAYS_IN_LEAP_YEAR	366
#define UTC_EPOCH_YEAR		1970
#define SECONDS_PER_HOUR  (SECONDS_PER_MINUTE * MINUTES_PER_HOUR)
#define SECONDS_PER_DAY   (SECONDS_PER_HOUR * HOURS_PER_DAY)
#define DAYS_IN_4YR_CYCLE (DAYS_IN_USUAL_YEAR * 3 + DAYS_IN_LEAP_YEAR) 
#define MONTHS_IN_CYCLE	  (MONTHS_PER_YEAR * 4)

char  monthlist[ ] = "JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC "\
		     "JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC "\
		     "JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC "\
		     "JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC ";

int  monthdays[ ] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
		      31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
		      31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31,
		      31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

int main( int argc, char **argv )
{
	if ( argc == 1 ) { printf( "argument is missing\n" ); return 1; }
	int	seconds = atoi( argv[1] );

	// adjust hours for timezone-delay, then calculate elapsed days
	int	hours_since_epoch_GMT = seconds / SECONDS_PER_HOUR; 
	int	hours_since_epoch_PST = hours_since_epoch_GMT - 8;
	int	days_since_epoch = hours_since_epoch_PST / HOURS_PER_DAY;

	// analyze the elapsed days in terms of repetitive 4-year cycles 
	int	cycles_since_epoch = days_since_epoch / DAYS_IN_4YR_CYCLE;
	int	days_into_cycle = days_since_epoch % DAYS_IN_4YR_CYCLE;

	// now this loop will determine the day, month, and year
	int	year = UTC_EPOCH_YEAR + (cycles_since_epoch * 4);
	int	day = days_into_cycle;
	int	month = 0;
	while ( month < MONTHS_IN_CYCLE )
		{
		if ( day < monthdays[ month ] ) break;
		day -= monthdays[ month ];
		++month;
		}
	year += month / MONTHS_PER_YEAR;

	// extract the month's name from the monthlist
	char	monthname[ 4 ] = {0};
	strncpy( monthname, &monthlist[ month*4 ], 3 ); 

	// adjust the day-number for "off-by-one" counting
	printf( " %02d %3s %4d \n", day+1, monthname, year );
}