//A program to calculate the no of days between 2 dates in format yyyy-mm-dd
//Takes 2 command line args- initial date and final date

//Copyright (C) 2013  Aaditya Bagga  aaditya_gnulinux@zoho.com

/*This program 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
  any later version.

  This program is distributed 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 this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "to_int.h"

int d=0;        //to store the output,ie, difference in days

//Make a structure for the date
struct date
{
	int year;
	int month;
	int day;
}d1,d2;

//Make a macro for checking leap year
#define is_leap(year) (((year%4==0)&&((year%100)!=0)) || ((year%400)==0))

//Make a macro for checking elapsed days
#define check_elapsed_days(days) (days)

//Protoypes for functions
void check_months(int,int,int,int,int); 
void diff_years(int,int);

int main(int argc, char* argv[])
{
        if(argc != 3 ) {
                printf("Invalid number of arguments\n");
                exit(1);
        }
	
	//Extract the year,month,day components from the date

	char yi[5],yf[5],mi[3],mf[3],di[3],df[3];	//size+1 for null character at the end
	
	strncpy (yi, argv[1], 4);
	strncpy (mi, argv[1]+5, 2);
	strncpy (di, argv[1]+8, 3);
	
	strncpy (yf, argv[2], 4);
        strncpy (mf, argv[2]+5, 2);
        strncpy (df, argv[2]+8, 3);

	yi[4]='\0';	//insert null character at the end
	mi[2]='\0';	
	yf[4]='\0';     //insert null character at the end
        mf[2]='\0';
	

	//Convert the string date components to int and assign them to the structure elements

	d1.year=to_int(yi);
        d1.month=to_int(mi);
        d1.day=to_int(di);
        d2.year=to_int(yf);
        d2.month=to_int(mf);
	d2.day=to_int(df);

        
	//store difference b/w the date elements
        int dy=d2.year-d1.year;
        int dm=d2.month-d1.month;
        int dd=d2.day-d1.day;


	//*Now the date part


	//int flag=0;	//to check for wrong dates: currently the program is non-interactive so its not being implemented yet


	//Procedure:find diff in year -> find diff in month -> find diff in days

	//diff in year: find diff in year -> diff in month -> diff in days
	//diff in months -> find diff in months -> diff in days	

	if(dy==0)	//Year same
	{
		//Check difference in months
		if(dm==0)	//Month same
		{
			d=dd;	//output=diff in days
			//done
		}
		else
		{
			check_months(d1.year,d1.month,d2.month,d1.day,d2.day);
		}
	}
	else 		//There is Difference in Years
	{
		//Check if there is diff in years = 1 year, or more
		if(dy==1)
		{
			//Only diff in months need to be checked
			
				check_months(d1.year,d1.month,12,d1.day,31);
				check_months(d2.year,1,d2.month,0,d2.day);	//0 is used as initial day to compensate for the day being lost due to 2 function calls to check_months
		}
		else
		{
			diff_years(d1.year,d2.year);	//Check diff in years
		}
	}

	//Print the output
	printf(" %d day(s)\n",d);
	exit(0);

}

int check_days(int year,int month)	//find the no of days in a month
{
	int leap=is_leap(year);		//check if its leap year
	
	switch(month)
	{
		case 1:case 3: case 5:case 7: case 8: case 10: case 12:
			return 31;

		case 4: case 6: case 9:case 11:
			return 30;

		case 2:
		if(leap)
			return 29;
		else
			return 28;

		default: return 0;
	}
}

int check_remaining_days(int year,int month,int day)
{
	int days_in_month=check_days(year,month);
	
	return(days_in_month-day);
}

void check_months(int yr,int mi,int mf,int di,int df)
{
	//This function has been made in such a way that mf>=mi, if the opposite is true, display a message to the effect.
	if(mi>mf)
	{
		printf("Did you enter the date correctly? Its like first earlier date, then later date.\n");
		exit(1);
	}

	if(mf==1)
	{
		//if mf=1,ie,final month is january, then the days calculated as days in current month which is done at the end
	}
	else
	{
		d=d+check_remaining_days(yr,mi,di);	//Add the days in current month
	}
	
	//Days in current month added, move on to the next months

	mi=mi+1;	//Increment the month as days in current month have been added

	while(mi<mf)
	{
		d=d+check_days(yr,mi);	//add the days of the month
		mi=mi+1;	//now increment to check for next month
	}

	//Days in in-between months added; now add days in final month

	if(mi==13)
	{
		//In this case initial month was 12,ie,december,and the days are calculated at the top as remaining days
	}
	else
	{
		d=d+check_elapsed_days(df);	//add the days from the final month
	}
}

void diff_years(int yi,int yf)
{
	int dy=yf-yi;	//Difference in years
	
	while(dy>1)
	{
		if (is_leap((yi+1)))	//Check if next year is leap or not
			d=d+366;	//Add the no of days in the year
		else
			d=d+365;	//Add the no of days in the year

		yi=yi+1;	//Increment the year to  check for next year
		dy=yf-yi;	//Recalculate the difference
	}
	//done with the difference in years

	//now need to check the difference b/w months
	
		check_months(yi,d1.month,12,d1.day,31);
		check_months(yf,1,d2.month,0,d2.day);	//0 is used as initial day to compensate for the day being lost due to 2 function calls to check_months
}

gcc -lm -o dur dur.c

./dur 2013-11-28 2013-11-29
 1 day(s)

gcc -o dur dur.c
/tmp/cc1dOfR2.o: In function `to_int':
dur.c:(.text+0xa8): undefined reference to `pow'
collect2: error: ld returned 1 exit status

void    main    (int argc, char *argv [])

/*        exit on a failure        */
exit (EXIT_FAILURE);
/*        exit successfully        */
exit (EXIT_SUCCESS);

        /*      open the input file             */
        if ((in = fopen (argv [optind], "r")) == (FILE *) NULL) {
                (void) fprintf (stderr,
                     "%s:\tcan't open %s\n",
                     argv [0], argv [optind]);
                 exit (EXIT_FAILURE);
        }

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

static	time_t	julday	(int, int, int);

void	main	(void)
{
	int	ldd, lmm, lyy;
	int	udd, umm, uyy;

	(void) fprintf (stdout, "Enter Lower Date (MM/DD/YYYY): ");
	(void) scanf ("%d/%d/%d", &lmm, &ldd, &lyy);
	(void) fprintf (stdout, "Enter Upper Date (MM/DD/YYYY): ");
	(void) scanf ("%d/%d/%d", &umm, &udd, &uyy);
	(void) fprintf (stdout, "Number of Days: %ld\n",
	    julday (umm, udd, uyy) - julday (lmm, ldd, lyy));
}

/*	Gregorian Calendar adopted 15 Oct 1582		*/
#define	IGREG	(15+31L*(10+12L*1582))

static	time_t	julday	(int mm, int id, int iyyy)
{
	time_t	jul;
	int	ja, jy, jm;

	if (iyyy == 0) {
		(void) fprintf (stderr, "julday:\tthere is no year zero\n");
		exit (1);
	}
	if (iyyy < 0)
		++iyyy;
	if (mm > 2) {
		jy = iyyy;
		jm = mm + 1;
	} else {
		jy = iyyy - 1;
		jm = mm + 13;
	}
	jul = (time_t) (floor (365.25 * jy) +
	    floor (30.6001 * jm) + id + 1720995);
	if (id + 31L * (mm + 12L * iyyy) >= IGREG) {
		ja = 0.01 * jy;
		jul += 2 - ja + (int) (0.25 * ja);
	}
	return (jul);
}
#undef	IGREG

cc -o ndays ndays.c -lm

ndays
Enter Lower Date (MM/DD/YYYY): 01/01/1901
Enter Upper Date (MM/DD/YYYY): 11/28/2013
Number of Days: 41239

gcc -ansi -pedantic [...]

 ./ndays 
Enter Lower Date (MM/DD/YYYY): 01/01/2013
Enter Upper Date (MM/DD/YYYY): 29/11/2013
Number of Days: 865

 ./ndays 
Enter Lower Date (MM/DD/YYYY): 01/01/2013
Enter Upper Date (MM/DD/YYYY): 11/29/2013
Number of Days: 332

dur 1901-01-01 `date +%F`
 41241 day(s)