/**********************************************
Demo Two - Use C to read and write files
Steve Dannelly
January 2011

The format for the input file must be
   firstname lastname mm/dd/yyyy
The program converts all dates to Julian (days since 4713BC)
in order to compute days between two dates.

*********************************************/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>

/******************************************************/
// Convert a Gregorian date to Julian date
// formula from http://en.wikipedia.org/wiki/Julian_day
int convert_to_JDN(int month, int day, int year)
{
   int JDN,a,y,m;
   a = (14 - month)/12;
   y = year + 4800 - a;
   m = month + (12 * a) - 3;
   JDN = day + ((153*m - 2)/5) + (365*y) + (y/4) + (y/100) + (y/400) - 32045;
   return JDN;
}
/******************************************************/
// get today's Julian date
int todays_Julian()
{
  int month,day,year;        // today's date
  struct tm *time_info;      // parts of today's date

  // get today's date from the system
  time_t current = time(0);
  time_info = localtime (&current);

  // correct offsets
  year  = time_info->tm_year + 1900;
  month = time_info->tm_mon + 1;
  day   = time_info->tm_mday;

  // convert to a julian number and return that value
  return convert_to_JDN( month, day, year);
}

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

int main ()
{
   FILE *infile, *outfile;
   time_t now;
   int month, day, year;                  // data from file
   char fname[20], lname[20];             // data from file
   char c1, c2;                           // skip slashes in dates
   int birth_JDN, today_JDN, difference;  // dates in Julian

   /***** open input and output files *****/
   infile = fopen ("dates.in", "r");
   if (infile == NULL)
     {
      fprintf(stderr, "Error opening input file\n\n");
      exit (1);
     }
   outfile = fopen ("dates.out", "w");
   if (outfile == NULL)
     {
      fprintf(stderr, "Error opening output file\n\n");
      exit (1);
     }

   // what day is today in Julian?
   today_JDN = todays_Julian();

   // add today's date to top of the output file
   now = time(0);
   fprintf (outfile, "As of %s\n", asctime(localtime(&now)));

   /***** loop through all the dates in the input file  ****/
   while (fscanf (infile, "%s %s %d %c %d %c %d", fname, lname, &month, &c1, &day, &c2, &year) != -1)
      {
       // how long ago was that?
       birth_JDN = convert_to_JDN (month, day, year);
       difference = today_JDN - birth_JDN;
       // write results to outfile
       fprintf(outfile, "%s %s was born %d days ago,", fname, lname, difference);
       fprintf(outfile, " or %.3f years\n", difference/365.25);
      }

   printf("\nfile succefully converted\n\n");
}