#include <stdio.h>
#include <stdlib.h>
#define RANGE_LIMIT 10000

/*
 *  Program to list ranged numbers from an input file.
 *  
 *  Comments
 *    1. For each entry in v_infile, we need to check that it is not larger than the previous by more than the RANGE_LIMIT.
 *    2. Output is formatted without decimal places (same as input) although it is stored in a double variable.
 *    3. Input is obtained using fgets, which returns a string, so needs casting to a double.
 *    4. Comments added through out the code, to make it easier to read!
 */
int main()
{
   FILE *v_infile, *v_outfile;
   char v_first_s[100], v_this_s[100];
   double v_first, v_this, v_last;

   v_infile=fopen("data","r");

   if (v_infile)
   {
      /*
       * If I managed to open the file for reading,
       * grab the first line into string: v_first_s
       * and check it was not an empty file
       */
      if(fgets(v_first_s,sizeof(v_first_s),v_infile) == NULL)
      {
         printf("No data in file!\n");
         fclose(v_infile);
         return(3);
      }

      /*
       * Input file was not empty so open the output file for writing!
       */
      v_outfile=fopen("output","w");

      if (v_outfile)
      {
        /*
	 * If I can write to the output file,
	 * convert the string: v_first_s to a double: v_first
	 * then copy the double: v_first to another double: v_last
	 */
         v_first = (double)atof(v_first_s);
         v_last = v_first;

         while (fgets(v_this_s,sizeof(v_this_s),v_infile)!=NULL)
         {
	    /*
	     * For each entry in the input file,
	     * grab the line into string: v_this_s
	     * and convert it to a double: v_this
	     */
	    v_this = (double)atof(v_this_s);
            if((v_this - v_first) > RANGE_LIMIT)
            {
	       /*
	        * if the value of double: v_this
		* is greater than the value of double: v_first
		* by more than RANGE_LIMIT
		* Add a line to the output file and
		* start a new range.
		*/
               fprintf(v_outfile, "Range: %10.0f to %10.0f\n", v_first, v_last);
               v_first = v_this;
            }
	    /*
	     * End of this loop change the value of double: v_last
	     */
            v_last = v_this;
         }

	 /*
	  * Following line catches the last piece of data
	  * and passes it to the output file!
	  */
         fprintf(v_outfile, "Range: %10.0f to %10.0f\n", v_first, v_last);

	 /*
	  * Finished writing to output file, close it!
	  */
	 fclose(v_outfile);
      } else {
         /*
	  * Could not open output file for writing!
	  * Print error message and close program.
	  */
         printf("Unable to write to output file!\n");
	 return(4);
      }

      /*
       * Finished reading from input file, close it!
       */
      fclose(v_infile);
   } else {
      /*
       * Could not open input file for reading!
       * Print error message and close program.
       */
      printf("Unable to open input file!\n");
      return(2);
   }
   
   /*
    * Program completed without any errors!
    */
   return(0);
}