#include "stdio.h"
  //9780000000000
  //9790000000000

  int main(int argc, char
      const * argv[]) {
      //Constant
      const long DIVISOR = 10000000000; //10 zero's
      const int LOWER_BOUND = 100;
      const int UPPER_BOUND = 999;
      const int FIRST_VALID_PREFIX = 978;
      const int SECOND_VALID_PREFIX = 979;
      const int NUMBER_OF_DIGITS = 13;
      const long FORMULA_DIVISOR = 1000000000000; //13 zero's
      const int VALID = 0;
      const int INVALID_LESS_THAN_13 = 1;
      const int INVALID_MORE_THAN_13 = 2;
      const int INVALID_PREFIX = 3;
      const int INVALID_FORMULA = 4;
      // Read the number
      while (1) {
          int parseResult = 0;
          long isbn; //store input
          printf("Enter an ISBN: ");
          scanf("%ld", & isbn);

          if (isbn < 0) {
              //Negative input - exit program
              printf("Program exits. Goodbye!!! \n");
              return 0;
          }

          //Divide isbn by divisor constant
          int result = isbn / DIVISOR;
          if (result < LOWER_BOUND) {
              parseResult = INVALID_LESS_THAN_13;
              //printf("Invalid ISBN. Not enough 13 digits.");
          } else if (result > UPPER_BOUND) {
              parseResult = INVALID_MORE_THAN_13;
              //printf("Invalid ISBN. More than 13 digits.");
          }

          if (result == FIRST_VALID_PREFIX || result == SECOND_VALID_PREFIX) {
              int result = 0; //store formula sum of digit
              long divisor = FORMULA_DIVISOR;
              for (int i = 0; i < NUMBER_OF_DIGITS; i++) {
                  //get the digit at index of i
                  int currentDigit = isbn / divisor;
                  // printf("%i \n", currentDigit);
                  if (i % 2 == 0) {
                      //if i is even
                      result += currentDigit;
                  } else {
                      //else i is odd
                      result += 3 * currentDigit;
                  }
                  isbn = isbn - currentDigit * divisor;
                  divisor = divisor / 10;
                  // printf("current isbn is: %ld \n", isbn);
              }
              // printf("reulst is: %i", result);
              if (result % 10 != 0) {
                  parseResult = INVALID_FORMULA;
                  //printf("Invalid ISBN. Failed validation test.");
              }
              else {
                  parseResult = VALID;
              }
          }
          //handle parse result
          if (parseResult == VALID) {
            printf("Valid ISBN. \n");
          } else if (parseResult == INVALID_LESS_THAN_13) {
            printf("Invalid ISBN. Not enough 13 digits. \n");
          } else if (parseResult == INVALID_MORE_THAN_13) {
            printf("Invalid ISBN. More than 13 digits. \n");
          } else if (parseResult == INVALID_PREFIX) {
            printf("Invalid ISBN. Wrong prefix element. \n");
          } else if (parseResult ==INVALID_FORMULA) {
            printf("Invalid ISBN. Failed validation test. \n");
          }
      }
      return 0;
  }