// Functions.h

#ifndef FUNCTIONS_H
#define FUNCTIONS_H

# include <string>
using namespace std;

void WriteHeader();
int GetDigit(int number);
int SumOfOddPlace(string cardNumber);
bool StartsWith(string cardNumber, string substr);
bool IsValid(string cardNumber);
int SumOfDoubleEvenPlace(string cardNumber);
bool ValidateInput(string cardNumber);

#endif

//end of Functions.h

// Functions.cpp

#include "Functions.h"
#include <iostream>

// Course header
void WriteHeader()
{
cout<<"CREDIT CARD NUMBER VALIDATION"<<endl<<endl;
}

// function which returns true if substr is the prefix for cardNumber
bool StartsWith(string cardNumber, string substr)
{
   // checks if substring of cardNumber starting from 1st position to the length of substr string is equal to substr
   if(substr == cardNumber.substr(0,substr.length()))
       return true;
   return false;
}

//function which returns the number if it is single digit else returns the sum of two digits
int GetDigit(int number)
{
   int sum = 0;
   int num = number;
   // loop to calculate the sum of digits in the number
   while(num!=0)
   {
       sum = sum + num%10;
       num=num/10;
   }

   return sum;
}

// function to calculate sum of double of every second digit from right to left
int SumOfDoubleEvenPlace(string cardNumber)
{
   int sum =0;
   // loop from second last digit to start in steps of -2
   for(int i=cardNumber.length()-2;i>-1;i=i-2)
{
       int n=cardNumber[i] - '0'; // convert ith character to int
       sum = sum + GetDigit(2*n);
   }

   return sum;
}

// return the sum of odd place digit in the card number from right to left
int SumOfOddPlace(string cardNumber)
{
   int sum =0;
   // loop from last digit to start in steps of -2
   for(int i=cardNumber.length()-1;i>-1;i=i-2)
{
       int n = cardNumber[i] - '0'; // convert ith character to int
       sum = sum + n;
   }
   return sum;
}

// returns true if the card number was entered with a valid format
bool ValidateInput(string cardNumber)
{
// if length is incorrect
   if(cardNumber.length()< 13 || cardNumber.length() > 16)
return false;
else
{
// loop to check that each character is a digit
for(size_t i=0;i<cardNumber.length();i++)
{
// ith character is not an digit, return false
if(cardNumber[i] < '0' || cardNumber[i] > '9')
return false;
}

return IsValid(cardNumber); // return the result of IsValid
}
}

// function to check if the card number is valid
bool IsValid(string cardNumber)
{
   int sum=0;

   // if prefix is incorrect
   if(!StartsWith(cardNumber,"4") && !StartsWith(cardNumber, "5") && !StartsWith(cardNumber, "37") && !StartsWith(cardNumber, "6"))
return false;

// calculate sum of double of even places
   sum = SumOfDoubleEvenPlace(cardNumber);

   // calculate sum of odd places
   sum += SumOfOddPlace(cardNumber);

   if(sum%10 == 0) // total sum is divisible by 10, valid card number
       return true;
   else // invalid card number
       return false;
}

//end of Functions.cpp

// Driver.cpp : C++ program to validate credit card numbers

# include <iostream>
#include "Functions.h"
using namespace std;


int main()
{
   string cardNumber ;
   char choice;

   WriteHeader();

   // loop that continues till the user wants
   do{
// input the card number
cout<<"Enter a credit card number(without any space or special character):"<<endl;
cin>>cardNumber;

// display if the card number is valid or not
if(ValidateInput(cardNumber))
cout<<cardNumber<<" is valid"<<endl;
else
cout<<cardNumber<<" is invalid"<<endl;
// ask if the user wants to continue
cout<<"Do you want to continue? (y/n) ";
cin>>choice;
   }while(choice == 'y' || choice == 'Y');

   cout<<"Good-bye"<<endl;
   return 0;
}

//end of Driver.cpp