Validate_credit_num.cpp

/*
In this Kata, you will implement the Luhn Algorithm, which is used to help validate credit card numbers.

Given a positive integer of up to 16 digits, return true if it is a valid credit card number, and false if it is not.

Here is the algorithm:

Double every other digit, scanning from right to left, starting from the second digit (from the right).

Another way to think about it is: if there are an even number of digits, double every other digit starting with the first; if there are an odd number of digits, double every other digit starting with the second:

1714 ==> [1*, 7, 1*, 4] ==> [2, 7, 2, 4]

12345 ==> [1, 2*, 3, 4*, 5] ==> [1, 4, 3, 8, 5]

891 ==> [8, 9*, 1] ==> [8, 18, 1]
If a resulting number is greater than 9, replace it with the sum of its own digits (which is the same as subtracting 9 from it):

[8, 18*, 1] ==> [8, (1+8), 1] ==> [8, 9, 1]

or:

[8, 18*, 1] ==> [8, (18-9), 1] ==> [8, 9, 1]
Sum all of the final digits:

[8, 9, 1] ==> 8 + 9 + 1 = 18
Finally, take that sum and divide it by 10. If the remainder equals zero, the original credit card number is valid.

  18 (modulus) 10 ==> 8 , which is not equal to 0, so this is not a valid credit card number
*/

class Kata {
  public:
  static int validate(long long int n) {
  
    std::string num (std::to_string(n));
    
    bool ne = num.size() & 1;
    int sum = 0;
    
    for (int i = 0; i < num.size(); ++i) 
      if (ne) {
        sum += (i + 1) & 1 ? num[i] - '0' : ( (num[i] - '0') * 2 > 9 ? (num[i] - '0') * 2 - 9 : (num[i] - '0') * 2);  
      }
      else {
        sum += !((i + 1) & 1) ? num[i] - '0' : ( (num[i] - '0') * 2 > 9 ? (num[i] - '0') * 2 - 9 : (num[i] - '0') * 2);
      }
    
    return !(sum % 10);
  }
};