gistfile1.txt

/*
 * CreditCard.cpp
 *
 *  Created on: May 26, 2015
 *      Author: Zak Taylor
 */

//4388576018410707 - VALID
//4388576018402626 - INVALID
#include <iostream>
#include <cmath>
using namespace std;

static bool isValid(long long number);
static int sumOfDoubleEvenPlace(long long number);
static int sumOfOddPlace(long long number);
static int getDigit(long long number);
static bool prefixMatched(long long number, int d);
static long long getPrefix(long long number, int k);
static int getSize(long d);
static long long number;

int main(){

	cout << "Enter a credit card number as a long integer: " << endl;

	cin >> number;

	cout << number<< " is ";
		if(isValid(number))
			cout <<"valid" << endl;
		else
			cout <<"invalid" << endl;
		return 0;
	}

static bool isValid(long long number){

		int numSize = getSize(number);

		if((numSize > 16) || (numSize < 13)) return false;

	int sum = sumOfDoubleEvenPlace(number) + sumOfOddPlace(number);

		if(sum % 10 != 0) return false;

		if(prefixMatched(number, 4)) return true;
		if(prefixMatched(number, 5)) return true;
		if(prefixMatched(number, 37)) return true;
		if(prefixMatched(number, 6)) return true;

		else return false;
}

static int sumOfDoubleEvenPlace(long long number){
		int sum = 0;

		while(number > 0)
		{
			int digit = ((number % 100) / 10);
			number /= 100;

			sum += getDigit(2 * digit);
		}

		return sum;
	}

static int sumOfOddPlace(long long number)
	{
		int sum = 0;

		while(number > 0)
		{
			int digit = (number % 10);
			number /= 100;

			sum += getDigit(digit);
		}

		return sum;
	}

static int getDigit(long long number)
	{
		return (number / 10) + (number % 10); // Still works the same if it's a single digit number.
	}

static int getSize(long d)
{
	int digitCount = 0;

	while(d > 0){
		d /= 10; // Chop off one digit.
		digitCount++;
	}

	return digitCount;
}


static bool prefixMatched(long long number, int d)
	{
		return getPrefix(number, getSize(d)) == d;
	}

static long long getPrefix(long long number, int k)
{
	int numberSize = getSize(number);
	int decimalMovements = numberSize - k;

	while(decimalMovements-- > 0){
		number /= 10;
	}

	return number;
}