credit card checking puzzle

README.md

Checking Credit Cards

Problem

Before submitting a credit card to a payment gateway it's important that we run some sanity checks on the number.

A common check that is performed upfront is to validate the card type based on the starting digits and length of card number. The main patterns that we care about are as follows:

+============+=============+===============+
| Card Type  | Begins With | Number Length |
+============+=============+===============+
| AMEX       | 34 or 37    | 15            |
+------------+-------------+---------------+
| Discover   | 6011        | 16            |
+------------+-------------+---------------+
| MasterCard | 51-55       | 16            |
+------------+-------------+---------------+
| Visa       | 4           | 13 or 16      |
+------------+-------------+---------------+

All of these card types also generate numbers such that they can be validated by the Luhn algorithm, so that's the second check systems usually try. The steps are:

1. Starting with the next to last digit and continuing with every other digit going back to the beginning of the card, double the digit
2. Sum all doubled and untouched digits in the number. For digits greater than 9 you will need to split them and sum the independently (i.e. "10", 1 + 0).
3. If that total is a multiple of 10, the number is valid.

Task

Your task is to write a ruby program that accepts credit card numbers. Card numbers must be passed in line by line (one set of numbers per line). The program should print the card in the following format "TYPE: NUMBERS (VALIDITY)".

Input and Output

Given the following credit cards:

4111111111111111
4111111111111
4012888888881881
378282246310005
6011111111111117
5105105105105100
5105 1051 0510 5106
9111111111111111

I would expect the following output:

VISA: 4111111111111111       (valid)
VISA: 4111111111111          (invalid)
VISA: 4012888888881881       (valid)
AMEX: 378282246310005        (valid)
Discover: 6011111111111117   (valid)
MasterCard: 5105105105105100 (valid)
MasterCard: 5105105105105106 (invalid)
Unknown: 9111111111111111    (invalid)

This is a great example. It's commonly used as a "coding" problem for some places to test out your skills