# Luhn

Given a number determine whether or not it is valid per the Luhn formula.

The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is
a simple checksum formula used to validate a variety of identification
numbers, such as credit card numbers and Canadian Social Insurance
Numbers.

The task is to check if a given string is valid.

Validating a Number
------

Strings of length 1 or less are not valid. Spaces are allowed in the input,
but they should be stripped before checking. All other non-digit characters
are disallowed.

## Example 1: valid credit card number

```
4539 1488 0343 6467
```

The first step of the Luhn algorithm is to double every second digit,
starting from the right. We will be doubling

```
4_3_ 1_8_ 0_4_ 6_6_
```

If doubling the number results in a number greater than 9 then subtract 9
from the product. The results of our doubling:

```
8569 2478 0383 3437
```

Then sum all of the digits:

```
8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80
```

If the sum is evenly divisible by 10, then the number is valid. This number is valid!

## Example 2: invalid credit card number

```
8273 1232 7352 0569
```

Double the second digits, starting from the right

```
7253 2262 5312 0539
```

Sum the digits

```
7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
```

57 is not evenly divisible by 10, so this number is not valid.

* * * *

To run the code in this exercise, you will only need to have [CommandBox CLI installed](https://ortus.gitbooks.io/commandbox-documentation/content/setup/installation.html).  This binary runs CFML code from the command line.

To run the tests, `cd` into the exercise folder and run the following:

```bash 
box task run TestRunner
# Or start up a test watcher that will rerun when files change
box task run TestRunner --:watcher
```

The tests leverage a library called TestBox which supports xUnit and BDD style of testing.  All test suites will be written in the [BDD style](https://testbox.ortusbooks.com/content/primers/bdd/specs.html) which uses closures to define test specs.  You won't need to worry about installing TestBox.  The CLI test runner will take care of that for you.  You just need to be connected to the internet the first time you run it.  You can read more about it here:

[https://testbox.ortusbooks.com/content/](https://testbox.ortusbooks.com/content/)
## Source

The Luhn Algorithm on Wikipedia [http://en.wikipedia.org/wiki/Luhn_algorithm](http://en.wikipedia.org/wiki/Luhn_algorithm)

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.