# 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.

## Running tests

Execute the tests with:

```bash
$ elixir bob_test.exs
```

(Replace `bob_test.exs` with the name of the test file.)

### Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by
commenting out the relevant `@tag :pending` with a `#` symbol.

For example:

```elixir
# @tag :pending
test "shouting" do
  assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
```

Or, you can enable all the tests by commenting out the
`ExUnit.configure` line in the test suite.

```elixir
# ExUnit.configure exclude: :pending, trace: true
```

For more detailed information about the Elixir track, please
see the [help page](http://exercism.io/languages/elixir).

## 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.