# Luhn in Erlang

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

1 | ```
exercism fetch erlang luhn
``` |

# Luhn

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

The 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

1 |
```
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

1 |
```
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:

1 |
```
8569 2478 0383 3437
``` |

Then sum all of the digits:

1 |
```
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

1 |
```
8273 1232 7352 0569
``` |

Double the second digits, starting from the right

1 |
```
7253 2262 5312 0539
``` |

Sum the digits

1 |
```
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

In order to run the tests, issue the following command from the exercise directory:

For running the tests provided, `rebar3`

is used as it is the official build and
dependency management tool for erlang now. Please refer to the tracks installation
instructions on how to do that.

In order to run the tests, you can issue the following command from the exercise directory.

1 |
```
$ rebar3 eunit
``` |

### Test versioning

Each problem defines a macro `TEST_VERSION`

in the test file and
verifies that the solution defines and exports a function `test_version`

returning that same value.

To make tests pass, add the following to your solution:

The benefit of this is that reviewers can see against which test version an iteration was written if, for example, a previously posted solution does not solve the current problem or passes current tests.

## Questions?

For detailed information about the Erlang track, please refer to the help page on the Exercism site. This covers the basic information on setting up the development environment expected by the exercises.

## Source

The Luhn Algorithm on Wikipedia 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.