# Luhn in Ruby

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

1 | ```
exercism fetch ruby 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.

For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

1 |
```
gem install minitest
``` |

If you would like color output, you can `require 'minitest/pride'`

in
the test file, or note the alternative instruction, below, for running
the test file.

In order to run the test, you can run the test file from the exercise
directory. For example, if the test suite is called
`hello_world_test.rb`

, you can run the following command:

1 |
```
ruby hello_world_test.rb
``` |

To include color from the command line:

1 |
```
ruby -r minitest/pride hello_world_test.rb
``` |

## 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.