# Luhn in Perl 6

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

1 | ```
exercism fetch perl6 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.

## Resources

Remember to check out the Perl 6 documentation and resources pages for information, tips, and examples if you get stuck.

## Running the tests

There is a test suite and module included with the exercise.
The test suite (a file with the extension `.t`

) will attempt to run routines
from the module (a file with the extension `.pm6`

).
Add/modify routines in the module so that the tests will pass! You can view the
test data by executing the command `perl6 --doc *.t`

(* being the name of the
test suite), and run the test suite for the exercise by executing the command
`prove . --exec=perl6`

in the exercise directory.
You can also add the `-v`

flag e.g. `prove . --exec=perl6 -v`

to display all
tests, including any optional tests marked as 'TODO'.

## 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.