# 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

```text
4539 3195 0343 6467
```

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

```text
4_3_ 3_9_ 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:

```text
8569 6195 0383 3437
```

Then sum all of the digits:

```text
8+5+6+9+6+1+9+5+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

```text
8273 1232 7352 0569
```

Double the second digits, starting from the right

```text
7253 2262 5312 0539
```

Sum the digits

```text
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 Raku [documentation](https://docs.raku.org/) and
[resources](https://raku.org/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 `.rakutest`) will attempt to run routines
from the module (a file with the extension `.rakumod`).
Add/modify routines in the module so that the tests will pass! You can view the
test data by executing the command `raku --doc *.rakutest` (\* being the name of the
test suite), and run the test suite for the exercise by executing the command
`prove6 .` in the exercise directory.

## 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.