# 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 1488 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_ 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:

```text
8569 2478 0383 3437
```

Then sum all of the digits:

```text
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

```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.

## Getting Started
For installation and learning resources, refer to the
[exercism help page](http://exercism.io/languages/ocaml).

## Installation
To work on the exercises, you will need `Opam` and `Core`. Consult [opam](https://opam.ocaml.org) website for instructions on how to install `opam` for your OS. Once `opam` is installed open a terminal window and run the following command to install core:

```bash
opam install core
```

To run the tests you will need `OUnit`. Install it using `opam`:

```bash
opam install ounit
```

## Running Tests
A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:

```bash
make
```

## Interactive Shell
`utop` is a command line program which allows you to run Ocaml code interactively. The easiest way to install it is via opam:
```bash
opam install utop
```
Consult [utop](https://github.com/diml/utop/blob/master/README.md) for more detail.

## Feedback, Issues, Pull Requests
The [exercism/ocaml](https://github.com/exercism/ocaml) repository on
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If you have feedback about an exercise, or want to help implementing a new
one, head over there and create an issue.  We'll do our best to help you!

## 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.