# ISBN Verifier

The [ISBN-10 verification process](https://en.wikipedia.org/wiki/International_Standard_Book_Number) is used to validate book identification
numbers. These normally contain dashes and look like: `3-598-21508-8`

## ISBN

The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only). In the case the check character is an X, this represents the value '10'. These may be communicated with or without hyphens, and can be checked for their validity by the following formula:

```
(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0
```

If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.

## Example

Let's take the ISBN-10 `3-598-21508-8`. We plug it in to the formula, and get:
```
(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0
```

Since the result is 0, this proves that our ISBN is valid.

## Task

Given a string the program should check if the provided string is a valid ISBN-10.
Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.

The program should be able to verify ISBN-10 both with and without separating dashes.


## Caveats

Converting from strings to numbers can be tricky in certain languages.
Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10'). For instance `3-598-21507-X` is a valid ISBN-10.

## Bonus tasks

* Generate a valid ISBN-13 from the input ISBN-10 (and maybe verify it again with a derived verifier).

* Generate valid ISBN, maybe even from a given starting ISBN.
## Rust Installation

Refer to the [exercism help page][help-page] for Rust installation and learning
resources.

## Writing the Code

Execute the tests with:

```bash
$ cargo test
```

All but the first test have been ignored.  After you get the first test to
pass, remove the ignore flag (`#[ignore]`) from the next test and get the tests
to pass again.  The test file is located in the `tests` directory.   You can
also remove the ignore flag from all the tests to get them to run all at once
if you wish.

Make sure to read the [Modules](https://doc.rust-lang.org/book/second-edition/ch07-00-modules.html) chapter if you
haven't already, it will help you with organizing your files.

## Feedback, Issues, Pull Requests

The [exercism/rust](https://github.com/exercism/rust) repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!

If you want to know more about Exercism, take a look at the [contribution guide](https://github.com/exercism/docs/blob/master/contributing-to-language-tracks/README.md).

[help-page]: http://exercism.io/languages/rust
[modules]: https://doc.rust-lang.org/book/second-edition/ch07-00-modules.html
[cargo]: https://doc.rust-lang.org/book/second-edition/ch14-00-more-about-cargo.html

## Source

Converting a string into a number and some basic processing utilizing a relatable real world example. [https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation](https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation)

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.