`1` ```exercism fetch elixir rail-fence-cipher ```

# Rail Fence Cipher

Implement encoding and decoding for the rail fence cipher.

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

 ```1 2 3``` ```W . . . E . . . C . . . R . . . L . . . T . . . E . E . R . D . S . O . E . E . F . E . A . O . C . . . A . . . I . . . V . . . D . . . E . . . N . . ```

 `1` ```WECRLTEERDSOEEFEAOCAIVDEN ```

To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.

 ```1 2 3``` ```? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . ```

The first row has seven spots that can be filled with "WECRLTE".

 ```1 2 3``` ```W . . . E . . . C . . . R . . . L . . . T . . . E . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . ```

Now the 2nd row takes "ERDSOEEFEAOC".

 ```1 2 3``` ```W . . . E . . . C . . . R . . . L . . . T . . . E . E . R . D . S . O . E . E . F . E . A . O . C . . . ? . . . ? . . . ? . . . ? . . . ? . . . ? . . ```

Leaving "AIVDEN" for the last row.

 ```1 2 3``` ```W . . . E . . . C . . . R . . . L . . . T . . . E . E . R . D . S . O . E . E . F . E . A . O . C . . . A . . . I . . . V . . . D . . . E . . . N . . ```

If you now read along the zig-zag shape you can read the original message.

## Running tests

Execute the tests with:

 `1` ```\$ elixir bob_test.exs ```

(Replace `bob_test.exs` with the name of the test file.)

### Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.

For example:

 ```1 2 3 4``` ```# @tag :pending test "shouting" do assert Bob.hey("WATCH OUT!") == "Whoa, chill out!" end ```

Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the test suite.

 `1` ```# ExUnit.configure exclude: :pending, trace: true ```

For more detailed information about the Elixir track, please see the help page.

## Submitting Incomplete Solutions

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