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## to Rotational Cipher in the Elixir Track

Published at Oct 06 2019 · 0 comments
Instructions
Test suite
Solution

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

The Caesar cipher is a simple shift cipher that relies on transposing all the letters in the alphabet using an integer key between `0` and `26`. Using a key of `0` or `26` will always yield the same output due to modular arithmetic. The letter is shifted for as many values as the value of the key.

The general notation for rotational ciphers is `ROT + <key>`. The most commonly used rotational cipher is `ROT13`.

A `ROT13` on the Latin alphabet would be as follows:

``````Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: nopqrstuvwxyzabcdefghijklm
``````

It is stronger than the Atbash cipher because it has 27 possible keys, and 25 usable keys.

Ciphertext is written out in the same formatting as the input including spaces and punctuation.

## Examples

• ROT5 `omg` gives `trl`
• ROT0 `c` gives `c`
• ROT26 `Cool` gives `Cool`
• ROT13 `The quick brown fox jumps over the lazy dog.` gives `Gur dhvpx oebja sbk whzcf bire gur ynml qbt.`
• ROT13 `Gur dhvpx oebja sbk whzcf bire gur ynml qbt.` gives `The quick brown fox jumps over the lazy dog.`

## Running tests

Execute the tests with:

``````\$ mix test
``````

### 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:

``````# @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.

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

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

## Submitting Incomplete Solutions

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

### rotational_cipher_test.exs

``````defmodule RotationalCipherTest do
use ExUnit.Case

# @tag :pending
test "rotate a by 1" do
plaintext = "a"
shift = 1
assert RotationalCipher.rotate(plaintext, shift) == "b"
end

@tag :pending
test "rotate a by 26, same output as input" do
plaintext = "a"
shift = 26
assert RotationalCipher.rotate(plaintext, shift) == "a"
end

@tag :pending
test "rotate a by 0, same output as input" do
plaintext = "a"
shift = 0
assert RotationalCipher.rotate(plaintext, shift) == "a"
end

@tag :pending
test "rotate m by 13" do
plaintext = "m"
shift = 13
assert RotationalCipher.rotate(plaintext, shift) == "z"
end

@tag :pending
test "rotate n by 13 with wrap around alphabet" do
plaintext = "n"
shift = 13
assert RotationalCipher.rotate(plaintext, shift) == "a"
end

@tag :pending
test "rotate capital letters" do
plaintext = "OMG"
shift = 5
assert RotationalCipher.rotate(plaintext, shift) == "TRL"
end

@tag :pending
test "rotate spaces" do
plaintext = "O M G"
shift = 5
assert RotationalCipher.rotate(plaintext, shift) == "T R L"
end

@tag :pending
test "rotate numbers" do
plaintext = "Testing 1 2 3 testing"
shift = 4
assert RotationalCipher.rotate(plaintext, shift) == "Xiwxmrk 1 2 3 xiwxmrk"
end

@tag :pending
test "rotate punctuation" do
plaintext = "Let's eat, Grandma!"
shift = 21
assert RotationalCipher.rotate(plaintext, shift) == "Gzo'n zvo, Bmviyhv!"
end

@tag :pending
test "rotate all letters" do
plaintext = "The quick brown fox jumps over the lazy dog."
shift = 13

assert RotationalCipher.rotate(plaintext, shift) ==
"Gur dhvpx oebja sbk whzcf bire gur ynml qbt."
end
end``````

### test_helper.exs

``````ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)``````
``````defmodule RotationalCipher do
@doc """
Given a plaintext and amount to shift by, return a rotated string.

Example:
iex> RotationalCipher.rotate("Attack at dawn", 13)
"Nggnpx ng qnja"
"""

@lowcase ?a..?z
@upcase ?A..?Z

@spec rotate(text :: String.t(), shift :: integer) :: String.t()
def rotate(text, shift) do
text
|> String.to_charlist
|> Enum.map(& gen(&1, shift))
|> to_string
end

defp gen(n, shift) when n in @lowcase do
n - ?a + shift |> rem(26) |> Kernel.+(?a)
end
defp gen(n, shift) when n in @upcase do
n - ?A + shift |> rem(26) |> Kernel.+(?A)
end
defp gen(n, _shift), do: n

end``````