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to Rotational Cipher in the Elixir Track

Published at Aug 19 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.

Source

Wikipedia https://en.wikipedia.org/wiki/Caesar_cipher

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
  @alphabet_length 26

  defguardp is_lowercase?(char) when char in ?a..?z
  defguardp is_uppercase?(char) when char in ?A..?Z

  @doc """
  Given a plaintext and amount to shift by, return a rotated string.

  Example:
  iex> RotationalCipher.rotate("Attack at dawn", 13)
  "Nggnpx ng qnja"
  """
  @spec rotate(text :: String.t(), shift :: integer) :: String.t()
  def rotate(text, shift) do
    text
    |> String.to_charlist()
    |> Enum.map(&rotate_char(&1, shift))
    |> List.to_string()
  end

  defp rotate_char(char, shift) when is_lowercase?(char) do
    rotate_char(?a, char, shift)
  end

  defp rotate_char(char, shift) when is_uppercase?(char) do
    rotate_char(?A, char, shift)
  end

  defp rotate_char(char, _shift), do: char

  defp rotate_char(base, char, shift) do
    rem(char - base + shift, @alphabet_length) + base
  end
end

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