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.
omg
gives trl
c
gives c
Cool
gives Cool
The quick brown fox jumps over the lazy dog.
gives Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
gives The quick brown fox jumps over the lazy dog.
Execute the tests with:
$ elixir rotational_cipher_test.exs
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
For more detailed information about the Elixir track, please see the help page.
Wikipedia https://en.wikipedia.org/wiki/Caesar_cipher
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
if !System.get_env("EXERCISM_TEST_EXAMPLES") do
Code.load_file("rotational_cipher.exs", __DIR__)
end
ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
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
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"
"""
@spec rotate(text :: String.t(), shift :: integer) :: String.t()
def rotate(text, shift) do
for point <- to_charlist(text), into: "" do
maybe_rotate(point, shift)
end
end
defp maybe_rotate(<<point::utf8>>, shift)
when point in ?A..?Z,
do: <<do_rotate(point, shift, ?A)::utf8>>
defp maybe_rotate(<<point::utf8>>, shift)
when point in ?a..?z,
do: <<do_rotate(point, shift, ?a)::utf8>>
defp maybe_rotate(point, _),
do: point
defp do_rotate(point, shift, start),
do: rem(point - start + shift, 26) + start
end
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