rotational_chipher without transliteration table

elixir/rotational-cipher/lib/rotational_cipher.ex

@@ -1,42 +1,15 @@
 defmodule RotationalCipher do
-  @moduledoc """
-  Generates a simple Caesar cipher by rotating the alphabet by a given amount.
-  Use for that shifted alphabet generation function and transliteration table based on two alphabets.
-  """
+  @lower ?a..?z
+  @upper ?A..?Z
 
-  @alphabet ?a..?z
-            |> Enum.to_list()
-            |> to_string()
-            |> String.split("", trim: true)
+  defguard is_lower(char) when char in @lower
+  defguard is_upper(char) when char in @upper
+  defguard is_letter(char) when char in @lower or char in @upper
 
-  # Generates a shifted alphabet based on the given shift amount.
-  # If swaped is true, the function will return a map with the index as the key and the character as the value,
-  # otherwise the function will return a map with the character as the key and the index as the value.
-
-  defp alphabet(shift \\ 0, swaped \\ false) when shift < 26 and shift >= 0 do
-    @alphabet
-    |> Enum.with_index(fn char, index ->
-      position = if index + shift >= 26, do: index + shift - 26, else: index + shift
-      if swaped == true, do: {position, char}, else: {char, position}
-    end)
-    |> Enum.into(%{})
-  end
-
-  # Generates a transliteration table based on the given shift amount.
-
-  defp transliterate_table(shift) do
-    transliterated = alphabet(shift, true)
-
-    alphabet()
-    |> Enum.reduce(%{}, fn {char, index}, acc ->
-      Map.merge(acc, %{
-        transliterated[index] => char,
-        String.upcase(transliterated[index]) => String.upcase(char)
-      })
-    end)
-  end
-
-  defp rotate_char(char, table), do: table[char] || char
+  defp rotate_char(char, shift) when is_lower(char) and is_lower(char + shift), do: char + shift
+  defp rotate_char(char, shift) when is_upper(char) and is_upper(char + shift), do: char + shift
+  defp rotate_char(char, shift) when is_letter(char), do: char + shift - 26
+  defp rotate_char(char, _shift) when not is_letter(char), do: char
 
   @doc """
   Given a plaintext and amount to shift by, return a rotated string.
@@ -47,14 +20,10 @@ defmodule RotationalCipher do
   """
 
   @spec rotate(text :: String.t(), shift :: integer) :: String.t()
-  def rotate(text, shift) when shift == 0 or shift == 26, do: text
-
   def rotate(text, shift) do
-    table = transliterate_table(shift)
-
     text
-    |> String.graphemes()
-    |> Enum.map(&rotate_char(&1, table))
-    |> Enum.join()
+    |> to_char_list()
+    |> Enum.map(&rotate_char(&1, shift))
+    |> to_string()
   end
 end