61 lines
1.8 KiB
Elixir
61 lines
1.8 KiB
Elixir
defmodule RotationalCipher do
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@moduledoc """
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Generates a simple Caesar cipher by rotating the alphabet by a given amount.
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Use for that shifted alphabet generation function and transliteration table based on two alphabets.
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"""
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@alphabet ?a..?z
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|> Enum.to_list()
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|> to_string()
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|> String.split("", trim: true)
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# Generates a shifted alphabet based on the given shift amount.
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# If swaped is true, the function will return a map with the index as the key and the character as the value,
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# otherwise the function will return a map with the character as the key and the index as the value.
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defp alphabet(shift \\ 0, swaped \\ false) when shift < 26 and shift >= 0 do
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@alphabet
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|> Enum.with_index(fn char, index ->
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position = if index + shift >= 26, do: index + shift - 26, else: index + shift
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if swaped == true, do: {position, char}, else: {char, position}
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end)
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|> Enum.into(%{})
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end
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# Generates a transliteration table based on the given shift amount.
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defp transliterate_table(shift) do
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transliterated = alphabet(shift, true)
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alphabet()
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|> Enum.reduce(%{}, fn {char, index}, acc ->
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Map.merge(acc, %{
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transliterated[index] => char,
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String.upcase(transliterated[index]) => String.upcase(char)
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})
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end)
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end
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defp rotate_char(char, table), do: table[char] || char
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@doc """
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Given a plaintext and amount to shift by, return a rotated string.
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Example:
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iex> RotationalCipher.rotate("Attack at dawn", 13)
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"Nggnpx ng qnja"
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"""
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@spec rotate(text :: String.t(), shift :: integer) :: String.t()
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def rotate(text, shift) when shift == 0 or shift == 26, do: text
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def rotate(text, shift) do
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table = transliterate_table(shift)
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text
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|> String.graphemes()
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|> Enum.map(&rotate_char(&1, table))
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|> Enum.join()
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end
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end
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