My Ruby implementation of a simple Caesar cipher

caesar_cipher.rb

# Caesar cipher module
#
# USAGE:
# =====
#   ```ruby
#   my_var = Plaintext.new 'TOO MANY SECRETS'
#   my_var.rotn # => "GBB ZNAL FRPERGF"
#   my_var # => "TOO MANY SECRETS"
#   ```
#   There is a bang method to make changes permanent:
#   ```ruby
#   my_var = Plaintext.new 'TOO MANY SECRETS'
#   my_var.rotn! # => "GBB ZNAL FRPERGF"
#   my_var # => "GBB ZNAL FRPERGF"
#   ```
#
# Parameters:
# -----------
# 1. **amount**: The number of positions to jump inside the alphabet.<br>
#    (defaults to `13`)
#   ```ruby
#   Plaintext.new('Caesar').rotn     # => "Pnrfne"
#   Plaintext.new('Caesar').rotn(3)  # => "Fdhvdu"
#   Plaintext.new('Caesar').rotn(0)  # => "Caesar"
#   Plaintext.new('Caesar').rotn(-3) # => "Zxbpxo"
#   ```
#
# #### INFO:
#  - https://en.wikipedia.org/wiki/Caesar_cipher
#  - https://es.wikipedia.org/wiki/ROT13
#
module CaesarCipher
  def rotn!(amount = 13)
    lut_hash = [*('a'..'z')].zip([*('a'..'z')].rotate(amount)).to_h
    lut_hash.merge! [*('A'..'Z')].zip([*('A'..'Z')].rotate(amount)).to_h
    lut_hash.merge! [*('0'..'9')].zip([*('0'..'9')].rotate(amount)).to_h

    hash_sub! lut_hash
  end

  def rotn(*args)
    dup.rotn!(*args)
  end

  private

  def hash_sub!(hash)
    chars.each_with_index do |char, i|
      self[i] = hash.fetch char, char
    end
    self
  end
end

class Plaintext < String
  include CaesarCipher
end