ROT13 in Ruby

rot13.rb

require 'byebug'

module Rot13
  extend self

  LOWERS = ("a".."z").to_a
  UPPERS = ("A".."Z").to_a

  def cypher(val)
    val.to_s.chars.map do |ch|
      cypher_char(ch, LOWERS)   ||
        cypher_char(ch, UPPERS) ||
        ch
    end.join
  end
  alias_method :decypher, :cypher

private

  def cypher_char(val, array)
    return unless index = array.index(val)
    array.reverse[index-13]
  end

end