caesar_cypher.rb

class CaesarCypher

  # https://en.wikipedia.org/wiki/Letter_frequency
  CHAR_DISTRIBUTION = {'a'=>8.167,'b'=>1.492,'c'=>2.782,'d'=>4.253,'e'=>12.702,'f'=>2.228,'g'=>2.015,'h'=>6.094,'i'=>6.966,'j'=>0.153,'k'=>0.772,'l'=>4.025,'m'=>2.406,'n'=>6.749,'o'=>7.507,'p'=>1.929,'q'=>0.095,'r'=>5.987,'s'=>6.327,'t'=>9.056,'u'=>2.758,'v'=>0.978,'w'=>2.361,'x'=>0.150,'y'=>1.974,'z'=>0.074}
  IDEAL_ORDER = Hash[CHAR_DISTRIBUTION.sort_by { |_, v| -v }].keys
  THRESHOLD_PERCENTAGE = 10

  def encode(message, shift)
    message.chars.map do |char|
      if char =~ /[a-z]/
        encode_char char, shift, 97
      elsif char =~ /[A-Z]/
        encode_char char, shift, 65
      else
        char
      end
    end.join('')
  end

  def decode(message, shift)
    encode(message, -shift)
  end

  def decode_guess(message)
    guesses = {}

    25.times do |shift|
      test_decode = decode message, shift
      letters = letters_order_by_count(test_decode)
      guesses[shift] = similarity_index(letters)
    end

    decode message, guesses.max_by{|k,v| v}[0]
  end

  private

  def encode_char(char, shift, ascii_diff)
    ((char.ord - ascii_diff + shift) % 26 + ascii_diff).chr
  end

  def letters_order_by_count(message)
    letters = count_letters(message)
    total = letters.values.inject(0, :+).to_f

    letters.each do |letter, count|
      letters[letter] = count / total * 100
    end

    Hash[letters.sort_by { |_, v| -v }].keys
  end

  def count_letters(message)
    letters_only = message.downcase.gsub /[^a-zA-Z]/, ''
    Hash[letters_only.split('').group_by{|x| x}.map{|k,v| [k,v.count] }]
  end

  # IDEAL_ORDER = ["e", "t", "a", "o", "i", "n", "s", "h", "r", "d", "l", "c", "u", "m", "w", "f", "g", "y", "p", "b", "v", "k", "j", "x", "q", "z"]
  # letters     = ["e", "n", "a", "t", "i", "o", "s", "r", "d", "l", "h", "c", "u", "p", "m", "y", "g", "w", "f", "b", "v", "k", "x", "j", "q"]
  def similarity_index(letters)
    matches = 0
    letters.each_with_index do |char, i|
      position_percentage = letters.find_index(char) / 26.0 * 100
      ideal_position_percentage = IDEAL_ORDER.find_index(char) / 26.0 * 100

      if (ideal_position_percentage - position_percentage).abs < THRESHOLD_PERCENTAGE
        matches += 1
      end
    end
    matches
  end

end

caesar_cypher =  CaesarCypher.new

original = "汉语 Aa - Village did removed enjoyed explain nor ham saw calling talking. Securing as informed declared or margaret. Joy horrible moreover man feelings own shy. Request norland neither mistake for yet. Between the for morning assured country believe. On even feet time have an no at. Relation so in confined smallest children unpacked delicate. Why sir end believe uncivil respect. Always get adieus nature day course for common. My little garret repair to desire he esteem.
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encoded = caesar_cypher.encode original, 3

puts "Original: #{original}"
puts '-----'
puts "Encoded: #{encoded}"
puts '-----'
puts "Decoded: #{caesar_cypher.decode encoded, 3}"
puts '-----'
puts "Guessed: #{caesar_cypher.decode_guess encoded}"