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to Rail Fence Cipher in the Ruby Track

Published at Jul 17 2020 · 0 comments
Instructions
Test suite
Solution

Implement encoding and decoding for the rail fence cipher.

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Then reads off:

WECRLTEERDSOEEFEAOCAIVDEN

To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.

? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The first row has seven spots that can be filled with "WECRLTE".

W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Now the 2nd row takes "ERDSOEEFEAOC".

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Leaving "AIVDEN" for the last row.

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

If you now read along the zig-zag shape you can read the original message.


For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby rail_fence_cipher_test.rb

To include color from the command line:

ruby -r minitest/pride rail_fence_cipher_test.rb

Source

Wikipedia https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

rail_fence_cipher_test.rb

require 'minitest/autorun'
require_relative 'rail_fence_cipher'

class RailFenceCipherTest < Minitest::Test
  def test_encode_with_empty_string
    assert_equal '', RailFenceCipher.encode('', 4)
  end

  def test_encode_with_one_rail
    skip
    assert_equal 'One rail, only one rail',
                 RailFenceCipher.encode('One rail, only one rail', 1)
  end

  def test_encode_with_two_rails
    skip
    assert_equal 'XXXXXXXXXOOOOOOOOO',
                 RailFenceCipher.encode('XOXOXOXOXOXOXOXOXO', 2)
  end

  def test_encode_with_three_rails
    skip
    assert_equal 'WECRLTEERDSOEEFEAOCAIVDEN',
                 RailFenceCipher.encode('WEAREDISCOVEREDFLEEATONCE', 3)
  end

  def test_encode_with_ending_in_the_middle
    skip
    assert_equal 'ESXIEECSR', RailFenceCipher.encode('EXERCISES', 4)
  end

  def test_encode_with_less_letters_than_rails
    skip
    assert_equal 'More rails than letters',
                 RailFenceCipher.encode('More rails than letters', 24)
  end

  def test_decode_with_empty_string
    skip
    assert_equal '', RailFenceCipher.decode('', 4)
  end

  def test_decode_with_one_rail
    skip
    assert_equal 'ABCDEFGHIJKLMNOP',
                 RailFenceCipher.decode('ABCDEFGHIJKLMNOP', 1)
  end

  def test_decode_with_two_rails
    skip
    assert_equal 'XOXOXOXOXOXOXOXOXO',
                 RailFenceCipher.decode('XXXXXXXXXOOOOOOOOO', 2)
  end

  def test_decode_with_three_rails
    skip
    assert_equal 'THEDEVILISINTHEDETAILS',
                 RailFenceCipher.decode('TEITELHDVLSNHDTISEIIEA', 3)
  end
end
# frozen_string_literal: true

class RailFenceCipher
  TEMPLATE_EMPTY_CHAR = '.'
  TEMPLATE_FILL_CHAR = '?'

  def self.encode(a_message, this_amount_of_rails)
    RailFenceCipherEncoder.new(a_message, this_amount_of_rails).encode
  end

  def self.decode(a_message, this_amount_of_rails)
    RailFenceCipherDecoder.new(a_message, this_amount_of_rails).decode
  end

  private

  def initialize(message, amount_of_rails)
    @message = message
    @message_chars = extract_chars(message)
    @message_length = @message_chars.length
    @amount_of_rails = amount_of_rails
  end

  attr_reader :message, :message_chars, :message_length, :amount_of_rails

  def extract_chars(a_string)
    a_string.gsub(/\W/, '').chars
  end

  # alternating_sequence of length 10 and max 3 -> 0,1,2,3,2,1,0,1,2
  def alternating_sequence
    max = amount_of_rails - 1
    direction = 1
    current = 0

    result = [current]

    (message_length - 1).times do
      current += direction
      direction *= -1 if current.zero? || current == max
      result << current
    end

    result
  end

  def generate_template(alphabet: template_default_alphabet)
    rails = amount_of_rails.times.collect { message_length.times.collect { TEMPLATE_EMPTY_CHAR } }
    insertion_sequence = alternating_sequence

    message_length.times do |column|
      rails[insertion_sequence.shift][column] = alphabet.next
    end

    rails
  end

  def template_default_alphabet
    (1..Float::INFINITY).lazy.map { TEMPLATE_FILL_CHAR }
  end

  def valid_number_of_rails?
    amount_of_rails > 1 && message_length > amount_of_rails
  end

  def join_template(template)
    template.map(&:join).join
  end

  def remove_empty_char(a_string)
    a_string.gsub(TEMPLATE_EMPTY_CHAR, '')
  end
end

class RailFenceCipherEncoder < RailFenceCipher
  def encode
    return message unless valid_number_of_rails?

    rails = generate_template(alphabet: message_chars.each)
    remove_empty_char(join_template(rails))
  end
end

class RailFenceCipherDecoder < RailFenceCipher
  def decode
    return message unless valid_number_of_rails?

    template = generate_template
    chars =  message_chars

    rails = template.map do |row|
      row.map do |column|
        column == TEMPLATE_FILL_CHAR && chars.shift || column
      end
    end

    remove_empty_char(join_template(rails.transpose))
  end
end

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