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to Rail Fence Cipher in the Ruby Track

Published at Aug 30 2019 · 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
class RailFenceCipher
  def self.encode(message, rails_number)
    cipher = new(message, rails_number)

    cipher.unappropriate_rails_number? ? message : cipher.encode
  end

  def self.decode(message, rails_number)
    cipher = new(message, rails_number)

    cipher.unappropriate_rails_number? ? message : cipher.decode
  end

  def initialize(message, rails_number)
    @message           = message
    @rails             = rails_number
    @encoding_iterator = 0
    @decoding_iterator = 0
  end

  def encode
    to_encode = message.chars
    cipher    = Array.new(rails) { [] }
    cipher[encoding_iterator] << to_encode.shift

    until to_encode.empty?
      encode_up_to_down(encoding_iterator, cipher, to_encode)
      encode_down_to_top(encoding_iterator, cipher, to_encode)
    end

    cipher
      .map(&:join)
      .join
  end

  def decode
    cipher = generate_decoding_rails_cipher
    cipher
      .transpose
      .each { |nested| nested.delete('.') }
      .join
  end

  def unappropriate_rails_number?
    rails == 1 || rails > message.size
  end

  protected

  def encode_up_to_down(encoding_iterator, cipher, to_encode)
    (encoding_iterator + 1...rails).each do |idx|
      self.encoding_iterator = idx
      cipher[idx] << to_encode.shift
    end
  end

  def encode_down_to_top(encoding_iterator, cipher, to_encode)
    (0..encoding_iterator - 1).reverse_each do |idx|
      self.encoding_iterator = idx
      cipher[idx] << to_encode.shift
    end
  end

  def generate_decoding_rails_cipher
    to_decode   = message.chars
    cipher      = Array.new(rails) { [] }
    number_dots = 2 * (rails - 2) + 1

    until to_decode.empty?
      flag = false
      count = 0
      cipher[decoding_iterator] << to_decode.shift
      if same_size?(cipher[decoding_iterator], message)
        flag        = true
        number_dots = update_number_of_dots(number_dots, rails)
        update_decoding_iterator
        return cipher if decoding_iterator == rails

        decoding_iterator.times { cipher[decoding_iterator] << '.' }
        count += decoding_iterator
      end

      while count <= number_dots - 1
        cipher[decoding_iterator] << '.' unless flag
        count += 1
        next unless same_size?(cipher[decoding_iterator], message)

        number_dots = update_number_of_dots(number_dots, rails)
        update_decoding_iterator
        return cipher if decoding_iterator == rails

        decoding_iterator.times { cipher[decoding_iterator] << '.' }
        count += decoding_iterator
      end
    end
  end

  def same_size?(object1, object2)
    object1.size == object2.size
  end

  def update_number_of_dots(dots, rails)
    (dots - 2).negative? ? 2 * (rails - 2) + 1 : dots - 2
  end

  def update_decoding_iterator
    self.decoding_iterator += 1
  end

  attr_reader :message, :rails
  attr_accessor :encoding_iterator, :decoding_iterator
end

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fbraza's Reflection

The solution works but is not beautiful.

I need to improve this. Feel free to let any feedbacks