Avatar of karpinovsky

karpinovsky's solution

to Rail Fence Cipher in the Ruby Track

Published at Nov 18 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
=begin
Write your code for the 'Rail Fence Cipher' exercise in this file. Make the tests in
`rail_fence_cipher_test.rb` pass.

To get started with TDD, see the `README.md` file in your
`ruby/rail-fence-cipher` directory.
=end

# Elapsed time: ~ 1 hour

class RailFenceCipher
  class << self
    def encode(text, rows)
      new(text, rows).send(:encode)
    end

    def decode(text, rows)
      new(text, rows).send(:decode)
    end
  end

  private

  def initialize(text, rows)
    @letters = text.chars
    @fence = Array.new(rows) { Array.new(text.length) }
    @cycle = Range.new(0, rows.pred).to_a.yield_self { |ary| ary[0..-1] + ary[1..-2].reverse }.cycle
  end

  attr_reader :letters, :fence, :cycle

  def encode
    letters.each_with_object(fence).with_index do |(char, fence), index|
      fence[cycle.next][index] = char
    end.join
  end

  def decode
    0.upto(letters.count.pred) do |i|
      fence[cycle.next][i] = '?'
    end

    cycle.rewind
    letters.each do |ch|
      j = fence[cycle.peek].index('?')

      if j.nil?
        cycle.next 
        j = fence[cycle.peek].index('?')
      end

      fence[cycle.peek][j] = ch
    end

    cycle.rewind
    0.upto(letters.count).map do |i|
      fence[cycle.next][i]
    end.join
  end
end

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?