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# joaofnds's solution

## 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 . .
``````

``````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
``````

## 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

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``````

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?