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# asiviero's solution

## to Rail Fence Cipher in the Ruby Track

Published at Jun 09 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``````
``````=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
class RailFenceCipher

def self.encode(message, rails)
return message if trivial_case?(message, rails)
rows = Array.new(rails) {[]}

message.chars.each_slice(2*(rails-1)) do |values|
#p values
rows[0] << values[0]
1.upto(rails-1) {|i| rows[i] << values[i]}
(rails-1).downto(2) {|i| rows[i-1] << values[2*rails-1 - i]}
end
rows.join
end

def self.decode(message, rails)
return message if trivial_case?(message, rails)

deciphered = '?' * message.length
cloned = message.clone
max_row_index = rails-1

#first line
base = 0
cloned.each_char do |char|
deciphered[base] = char
base += 2*max_row_index
break if base >= message.length
end
# remove from values
cloned[0...(deciphered.tr("?", "").length)] = ''

# Middle rows, who have symmetrical values
1.upto(max_row_index-1) do |row_index|
row_base = row_index

while cloned.length do
# left-side
deciphered[row_base] = cloned[0]
cloned[0] = ''
# symmetrical value
symmetrical_index = row_base+(max_row_index-row_index)*2
if symmetrical_index < message.length
deciphered[symmetrical_index] = cloned[0]
cloned[0] = ''
end

# step jump
row_base += 2*max_row_index
break if row_base >= message.length
end
end

# last row
base = 0
cloned.each_char do |char|
break if base+max_row_index >= message.length
deciphered[base+max_row_index] = char
base += 2*max_row_index
end

deciphered
end

def self.trivial_case?(message, rails)
rails == 1 || message.empty? || message.length < rails
end
end``````