ruby - rail-fence-cipher

ruby/rail-fence-cipher/README.md

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+# Rail Fence Cipher
+
+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:
+
+```text
+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:
+
+```text
+WECRLTEERDSOEEFEAOCAIVDEN
+```
+
+To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.
+
+```text
+? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+The first row has seven spots that can be filled with "WECRLTE".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Now the 2nd row takes "ERDSOEEFEAOC".
+
+```text
+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.
+
+```text
+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](http://exercism.io/languages/ruby/resources).
+
+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](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.

ruby/rail-fence-cipher/rail_fence_cipher.rb

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+# RailFenceCipher class
+class RailFenceCipher
+  def self.encode(message, rails)
+    rows = Array.new(rails, '')
+    pattern(rails, message.length).each.with_index do |row, char|
+      rows[row] += message[char]
+    end
+    rows.join
+  end
+
+  def self.decode(message, rails)
+    chars = message.chars
+    counts = get_counts(rails, message.length)
+    rows = Array.new(rails) { |row| chars.shift(counts[row]) }
+    pattern(rails, message.length).map { |i| rows[i].shift }.join
+  end
+
+  def self.pattern(rails, size)
+    zigzag = 0.upto(rails - 1).to_a + (rails - 2).downto(1).to_a
+    zigzag.cycle.take(size)
+  end
+
+  def self.get_counts(rails, size)
+    Array.new(rails) { |row| pattern(rails, size).count { |i| i == row } }
+  end
+end

ruby/rail-fence-cipher/rail_fence_cipher_test.rb

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+require 'minitest/autorun'
+require_relative 'rail_fence_cipher'
+
+class RailFenceCipherTest < Minitest::Test
+  def test_encode_with_empty_string
+    # skip
+    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