98 lines
3.0 KiB
Markdown
98 lines
3.0 KiB
Markdown
# Rail Fence Cipher
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Implement encoding and decoding for the rail fence cipher.
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The Rail Fence cipher is a form of transposition cipher that gets its name from
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the way in which it's encoded. It was already used by the ancient Greeks.
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In the Rail Fence cipher, the message is written downwards on successive "rails"
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of an imaginary fence, then moving up when we get to the bottom (like a zig-zag).
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Finally the message is then read off in rows.
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For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE",
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the cipherer writes out:
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. E . R . D . S . O . E . E . F . E . A . O . C .
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. . A . . . I . . . V . . . D . . . E . . . N . .
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```
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Then reads off:
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```text
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WECRLTEERDSOEEFEAOCAIVDEN
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```
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To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.
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```text
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? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
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. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
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. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
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```
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The first row has seven spots that can be filled with "WECRLTE".
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
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. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
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```
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Now the 2nd row takes "ERDSOEEFEAOC".
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. E . R . D . S . O . E . E . F . E . A . O . C .
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. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
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```
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Leaving "AIVDEN" for the last row.
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```text
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W . . . E . . . C . . . R . . . L . . . T . . . E
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. E . R . D . S . O . E . E . F . E . A . O . C .
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. . A . . . I . . . V . . . D . . . E . . . N . .
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```
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If you now read along the zig-zag shape you can read the original message.
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## Getting Started
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Make sure you have read the "Guides" section of the
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[C track][c-track] on the Exercism site. This covers
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the basic information on setting up the development environment expected
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by the exercises.
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## Passing the Tests
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Get the first test compiling, linking and passing by following the [three
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rules of test-driven development][3-tdd-rules].
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The included makefile can be used to create and run the tests using the `test`
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task.
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make test
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Create just the functions you need to satisfy any compiler errors and get the
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test to fail. Then write just enough code to get the test to pass. Once you've
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done that, move onto the next test.
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As you progress through the tests, take the time to refactor your
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implementation for readability and expressiveness and then go on to the next
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test.
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Try to use standard C99 facilities in preference to writing your own
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low-level algorithms or facilities by hand.
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## Source
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Wikipedia [https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher](https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher)
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## Submitting Incomplete Solutions
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It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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[c-track]: https://exercism.io/my/tracks/c
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[3-tdd-rules]: http://butunclebob.com/ArticleS.UncleBob.TheThreeRulesOfTdd
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