Implementing rail-fence-cipher

config.json

@@ -31,6 +31,15 @@
       "topics": [
       ]
     },
+    {
+      "uuid": "5bf7612c-427e-484d-bc67-8553fac4f64d",
+      "slug": "rail-fence-cipher",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 1,
+      "topics": [
+      ]
+    },
     {
       "uuid": "6cbb7841-9d96-4528-88e3-6e98a450fd7c",
       "slug": "space-age",

exercises/rail-fence-cipher/README.md

@@ -0,0 +1,119 @@
+# 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.
+
+
+## Getting Started
+
+For installation and learning resources, refer to the
+[exercism help page](http://exercism.io/languages/haskell).
+
+## Running the tests
+
+To run the test suite, execute the following command:
+
+```bash
+stack test
+```
+
+#### If you get an error message like this...
+
+```
+No .cabal file found in directory
+```
+
+You are probably running an old stack version and need
+to upgrade it.
+
+#### Otherwise, if you get an error message like this...
+
+```
+No compiler found, expected minor version match with...
+Try running "stack setup" to install the correct GHC...
+```
+
+Just do as it says and it will download and install
+the correct compiler version:
+
+```bash
+stack setup
+```
+
+## Running *GHCi*
+
+If you want to play with your solution in GHCi, just run the command:
+
+```bash
+stack ghci
+```
+
+## Feedback, Issues, Pull Requests
+
+The [exercism/haskell](https://github.com/exercism/haskell) repository on
+GitHub is the home for all of the Haskell exercises.
+
+If you have feedback about an exercise, or want to help implementing a new
+one, head over there and create an issue.  We'll do our best to help you!
+
+## 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.

exercises/rail-fence-cipher/examples/success-standard/package.yaml

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+name: rail-fence-cipher
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: RailFenceCipher
+  source-dirs: src
+
+tests:
+  test:
+    main: Tests.hs
+    source-dirs: test
+    dependencies:
+      - rail-fence-cipher
+      - hspec

exercises/rail-fence-cipher/examples/success-standard/src/RailFenceCipher.hs

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+module RailFenceCipher (encode, decode) where
+
+import Data.List (sortBy)
+import Data.Function (on)
+
+encode :: Int -> [a] -> [a]
+encode n xs = concat [[a | (a, b) <- zip xs c, b == i] | i <- [1..n]]
+  where c = cycle $ [1..n] ++ [n-1,n-2..2]
+
+decode :: Int -> [a] -> [a]
+decode n xs = map snd $ sortBy (compare `on` fst) zippedL
+  where zippedL = zip (encode n [0..length xs - 1]) xs

exercises/rail-fence-cipher/package.yaml

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+name: rail-fence-cipher
+version: 1.1.0.1
+
+dependencies:
+  - base
+
+library:
+  exposed-modules: RailFenceCipher
+  source-dirs: src
+  dependencies:
+  # - foo       # List here the packages you
+  # - bar       # want to use in your solution.
+
+tests:
+  test:
+    main: Tests.hs
+    source-dirs: test
+    dependencies:
+      - rail-fence-cipher
+      - hspec

exercises/rail-fence-cipher/src/RailFenceCipher.hs

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+module RailFenceCipher (encode, decode) where
+
+encode :: Int -> String -> String
+encode = error "You need to implement this function!"
+
+decode :: Int -> String -> String
+decode = error "You need to implement this function!"

exercises/rail-fence-cipher/stack.yaml

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+resolver: lts-9.11

exercises/rail-fence-cipher/test/Tests.hs

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+{-# LANGUAGE RecordWildCards #-}
+
+import Data.Foldable     (for_)
+import Test.Hspec        (Spec, describe, it, shouldBe)
+import Test.Hspec.Runner (configFastFail, defaultConfig, hspecWith)
+
+import RailFenceCipher (encode, decode)
+
+main :: IO ()
+main = hspecWith defaultConfig {configFastFail = True} specs
+
+specs :: Spec
+specs = do
+          describe "encode" $ for_ encodeCases testE
+          describe "decode" $ for_ decodeCases testD
+  where
+    testE Case{..} = it description $ encode key text `shouldBe` expected
+    testD Case{..} = it description $ decode key text `shouldBe` expected
+
+data Case = Case { description :: String
+                 , key         :: Int
+                 , text        :: String
+                 , expected    :: String
+                 }
+
+encodeCases :: [Case]
+encodeCases = [ Case { description = "encode with two rails"
+                     , key         = 2
+                     , text        = "XOXOXOXOXOXOXOXOXO"
+                     , expected    = "XXXXXXXXXOOOOOOOOO"
+                     }
+              , Case { description = "encode with three rails"
+                     , key         = 3
+                     , text        = "WEAREDISCOVEREDFLEEATONCE"
+                     , expected    = "WECRLTEERDSOEEFEAOCAIVDEN"
+                     }
+              , Case { description = "encode with ending in the middle"
+                     , key         = 4
+                     , text        = "EXERCISES"
+                     , expected    = "ESXIEECSR"
+                     }
+              ]
+
+decodeCases :: [Case]
+decodeCases = [ Case { description = "decode with three rails"
+                     , key         = 3
+                     , text        = "TEITELHDVLSNHDTISEIIEA"
+                     , expected    = "THEDEVILISINTHEDETAILS"
+                     }
+              , Case { description = "decode with five rails"
+                     , key         = 5
+                     , text        = "EIEXMSMESAORIWSCE"
+                     , expected    = "EXERCISMISAWESOME"
+                     }
+              , Case { description = "decode with six rails"
+                     , key         = 6
+                     , text        = "133714114238148966225439541018335470986172518171757571896261"
+                     , expected    = "112358132134558914423337761098715972584418167651094617711286"
+                     }
+              ]