Solve 'Circle cipher' kata

Author: borisskert

src/CircleCipher.hs

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+module CircleCipher (encode, decode) where
+
+-- https://www.codewars.com/kata/634d0723075de3f97a9eb604/train/haskell
+
+import Data.List (sortOn)
+
+encode :: [a] -> [a]
+encode xs = take size . concatMap (\(c1, c2) -> [c1, c2]) . zip xs . reverse $ xs
+  where
+    size = length xs
+
+decode :: [a] -> [a]
+decode xs = map snd . sortOn fst . zip indices $ xs
+  where
+    size = length xs
+    indices = encode [0 .. size - 1]

test/CircleCipherSpec.hs

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+module CircleCipherSpec where
+
+import CircleCipher (decode, encode)
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  describe "Fixed Tests" $ do
+    let tests =
+          [ ("codewars", "csordaew"),
+            ("white", "wehti"),
+            ("Assert", "Atsrse"),
+            ("Hello world!", "H!edlllroo w"),
+            ("You have chosen to translate this kata.", "Y.oaut ahka vsei hcth oesteanl stnoa rt")
+          ]
+    it "encodes properly" $ do
+      mapM_ (\(s, s') -> encode s `shouldBe` s') tests
+    it "decodes properly" $ do
+      mapM_ (\(s, s') -> decode s' `shouldBe` s) tests
+
+  describe "Additional Tests" $ do
+    it "encodes ints" $ do
+      encode [0, 1, 2, 3, 4, 5, 6, 7] `shouldBe` [0, 7, 1, 6, 2, 5, 3, 4]
+    it "decodes ints" $ do
+      decode [0, 7, 1, 6, 2, 5, 3, 4] `shouldBe` [0, 1, 2, 3, 4, 5, 6, 7]