Solve 'The Speed of Letters' kata

Author: borisskert

src/SpeedOfLetters.hs

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+{-# LANGUAGE TupleSections #-}
+
+module SpeedOfLetters (speedify) where
+
+-- https://www.codewars.com/kata/5fc7caa854783c002196f2cb/train/haskell
+
+import Data.List (groupBy, sortOn)
+import Data.Maybe (fromJust, fromMaybe, isJust)
+
+speedify :: String -> String
+speedify = makeText . rebuildLetters
+
+makeText :: [(Int, Char)] -> String
+makeText = go 0
+  where
+    go :: Int -> [(Int, Char)] -> String
+    go _ [] = []
+    go index chars@((i, c) : xs)
+      | i == index = c : go (index + 1) xs
+      | i < index = go index xs
+      | otherwise = ' ' : go (index + 1) chars
+
+rebuildLetters :: String -> [(Int, Char)]
+rebuildLetters = map last . groupOn fst . sortOn fst . zipWith (curry run) [0 ..]
+
+run :: (Int, Char) -> (Int, Char)
+run (index, c) = (,c) . (+ index) . lookupLetter $ c
+
+lookupLetter :: Char -> Int
+lookupLetter c
+  | isJust mIndex = fromJust mIndex
+  | otherwise = error "Letter not found"
+  where
+    mIndex = lookup c letterIndices
+
+letterIndices :: [(Char, Int)]
+letterIndices = zip ['A' .. 'Z'] [0 ..]
+
+groupOn :: (Eq b) => (a -> b) -> [a] -> [[a]]
+groupOn get = groupBy (\a b -> get a == get b)

test/SpeedOfLettersSpec.hs

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+module SpeedOfLettersSpec where
+
+import SpeedOfLetters (speedify)
+import Test.HUnit
+import Test.Hspec
+
+spec :: Spec
+spec = do
+  describe "Fixed Tests" $ do
+    it "Works for some example strings" $ do
+      assertEqual (show "AZ") "A                         Z" $
+        speedify "AZ"
+      assertEqual (show "ABC") "A B C" $
+        speedify "ABC"
+      assertEqual (show "ACE") "A  C  E" $
+        speedify "ACE"
+      assertEqual (show "CBA") "  A" $
+        speedify "CBA"
+      assertEqual (show "HELLOWORLD") "     E H    DLL   OLO   R  W" $
+        speedify "HELLOWORLD"