Solution for 5

5/5.hs

@@ -0,0 +1,44 @@
+module Main where
+
+import Data.Char
+import Data.Foldable (toList)
+import Data.Function (on)
+import Data.List (minimumBy)
+import qualified Data.Sequence as S
+import Data.Sequence ((><), (|>))
+
+xor :: Bool -> Bool -> Bool
+xor True False = True
+xor False True = True
+xor _ _ = False
+
+isReactive :: Char -> Char -> Bool
+isReactive x y = toLower x == toLower y && (isUpper x `xor` isUpper y)
+
+simplify :: S.Seq Char -> S.Seq Char -> S.Seq Char
+simplify acc s
+  | S.null s        = acc
+  | S.length s == 1 = acc >< s
+  | S.length s >= 2  =
+      let [x,y] = toList (S.take 2 s)
+      in if isReactive x y 
+         then simplify acc (S.drop 2 s)
+         else simplify (acc |> x) (S.drop 1 s)
+
+collapse :: S.Seq Char -> Int
+collapse s =
+  let simplified = simplify S.empty s
+  in if S.length simplified == S.length s
+     then S.length s
+     else collapse simplified
+
+betterCollapse :: S.Seq Char -> (Char, Int)
+betterCollapse s =
+  minimumBy (compare `on` snd)
+  . map (\c -> (c, collapse $ S.filter (\c' -> toLower c' /= toLower c) s))
+  $ ['a' .. 'z']
+
+main = do
+  input <- S.fromList . filter (/= '\n') <$> getContents
+  putStrLn $ "Size = " ++ show (collapse input)
+  putStrLn $ "Better Size = " ++ show (betterCollapse input)