Optimize day 5

Day5/Day5A.hs

@@ -2,19 +2,18 @@
 -- stack --resolver lts-12.5 script
 
 module Day5A where
+import Data.Bits (xor)
 import Data.Char
 import qualified Data.Foldable as F
 
 invertCase :: Char -> Char
-invertCase c
-  | isUpper c = toLower c
-  | otherwise = toUpper c
+invertCase = chr . xor 0x20 . ord
 
 calcLength :: String -> String -> Int
 calcLength ys (x:y:xs)
-  | x == invertCase y = case length ys of
-      0 -> calcLength ys xs
-      _ -> calcLength (tail ys) (head ys:xs)
+  | x == invertCase y = if null ys
+      then calcLength ys xs
+      else calcLength (tail ys) (head ys:xs)
   | otherwise        = calcLength (x:ys) (y:xs)
 calcLength ys [x]    = calcLength (x:ys) []
 calcLength ys []     = length ys