Day6 with better algorithm

src/Day6.hs

@@ -1,32 +1,35 @@
 module Day6 (
   findEnd,
   findStart,
-  isUnique,
   day6
 ) where
 
-findMarker :: Int -> String -> Maybe Int
-findMarker windowSize = findMarker' 0 ""
-  where
-  findMarker' :: Int -> String -> String -> Maybe Int
-  findMarker' offset window  []
-    | isUnique window && length window == windowSize = Just offset
-    | otherwise = Nothing
-  findMarker' offset window (c:cs)
-    | length window < windowSize = findMarker' (offset + 1) (c:window) cs
-    | not (isUnique window) = findMarker' (offset + 1) (c:init window) cs
-    | isUnique window = Just offset
-    | otherwise = Nothing
+import qualified Data.Sequence as S
+import Data.Sequence (Seq (..), (|>))
 
 findStart :: String -> Maybe Int
-findStart = findMarker 14 
+findStart = findUniqueSubstring 14
 
 findEnd :: String -> Maybe Int
-findEnd = findMarker 4
+findEnd = findUniqueSubstring 4
 
-isUnique :: (Eq a) => [a] -> Bool
-isUnique [] = True
-isUnique (c:cs) = c `notElem` cs && isUnique cs
+findUniqueSubstring :: Int -> String -> Maybe Int
+findUniqueSubstring size input
+ | length input < size = Nothing -- imposible to find a substring because input is too small
+ | otherwise = find 0 S.empty $ S.fromList input
+  where
+    find :: Int -> Seq Char -> Seq Char -> Maybe Int
+    find _ _ Empty = Nothing
+    find off window (r :<| rs)
+      = case S.elemIndexL r window of -- is new element in window?
+        Nothing -> if S.length window == size -1            -- Element not found, but is our window large nough?
+                      then Just nextOff                     -- Large enough, we got a winner!
+                      else find nextOff enlargedWindow rs   -- Not large enough, push new element to the end of the window
+        Just i -> find nextOff  (removeConflict (i + 1)) rs -- continue after clearing window
+      where
+        nextOff = off + 1
+        enlargedWindow = window |> r
+        removeConflict i = S.drop i enlargedWindow
 
 day6 :: IO ()
 day6 = do