f.hs

findMarker :: Int -> Int -> Int -> BS.ByteString -> Int
findMarker l o n buf = do
  case BS.elemIndexEnd c window of
    Nothing | o <= 0    -> n
            | otherwise -> findMarker l (o - 1) (n + 1) buf
    Just i  -> let o' = max i (o - 1)
               in findMarker l o' (n + 1) buf
  where
    c = BS.index buf (n - 1)
    window = BS.take windowW (BS.drop windowL buf)
    windowW = min l n - 1
    windowL = max (n - l) 0

main = do
  buf <- BS.readFile "day6/input.txt"
  let l = 14
  print (findMarker l 14 1 buf)