f.hs

longestSuffix :: Odin.Domain -> DomTable -> Maybe Odin.Domain
longestSuffix (Odin.Domain labels) table = case go (reverse labels) table of
    [] -> Nothing
    xs -> Just (Odin.Domain (reverse xs))
  where
    go []     _                = []
    go (x:xs) (DomTable table) = case M.lookup x table of
      Just sub -> x : go xs sub
      Nothing  -> []

emptyDomTable :: DomTable
emptyDomTable = DomTable M.empty

insert :: Odin.Domain -> DomTable -> DomTable
insert (Odin.Domain labels) table = go (reverse labels) table
  where
    go [] table = table
    go (x:xs) (DomTable table)
      | Just sub <- M.lookup x table
      = DomTable (M.insert x (go xs sub) table)

      | otherwise
      = let chain = foldr (\x t -> DomTable (M.singleton x t)) (DomTable M.empty) xs
         in DomTable (M.insert x chain table)