{-# LANGUAGE
  GADTs
  #-}

module Data.Trie.Pred.Disjoint.Tail
  ( NDPTrie (..)
  , lookup
  , merge
  , areDisjoint
  ) where

import Prelude hiding (lookup)
import Data.List.NonEmpty hiding (map)
import Data.List.NonEmpty as NE hiding (map)



data NDPTrie p t x where
  NDMore :: t
         -> Maybe x
         -> [NDPTrie p t x]
         -> NDPTrie p t x
  NDPred :: p
         -> (t -> Maybe r)
         -> Maybe (r -> x)
         -> [NDPTrie p t (r -> x)]
         -> NDPTrie p t x


-- | Overwrites when similar, leaves untouched when not
merge :: (Eq p, Eq t) => NDPTrie p t x -> NDPTrie p t x -> NDPTrie p t x
merge xx@(NDMore t mx xs) yy@(NDMore p my ys)
  | t == p = NDMore p my $ foldr go [] $ xs ++ ys
  | otherwise = xx
  where
    go :: (Eq p, Eq t) => NDPTrie p t x -> [NDPTrie p t x] -> [NDPTrie p t x]
    go a [] = [a]
    go a (b:bs) | areDisjoint a b =       a : b : bs
                | otherwise       = (merge a b) : bs
merge xx@(NDPred t q mrx xrs) yy@(NDPred p w mry yrs)
  | t == p = yy
  | otherwise = xx
merge xx@(NDMore t mx xs) yy@(NDPred p w mrx xrs) = yy
merge xx@(NDPred t q mrx xrs) yy@(NDMore p my ys) = yy


areDisjoint :: (Eq p, Eq t) => NDPTrie p t x -> NDPTrie p t x -> Bool
areDisjoint (NDMore t _ _)    (NDMore p _ _)    = t == p
areDisjoint (NDPred t _ _ _)  (NDPred p _ _ _)  = t == p
areDisjoint _ _ = True


lookup :: Eq t => NonEmpty t -> NDPTrie p t x -> Maybe x
lookup (t:|ts) (NDMore t' mx xs)
  | t == t' = case ts of
    [] -> mx
    _  -> getFirst $ map (lookup $ NE.fromList ts) xs
  | otherwise = Nothing
lookup (t:|ts) (NDPred _ p mrx xrs) =
  p t >>=
    \r -> case ts of
      [] -> ($ r) <$> mrx
      _  -> ($ r) <$> (getFirst $ map (lookup $ NE.fromList ts) xrs)


getFirst :: [Maybe a] -> Maybe a
getFirst [] = Nothing
getFirst (Nothing:xs) = getFirst xs
getFirst (Just x :xs) = Just x