content: {-# LANGUAGE ScopedTypeVariables #-}
  {-# LANGUAGE ApplicativeDo #-}
  {-# OPTIONS_GHC -Wall #-}
  
  -- | Like a 'UniqDFM', but maintains equivalence classes of keys sharing the
  -- same entry. See 'UniqSDFM'.
  module GHC.Types.Unique.SDFM (
          -- * Unique-keyed, /shared/, deterministic mappings
          UniqSDFM,
  
          emptyUSDFM,
          lookupUSDFM,
          equateUSDFM, addToUSDFM,
          traverseUSDFM
      ) where
  
  import GHC.Prelude
  
  import GHC.Types.Unique
  import GHC.Types.Unique.DFM
  import GHC.Utils.Outputable
  
  -- | Either @Indirect x@, meaning the value is represented by that of @x@, or
  -- an @Entry@ containing containing the actual value it represents.
  data Shared key ele
    = Indirect !key
    | Entry !ele
  
  -- | A 'UniqDFM' whose domain is /sets/ of 'Unique's, each of which share a
  -- common value of type @ele@.
  -- Every such set (\"equivalence class\") has a distinct representative
  -- 'Unique'. Supports merging the entries of multiple such sets in a union-find
  -- like fashion.
  --
  -- An accurate model is that of @[(Set key, Maybe ele)]@: A finite mapping from
  -- sets of @key@s to possibly absent entries @ele@, where the sets don't overlap.
  -- Example:
  -- @
  --   m = [({u1,u3}, Just ele1), ({u2}, Just ele2), ({u4,u7}, Nothing)]
  -- @
  -- On this model we support the following main operations:
  --
  --   * @'lookupUSDFM' m u3 == Just ele1@, @'lookupUSDFM' m u4 == Nothing@,
  --     @'lookupUSDFM' m u5 == Nothing@.
  --   * @'equateUSDFM' m u1 u3@ is a no-op, but
  --     @'equateUSDFM' m u1 u2@ merges @{u1,u3}@ and @{u2}@ to point to
  --     @Just ele2@ and returns the old entry of @{u1,u3}@, @Just ele1@.
  --   * @'addToUSDFM' m u3 ele4@ sets the entry of @{u1,u3}@ to @Just ele4@.
  --
  -- As well as a few means for traversal/conversion to list.
  newtype UniqSDFM key ele
    = USDFM { unUSDFM :: UniqDFM key (Shared key ele) }
  
  emptyUSDFM :: UniqSDFM key ele
  emptyUSDFM = USDFM emptyUDFM
  
  lookupReprAndEntryUSDFM :: Uniquable key => UniqSDFM key ele -> key -> (key, Maybe ele)
  lookupReprAndEntryUSDFM (USDFM env) = go
    where
      go x = case lookupUDFM env x of
        Nothing           -> (x, Nothing)
        Just (Indirect y) -> go y
        Just (Entry ele)  -> (x, Just ele)
  
  -- | @lookupSUDFM env x@ looks up an entry for @x@, looking through all
  -- 'Indirect's until it finds a shared 'Entry'.
  --
  -- Examples in terms of the model (see 'UniqSDFM'):
  -- >>> lookupUSDFM [({u1,u3}, Just ele1), ({u2}, Just ele2)] u3 == Just ele1
  -- >>> lookupUSDFM [({u1,u3}, Just ele1), ({u2}, Just ele2)] u4 == Nothing
  -- >>> lookupUSDFM [({u1,u3}, Just ele1), ({u2}, Nothing)] u2 == Nothing
  lookupUSDFM :: Uniquable key => UniqSDFM key ele -> key -> Maybe ele
  lookupUSDFM usdfm x = snd (lookupReprAndEntryUSDFM usdfm x)
  
  -- | @equateUSDFM env x y@ makes @x@ and @y@ point to the same entry,
  -- thereby merging @x@'s class with @y@'s.
  -- If both @x@ and @y@ are in the domain of the map, then @y@'s entry will be
  -- chosen as the new entry and @x@'s old entry will be returned.
  --
  -- Examples in terms of the model (see 'UniqSDFM'):
  -- >>> equateUSDFM [] u1 u2 == (Nothing, [({u1,u2}, Nothing)])
  -- >>> equateUSDFM [({u1,u3}, Just ele1)] u3 u4 == (Nothing, [({u1,u3,u4}, Just ele1)])
  -- >>> equateUSDFM [({u1,u3}, Just ele1)] u4 u3 == (Nothing, [({u1,u3,u4}, Just ele1)])
  -- >>> equateUSDFM [({u1,u3}, Just ele1), ({u2}, Just ele2)] u3 u2 == (Just ele1, [({u2,u1,u3}, Just ele2)])
  equateUSDFM
    :: Uniquable key => UniqSDFM key ele -> key -> key -> (Maybe ele, UniqSDFM key ele)
  equateUSDFM usdfm@(USDFM env) x y =
    case (lu x, lu y) of
      ((x', _)    , (y', _))
        | getUnique x' == getUnique y' -> (Nothing, usdfm) -- nothing to do
      ((x', _)    , (y', Nothing))     -> (Nothing, set_indirect y' x')
      ((x', mb_ex), (y', _))           -> (mb_ex,   set_indirect x' y')
    where
      lu = lookupReprAndEntryUSDFM usdfm
      set_indirect a b = USDFM $ addToUDFM env a (Indirect b)
  
  -- | @addToUSDFM env x a@ sets the entry @x@ is associated with to @a@,
  -- thereby modifying its whole equivalence class.
  --
  -- Examples in terms of the model (see 'UniqSDFM'):
  -- >>> addToUSDFM [] u1 ele1 == [({u1}, Just ele1)]
  -- >>> addToUSDFM [({u1,u3}, Just ele1)] u3 ele2 == [({u1,u3}, Just ele2)]
  addToUSDFM :: Uniquable key => UniqSDFM key ele -> key -> ele -> UniqSDFM key ele
  addToUSDFM usdfm@(USDFM env) x v =
    USDFM $ addToUDFM env (fst (lookupReprAndEntryUSDFM usdfm x)) (Entry v)
  
  traverseUSDFM :: forall key a b f. Applicative f => (a -> f b) -> UniqSDFM key a -> f (UniqSDFM key b)
  traverseUSDFM f = fmap (USDFM . listToUDFM_Directly) . traverse g . udfmToList . unUSDFM
    where
      g :: (Unique, Shared key a) -> f (Unique, Shared key b)
      g (u, Indirect y) = pure (u,Indirect y)
      g (u, Entry a)    = do
          a' <- f a
          pure (u,Entry a')
  
  instance (Outputable key, Outputable ele) => Outputable (Shared key ele) where
    ppr (Indirect x) = ppr x
    ppr (Entry a)    = ppr a
  
  instance (Outputable key, Outputable ele) => Outputable (UniqSDFM key ele) where
    ppr (USDFM env) = ppr env
  