GHC/Types/Unique/SDFM.hs

    {-# 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