src/Distribution/Utils/UnionFind.hs

    {-# LANGUAGE NondecreasingIndentation #-}
    -- | A simple mutable union-find data structure.
    --
    -- It is used in a unification algorithm for backpack mix-in linking.
    --
    -- This implementation is based off of the one in \"The Essence of ML Type
    -- Inference\". (N.B. the union-find package is also based off of this.)
    --
    module Distribution.Utils.UnionFind (
        Point,
        fresh,
        find,
        union,
        equivalent,
    ) where

    import Data.STRef
    import Control.Monad
    import Control.Monad.ST

    -- | A variable which can be unified; alternately, this can be thought
    -- of as an equivalence class with a distinguished representative.
    newtype Point s a = Point (STRef s (Link s a))
        deriving (Eq)

    -- | Mutable write to a 'Point'
    writePoint :: Point s a -> Link s a -> ST s ()
    writePoint (Point v) = writeSTRef v

    -- | Read the current value of 'Point'.
    readPoint :: Point s a -> ST s (Link s a)
    readPoint (Point v) = readSTRef v

    -- | The internal data structure for a 'Point', which either records
    -- the representative element of an equivalence class, or a link to
    -- the 'Point' that actually stores the representative type.
    data Link s a
        -- NB: it is too bad we can't say STRef Int#; the weights remain boxed
        = Info {-# UNPACK #-} !(STRef s Int) {-# UNPACK #-} !(STRef s a)
        | Link {-# UNPACK #-} !(Point s a)

    -- | Create a fresh equivalence class with one element.
    fresh :: a -> ST s (Point s a)
    fresh desc = do
        weight <- newSTRef 1
        descriptor <- newSTRef desc
        Point `fmap` newSTRef (Info weight descriptor)

    -- | Flatten any chains of links, returning a 'Point'
    -- which points directly to the canonical representation.
    repr :: Point s a -> ST s (Point s a)
    repr point = readPoint point >>= \r ->
      case r of
        Link point' -> do
            point'' <- repr point'
            when (point'' /= point') $ do
                writePoint point =<< readPoint point'
            return point''
        Info _ _ -> return point

    -- | Return the canonical element of an equivalence
    -- class 'Point'.
    find :: Point s a -> ST s a
    find point =
        -- Optimize length 0 and 1 case at expense of
        -- general case
        readPoint point >>= \r ->
          case r of
            Info _ d_ref -> readSTRef d_ref
            Link point' -> readPoint point' >>= \r' ->
              case r' of
                Info _ d_ref -> readSTRef d_ref
                Link _ -> repr point >>= find

    -- | Unify two equivalence classes, so that they share
    -- a canonical element. Keeps the descriptor of point2.
    union :: Point s a -> Point s a -> ST s ()
    union refpoint1 refpoint2 = do
        point1 <- repr refpoint1
        point2 <- repr refpoint2
        when (point1 /= point2) $ do
        l1 <- readPoint point1
        l2 <- readPoint point2
        case (l1, l2) of
            (Info wref1 dref1, Info wref2 dref2) -> do
                weight1 <- readSTRef wref1
                weight2 <- readSTRef wref2
                -- Should be able to optimize the == case separately
                if weight1 >= weight2
                    then do
                        writePoint point2 (Link point1)
                        -- The weight calculation here seems a bit dodgy
                        writeSTRef wref1 (weight1 + weight2)
                        writeSTRef dref1 =<< readSTRef dref2
                    else do
                        writePoint point1 (Link point2)
                        writeSTRef wref2 (weight1 + weight2)
            _ -> error "UnionFind.union: repr invariant broken"

    -- | Test if two points are in the same equivalence class.
    equivalent :: Point s a -> Point s a -> ST s Bool
    equivalent point1 point2 = liftM2 (==) (repr point1) (repr point2)