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Faster disjoint/isSubsetOf for Set via unbalanced splitting. #865

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Faster disjoint/isSubsetOf for Set via unbalanced splitting. #865

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2 changes: 2 additions & 0 deletions containers-tests/benchmarks/Set.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,8 @@ main = do
, bench "fromDistinctAscList" $ whnf S.fromDistinctAscList elems
, bench "disjoint:false" $ whnf (S.disjoint s) s_even
, bench "disjoint:true" $ whnf (S.disjoint s_odd) s_even
, bench "isSubsetOf:true" $ whnf (s_even `S.isSubsetOf`) s
, bench "isSubsetOf:false" $ whnf (s_even `S.isSubsetOf`) s_odd
, bench "null.intersection:false" $ whnf (S.null. S.intersection s) s_even
, bench "null.intersection:true" $ whnf (S.null. S.intersection s_odd) s_even
, bench "alterF:member" $ whnf (alterF_member elems) s
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22 changes: 20 additions & 2 deletions containers/src/Data/Set/Internal.hs
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Expand Up @@ -712,7 +712,7 @@ isSubsetOfX (Bin _ x l r) t
size l <= size lt && size r <= size gt &&
isSubsetOfX l lt && isSubsetOfX r gt
where
(lt,found,gt) = splitMember x t
(lt,found,gt) = splitMemberUnbalanced x t
#if __GLASGOW_HASKELL__
{-# INLINABLE isSubsetOfX #-}
#endif
Expand Down Expand Up @@ -743,7 +743,7 @@ disjoint (Bin _ x l r) t
-- Analogous implementation to `subsetOfX`
= not found && disjoint l lt && disjoint r gt
where
(lt,found,gt) = splitMember x t
(lt,found,gt) = splitMemberUnbalanced x t

{--------------------------------------------------------------------
Minimal, Maximal
Expand Down Expand Up @@ -1308,6 +1308,24 @@ splitMember x (Bin _ y l r)
{-# INLINABLE splitMember #-}
#endif

-- Same as 'splitMember' but skips re-balancing by using 'bin' instead of 'link'.
-- Attempting to build new trees out of these will error when re-balancing but
-- this can improve performance when the resulting trees are disposable.
splitMemberUnbalanced :: Ord a => a -> Set a -> (Set a,Bool,Set a)
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Tried to factor out bin/link to avoid code duplication but that made the involved functions 2-3x slower.

splitMemberUnbalanced _ Tip = (Tip, False, Tip)
splitMemberUnbalanced x (Bin _ y l r)
= case compare x y of
LT -> let (lt, found, gt) = splitMemberUnbalanced x l
!gt' = bin y gt r
in (lt, found, gt')
GT -> let (lt, found, gt) = splitMemberUnbalanced x r
!lt' = bin y l lt
in (lt', found, gt)
EQ -> (l, True, r)
#if __GLASGOW_HASKELL__
{-# INLINABLE splitMemberUnbalanced #-}
#endif

{--------------------------------------------------------------------
Indexing
--------------------------------------------------------------------}
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