Optimization idea for `submapBitmapIndexed`

[sjakobi]

unordered-containers/Data/HashMap/Internal.hs

Lines 1473 to 1495 in 807e3a4

	-- | \(O(\min n m))\) Checks if a bitmap indexed node is a submap of another.
	submapBitmapIndexed :: (HashMap k v1 -> HashMap k v2 -> Bool) -> Bitmap -> A.Array (HashMap k v1) -> Bitmap -> A.Array (HashMap k v2) -> Bool
	submapBitmapIndexed comp !b1 !ary1 !b2 !ary2 = subsetBitmaps && go 0 0 (b1Orb2 .&. negate b1Orb2)
	where
	go :: Int -> Int -> Bitmap -> Bool
	go !i !j !m
	| m > b1Orb2 = True
	-- In case a key is both in ary1 and ary2, check ary1[i] <= ary2[j] and
	-- increment the indices i and j.
	| b1Andb2 .&. m /= 0 = comp (A.index ary1 i) (A.index ary2 j) &&
	go (i+1) (j+1) (m `unsafeShiftL` 1)
	-- In case a key occurs in ary1, but not ary2, only increment index j.
	| b2 .&. m /= 0 = go i (j+1) (m `unsafeShiftL` 1)
	-- In case a key neither occurs in ary1 nor ary2, continue.
	| otherwise = go i j (m `unsafeShiftL` 1)
	b1Andb2 = b1 .&. b2
	b1Orb2 = b1 .|. b2
	subsetBitmaps = b1Orb2 == b2
	{-# INLINABLE submapBitmapIndexed #-}

Currently the bit mask used to check for subkeys that appear in both maps is incremented by one. It might be more efficient to use countTrailingZeros to check only the bits that actually overlap, similar to the code in unionArraysBy introduced in #395.