Programming Praxis – Priority Queues

Today’s Programming Praxis problem is about priority queues. Specifically, we have to implement one using a Leftist Heap.

We define a priority queue as follows. It’s basically a binary tree, but with an extra field in which we store the rank.

data PQueue a = Node Int a (PQueue a) (PQueue a) | Empty

Empty nodes have rank 0.

rank :: PQueue a -> Int
rank Empty          = 0
rank (Node r _ _ _) = r

A convenience function for node creation that calculates the rank automatically:

node :: a -> PQueue a -> PQueue a -> PQueue a
node i l r = if rank l > rank r then node i r l else Node (1 + rank r) i l r

Two priority queues can be merged as follows:

merge :: (a -> a -> Bool) -> PQueue a -> PQueue a -> PQueue a
merge _ Empty q = q
merge _ q Empty = q
merge p l@(Node _ il _ _) r@(Node _ ir lc rc) =
    if p ir il then node ir lc (merge p l rc) else merge p r l

To insert an item into a priority queue we make a new queue out of it and merge it into our original queue.

insert :: (a -> a -> Bool) -> a -> PQueue a -> PQueue a
insert p i = merge p (node i Empty Empty)

We convert a list to a priority queue by inserting all the items into an empty queue.

fromList :: (a -> a -> Bool) -> [a] -> PQueue a
fromList p = foldr (insert p) Empty

And to do the opposite we keep taking the root of the queue and merging its branches.

toList :: (a -> a -> Bool) -> PQueue a -> [a]
toList _ Empty          = []
toList p (Node _ i l r) = i : toList p (merge p l r)

With these functions, we can easily sort a list on priority by converting it to a priority queue and back.

pqSort :: (a -> a -> Bool) -> [a] -> [a]
pqSort p = toList p . fromList p

And finally we test if everything works ok.

main :: IO ()
main = print $ pqSort (<) [3, 7, 8, 1, 2, 9, 6, 4, 5]

30 lines counting white space. Not bad.

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