[erlang-questions] [ANN] Priority Queue Implementation

[Michael Truog]

I think the skew heap I have needs some work, because it seems to come only from Okasaki's code example, so it doesn't take into consideration his suggestions/exercises.  So, only insert is O(1), and the min would need to be stored separately to get O(1) instead of O(log(N)). He had a suggestion for making the merge of two heaps O(1), but I wasn't as concerned about that operation.  It seems hard to get an "out" operation that is O(1) amortized, that is removing the min from the heap (hopefully O(log(N)) worst case).  I will look at testing a heap implementation to see how it might work out.  Thanks for the information.

On 11/09/2011 01:00 AM, Ulf Wiger wrote:
>
> Yeah, obviously, mine was just a sketch, thrown down as an executable comment and optimized for brevity. :)
>
> (Although I'm not convinced, from reading, that Michael's implementation is faster than mine. Anyone who cares deeply enough could of course measure. I am currently not shopping for a faster priority queue, so I will pass on that.)
>
> As an aside, it was a simple skew heap exercise, presented by Chris Okasaki, that made me invite Quviq to Ericsson for the first Erlang QuickCheck pilots. 
>
> The task was to reverse-engineer the insertion order of a particular skew heap. John Hughes solved it with a "brute force approach", using QuickCheck to test his assumptions. Watching him do exploratory hacking with QuickCheck was so much fun that, once he ported QuickCheck to Erlang, I had to try to find out if it could be put to use in a commercial project.
>
> Unfortunately - or fortunately - for Quviq, the only candidate for a useful pilot was stateful, and QuickCheck had no support for that. For lesser minds, that might have been a problem, but John and Thomas quickly invented the statem model. :)
>
> BR,
> Ulf W
>
> On 9 Nov 2011, at 09:45, Zabrane Mickael wrote:
>
>> Hi Ulf,
>>
>> Michael Truog already has a SkewBinHeap impelmentation here:
>> https://github.com/okeuday/skewbinheap
>>
>> Regards,
>> Zabrane
>>
>> On Nov 9, 2011, at 9:42 AM, Ulf Wiger wrote:
>>
>>> I'm partial to skew heaps, mainly because they are so elegant.
>>>
>>> http://www.cse.yorku.ca/~andy/courses/4101/lecture-notes/LN5.pdf <http://www.cse.yorku.ca/%7Eandy/courses/4101/lecture-notes/LN5.pdf>
>>>
>>> Something like this (although I've done only basic testing):
>>>
>>> -module(skew).
>>> -export([new/0, in/2, out/1]).
>>>
>>> new() ->
>>>     [].
>>>
>>> in(X, Heap) ->
>>>     merge({X,[],[]}, Heap).
>>>
>>> out([]) ->
>>>     error;
>>> out({X, L, R}) ->
>>>     {X, merge(L, R)}.
>>>
>>> merge({P0,Pl,Pr}, {Q0,_,_} = Q) when P0 < Q0 ->
>>>     {P0, Pr, merge(Pl,Q)};
>>> merge({P0,_,_} = P, {Q0,Ql,Qr}) when P0 > Q0 ->
>>>     {Q0, Qr, merge(Ql,P)};
>>> merge({P0,Pl,Pr} = P,{P0,Ql,Qr}) ->   % equal roots
>>>     merge(P, merge(merge(Pl,Pr), merge(Ql,Qr)));
>>> merge([], Q) -> Q;
>>> merge(P, []) -> P.
>>>
>>> The cost is amortized O(log N) for in/2 and out/1. For peeking at the min, it's O(1).
>>>
>>> BR,
>>> Ulf W
>>>
>>> On 9 Nov 2011, at 04:33, Michael Truog wrote:
>>>
>>>> I was looking at Erlang priority queue implementations and the Riak/RabbitMQ one seemed a bit slow.  I have a different implementation with the same API here: https://github.com/okeuday/pqueue/blob/master/src/pqueue.erl
>>>>
>>>> The results from my test are here: http://okeuday.livejournal.com/19187.html
>>>>
>>>> The implementation has "in" operations that are roughly 3 times faster (300%), however, the "out" operation became roughly 30% slower.  So, as long as the priority queue is storing a decent amount of items, this data structure should provide better speed.  The implementation is limited to a specific priority range: -20 (high) to 20 (low).
>>>> _______________________________________________
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>>>
>>> Ulf Wiger, CTO, Erlang Solutions, Ltd.
>>> http://erlang-solutions.com <http://erlang-solutions.com/>
>>>
>>>
>>>
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>>
>>
>>
>
> Ulf Wiger, CTO, Erlang Solutions, Ltd.
> http://erlang-solutions.com
>
>
>

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