# [erlang-questions] [ANN] Priority Queue Implementation

Zabrane Mickael zabrane3@REDACTED
Wed Nov 9 09:45:06 CET 2011

```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
>
> 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).
>> _______________________________________________
>> erlang-questions mailing list
>> erlang-questions@REDACTED
>> http://erlang.org/mailman/listinfo/erlang-questions
>
> Ulf Wiger, CTO, Erlang Solutions, Ltd.
> http://erlang-solutions.com
>
>
>
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