[erlang-questions] [ANN] Priority Queue Implementation

Hynek Vychodil <>
Sun Nov 13 01:03:33 CET 2011


Keeping separate implementations for each edge cases even they do same
thing will never help improve quality of code. DRY

On Sat, Nov 12, 2011 at 8:14 PM, Michael Truog <> wrote:
> On 11/12/2011 05:19 AM, Hynek Vychodil wrote:
>> There is not need make special priority implementation in Erlang.
>
> Oh, but there is!  I think it is nice to have efficient data structures, just to keep software efficient.  So, there is a need, especially in a language like Erlang that happens to be slower than more native code execution.  Pursuing quality can help us avoid software that just becomes slower as it grows (see http://en.wikipedia.org/wiki/Wirth%27s_law).
>
> So, yes, there is no need to care, unless you care about quality.
>
>> On Sat, Nov 12, 2011 at 6:06 AM, Michael Truog <> wrote:
>>> I was looking for a priority queue implementation where the priority is separate from the value being queued, where the order is preserved for the same priority.  That might not be a typical application of a heap to a priority queue, because it might be expected to lose order within the heap structure.  Since I want the priority to be separate from the value, the data structure I am trying to pursue is a bit different.
>>>
>>> On 11/11/2011 09:08 AM, Hynek Vychodil wrote:
>>>> I have used this pretty raw but simple priority queue implementation
>>>> for mine primes generation with true sieve:
>>>>
>>>> -module(pair_heap).
>>>>
>>>> -compile(inline).
>>>>
>>>> -export([new/0, insert/2, find_min/1, delete_min/1, merge/2, to_list/1,
>>>>     from_list/1, insert_list/2]).
>>>>
>>>> new() -> [].
>>>>
>>>> find_min([H|_]) -> H;
>>>> find_min([]) -> empty.
>>>>
>>>> %insert(E, H) -> merge([E], H).
>>>> insert(E, []) -> [E];
>>>> insert(E, [EH|SH]) when EH < E -> [EH|[[E]|SH]];
>>>> insert(E, [_|_]=H) -> [E|[H]].
>>>>
>>>> merge([EA|_]=A, [EB|SB]) when EB < EA -> [EB|[A|SB]];
>>>> merge([EA|SA], [_|_]=B) -> [EA|[B|SA]];
>>>> merge([], B) -> B;
>>>> merge(A, []) -> A.
>>>>
>>>> delete_min([]) -> [];
>>>> delete_min([_|SH]) ->
>>>>   merge_pairs(SH).
>>>>
>>>> merge_pairs([]) -> [];
>>>> merge_pairs([X]) -> X;
>>>> merge_pairs([A,B|T]) -> merge(merge(A,B), merge_pairs(T)).
>>>>
>>>> to_list([]) -> [];
>>>> to_list(H) -> [find_min(H) | to_list(delete_min(H))].
>>>>
>>>> from_list([]) -> [];
>>>> from_list(L) -> insert_list(L, new()).
>>>>
>>>> insert_list([E|T], H) -> insert_list(T, insert(E, H));
>>>> insert_list([], H) -> H.
>>>>
>>>> It worked pretty well and fast for mine purposes.
>>>>
>>>> On Fri, Nov 11, 2011 at 4:42 PM, Jesper Louis Andersen
>>>> <> wrote:
>>>>> On Thu, Nov 10, 2011 at 09:12, Michael Truog <> wrote:
>>>>>> I previously added code that took care of that case, where two nodes needed
>>>>>> to be merged that both have queues.  However, I convinced myself at the
>>>>>> time, that the case would never happen.  So, the code probably needs to be
>>>>>> thought-through a bit more with more testing, but my hope is that merging
>>>>>> the queues isn't necessary.
>>>>> I have tested (tasted, but my typo was funnier) the forbidden fruit
>>>>> that is QuickCheck/PropEr. Your repository now has a pull-request in
>>>>> which I add partial testing via proper_statem. It generates an
>>>>> internal crash of the data structure code if we makes a bunch of
>>>>> inserts and then call len(), see
>>>>>
>>>>> https://github.com/okeuday/pqueue/issues/4
>>>>>
>>>>> Only your pqueue2 implementation is affected. pqueue is not shown to
>>>>> have any errors (yet). The crash is naturally in the "merge" part of the code :P
>>>>>
>>>>> --
>>>>> J.
>>>>> _______________________________________________
>>>>> erlang-questions mailing list
>>>>> 
>>>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>>>
>>>>
>>>
>>
>>
>
>



-- 
Hynek Vychodil
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