Re: [boost] [graph] dijkstra pull request ping

Subject: Re: [boost] [graph] dijkstra pull request ping
From: alex (alexhighviz_at_[hidden])
Date: 2015-10-23 10:47:00

>-----Original Message-----
>From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Marcin
>Zalewski
>Sent: 23 October 2015 14:33
>To: boost_at_[hidden]
>Subject: Re: [boost] [graph] dijkstra pull request ping
>
>On Thu, Oct 22, 2015 at 4:56 PM alex <alexhighviz_at_[hidden]> wrote:
>
>> > > > What do you mean by not evaluating decreased? Then we use
>decreased
>> > > > to check which visitor function we should call, so I am not sure if
>> > > > one
>> > > could
>> > > get rid
>> > > > of that without having two versions of the code (one for the
>> "classical"
>> > > version,
>> > > > and one for an arbitrary distance map).
>> > > >
>> > >
>> > > Sorry I am not clear. Yes, I do think that you should get rid of the
>> > > redundant check. And, yes that would mean that if you want the
>> > > arbitrary distance map, you should have two versions.
>> > >
>> >
>> > I would  only agree with that if we could demonstrate that there is a
>> statistically
>> > significant performance advantage of doing so. Looking at the code, I
am
>> > skeptical that this check has much or any impact on performance on most
>> > modern platforms. Do you have any evidence that shows there would be a
>> > benefit to having two versions of the code?
>> >
>>
>> There are the following advantages:
>>
>> 1. you avoid one unnecessary check for each vertex
>> 2. you do not need to initialize values in the distance map
>> 3. you do not need to specify an "inf" value for distance
>>
>> (1) and (2) should bring some performance gain, but it is so little that
I
>> did not notice it and did not feel the urge to measure it. I don't think
>> performance is a good reason to take out the redundant check.
>>
>
>The check only becomes redundant if there are two versions of the code, one
>for dijkstra_shortest_paths_no_init and one for dijkstra_shortest_paths.
>The code that covers dijkstra_shortest_paths_no_init automatically covers
>dijkstra_shortest_paths. 

Now you lost me. I thought the dilemma is to have either one version of
dijkstra_shortest_paths_no_init based on the patch by Piotr, or two have two
versions, one as in the patch by Piotr and one as currently implemented. In
either case, dijkstra_shortest_paths can remain just as it is. 

The patch by Piotr stands in the way of some other optimizations that were
proposed 3 years ago. The benefit of this optimizations in terms of
performance is only marginal, but they do make the dijkstra_shortest_path
simpler and cleaner. Also, to me it seems his patch is no longer really
Dijkstra's but some extension of it. To me it is natural to make that a
separate function and give it another name. 

However since Piotr's patch doesn't break anything, but just extends the
function, I can see that it is really just an issue of preference.

>There is an advantage to it, and if one would provide separate code for the
two, there should be a compelling reason. In
>my opinion, in this case the reason should be performance. Otherwise, why
>do this?
>
>> (2) and (3) however make the algorithm simpler and cleaner to use. It
>> becomes easier / possible to apply the method when there is no good value
>> for "inf". I do think that is a good reason to take out the redundant
>> check.
>>
>
>Yes, I agree with you that it makes *one version* of the algorithm cleaner,
>but we have to make a decision that we want to provide that version in the
>first place. I think that providing an elegant version at the cost of two
>different implementations is actually additional complexity.

In my opinion it would not really be two versions of the same algorithm, but
two functions for two similar algorithms. You can see that they are two
different algorithms because in the application that Piotr references
(http://paal.mimuw.edu.pl/docs/do_vv_2kminus1.html) it is used as a means of
computing approximate shortest paths rather than exact shortest paths (if I
understand correctly, because the precalculated distances supplied to the
algorithm are approximate rather than exact). 

>
>> If you take out the redundant checks in the relax function, you no longer
>> need to use boost::closed_plus and can do with std::plus. Again, to me
that
>> makes the function simpler and cleaner. But that is a separate issue,
>> because you can do that whether you make two separate functions or not.
>>
>
>True, agreed. Still, the same as above: do we want to have two
>implementations instead of one?
>

In this case there is not really that dilemma. All that is required here is
an additional variation of the relax function (relax_target_only), but that
is an implementation detail. This works with and without Piotr's patch.

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