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From: Jens Müller (jens.mueller_at_[hidden])
Date: 20070705 10:30:29
Aaron Windsor wrote:
> On 7/2/07, Jens Müller <jens.mueller_at_[hidden]> wrote:
>
>>Aaron Windsor wrote:
>>
>>>I've put two sets of files in the vault under Home/Algorithms/graph: the
>>>first is called planar_graphs.zip (also planar_graphs.tar.gz) and contains
>>>the .hpp files, the documentation, and the examples. The second is called
>>>planar_graph_testing.zip (also planar_graph_testing.tar.gz) and contains
>>>over 1000 test graphs and a small program that can be compiled to give an
>>>example of how these planar graph tools can be used. I'd appreciate any
>>>comments!
>>
>>
>>1. It would be nice if bidirectional graphs could be supported as well.
>
>
> Hi Jens,
>
> Thanks for taking a look at my implementation. Yes, it would be nice
> if bidirectional graphs were supported  they should be, and if it
> doesn't turn out to be too much trouble, I'd like to support them.
> I'll check on this when I have a little more time over the next couple
> of days.
IMO, the only difference between undirected and bidirectional graphs is
(see
http://www.boost.org/libs/graph/doc/graph_concepts.html#sec:undirectedgraphs):
that in bidirectional graphs, you have to iterate over in and out edges
seperately. Even source(e) and target(e) are used the same ...
[...]
>
> It's because you've got parallel edges  each edge is repeated twice
> (for example, {n0,n2} shows up as source=n0, target=n2 and source=n2,
> target=n0). I cleaned up the file and ran it through my code and it
> was recognized as a planar graph.
>
Args  sorry, that is a "feature" of the generators we use here ...
>
>>3. What is your implementation doing when the graph is not undirected or
>>when there are selfloops?
>
>
> It's undefined on anything that isn't an undirected graph with no
> selfloops and no parallel edges. The BoyerMyrvold algorithm relies
> on several computations on an undirected DFS tree like lowpoint, least
> ancestor, and the full suite of planar testing/embedding/drawing
> algorithms rely on some algorithms that are only defined on undirected
> graphs, like testing simple connectivity and finding biconnected
> components. This is a case when it would be nice to have some adapters
> in the BGL to treat directed graphs as undirected or viseversa.
You mean bidirectional graphs ;) Yes, that would be nice, and not that
very much complicated, I suppose. I just replied to a post by Benoit
from November 2006 who asked for a bidirectional>undirected adapter as
well. Maybe my bidirectional>"bidirected" adapter can serve as a
starting point, which is in turn based on the graph reversal adapter by
David Abrahams.
When I now think of it, I wonder wether it would have been feasible to
use a undirected graph for this  I suppose yes ... Probably my design
choice was due to the original LEDAbased implementation I'm working
with, which called G.make_bidirected(). Gotta take a look on it.
That, of course, does not solve the problem with selfloops and parallel
edges. IMO, it should be possible to construct planar embeddings for
graphs containing them, as well. Maybe deleting them from the graph
first and later "multiplying" edges again? This would mean adding them
in opposite order in the adjacency lists of the two endpoint nodes, I
suppose.
Do you know which steps of the algorithm rely on there being no
selfloops or parallel edges?
Regards,
Jens
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