#8714: add Bellman-Ford algorithm for shortest paths
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Reporter: mvngu | Owner: jason, ncohen, rlm
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-5.13
Component: graph theory | Resolution:
Keywords: | Merged in:
Authors: David Coudert | Reviewers:
Report Upstream: N/A | Work issues:
Branch: u/ncohen/8714 | Commit:
Dependencies: #12806 | Stopgaps:
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Changes (by ncohen):
* branch: => u/ncohen/8714
Comment:
Yoooooooooooo !
I just reviewed this patch, and I don't see anything wrong. Several
questions/remarks, though :
- the `weights` dictionary is actually (in memory) a copy of the whole
graph. Isn't it better to create a normalized copy of the graph and work
on this only ?
- Why `__bellman_ford__` and not just `_bellman_ford` ? Isn't the `__`
only for special Python functions ?
- You say that loops are supported, but I don't see how. Where do you
detect if there is a negative loop somewhere, for instance ?
- your function `ee` compares the vertices' name, and that's tricky.
Sometimes the vertices of a graph are not integers, can be sets
(KneserGraph for instance) and comparing them doesn't work as expected.
Besides:
- I modified a bit the behaviour of your modifications to
`shortest_paths`, as this dictionary only needs to contain an entry for a
target when such a path exists. I also used the fact that your Bellman
Ford implementation satisfies `pred[t] = [t]` whenever `dist[t] ==
Infinity`.
- From the looks of `.all_pairs_shortest_path`, perhaps we should create a
hidden function for the code of `Floyd-Warshall_Python`. Not urgent, just
cleaning stuff.
I added my patch as a git commit, to which the two previous hg patches
belong. The new branch is u/ncohen/8714.
Have fuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuun ! `;-)`
Nathann
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Ticket URL: <http://trac.sagemath.org/ticket/8714#comment:18>
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