Test fails with the development version of igraph about sfnetworks

[szhorvat]

For context on this, please see igraph/rigraph#571. It's not quite settled yet how this will be dealt with in igraph. The original fix restricts the Johnson method to using only mode='out' on directed graphs. The failure here was caused by using mode='in'.

All this said, the test used in sfnetwork does not achieve anything, and I recommend removing or changing it. Johnson's algorithm is intended specifically for directed graphs with some negative edge weights. When there are no negative weights, igraph will just use Dijsktra's algorithm, even if you specified Johnson. Thus the test, in its current form, compares Dijsktra against Dijkstra and can never fail.

Why does igraph just use Dijkstra when there are no negative weights? This makes perfect sense if we look at how Johnson's algorithm works: it simply transforms edge weights to ensure that they are all non-negative, allowing the use of Dijkstra's method instead of the Bellman-Ford algorithm, which has worse asymptotic complexity.

To sum up, I recommend removing the test that @krlmlr referenced, irrespective of how this will be dealt with in igraph 1.3.6. Comparing results between Johnson and Dijkstra makes no sense as the former is only used with negative edge weights, while the latter is only used with non-negative ones.

[loreabad6]

Hi @krlmlr and @szhorvat, thank you for pointing us to this failing test. The aim of the test is jsut to check that parameters are passed correctly onto igraph::distances(), and during creating this test we probably overlooked the conceptual theory of the algorithms. To keep the test alive, I switched the algorithm to unweighted, which should definitely result in different distances.