Distance is faster than finding shared edge

[m-kuhn]

Description

Followup #39961
This was still quite slow, this delays the expensive shared edge length test to when a fix requiring it is applied to make the checks themselves run fast.

CC @olivierdalang do you see an issue with this?

[olivierdalang]

Yes it makes sense... I'd also filter out the list of neighbours for the snapping that happens just after (lines 374).

Still, this all seems a bit risky, as it's also used in other methods like isEqual, or involvedFeatures, which I'm not sure how they are used ? Isn't there some room for improvement in QgsGeometryCheckerUtils::sharedEdgeLength itself ? Then, I haven't tested how bad perfs is currently, so can't really tell if it's worth the risk or not :-)

[m-kuhn]

isEqual() is a utility function, important is that it returns stable results (which it still does), involvedFeatures is used to zoom to context (visually), so not something that leads to data corruption.

The speed impact is considerable, 4.5s vs 18s on a test data set here ~4 times.

I am sure that sharedEdgeLength could be improved, it's a myriad of nested loops, I tried to optimize it, unsuccessfully so far. So for the moment, this gives the best bang for the buck.

[nyalldawson]

@m-kuhn there's a geos method for extracting shared edges - maybe that is faster?

[m-kuhn]

I could very well imagine that it's faster, but probably still slower than distance.
Still I'd like to continue to merge this and keep other improvements separate.

[nyalldawson]

Fair enough!