Add `GeometryCollection::toSet()` method

[Kolyunya]

I'd like to propose to add a functionality of making geometry collections unique.

This is just a quick proof of concept. Maybe the method should be moved to Geometry. Maybe it should be named differently. Maybe it should modify the collection instead of creating a new one. Maybe it should be a flag in one of the factory methods. There are really a lot of possible options to implement this...

I would be glad to hear any comments and/or recommendations.

P.S. Figuring out that splatting gotcha was a nightmare :rofl:

[BenMorel]

Hi, I'm not sure to want to add this feature to the library: where do you get duplicate geometries from? Understanding the use case might help here.

If we do add this feature:

• I'd rather call it deduplicate() or something like that
• I think returning a new collection is the right thing to do
• I don't like the idea of making it an option in the factory method
• I don't think it belongs to Geometry, as it does not make sense outside of GeometryCollection (apart maybe from CompoundCurve)

So, apart maybe from the naming, your implementation looks OK to me. The only things that bother me really, are:

• is there a common enough use case for deduplicating, that motivates adding the feature to the library itself?
• a tiny difference in coordinates will make the method consider the geometries different; considering all the gotchas with floats (like 0.1 + 0.2 !== 0.3), maybe such a method should allow a delta parameter, somehow like PHPUnit's assertEqualsWithDelta()?

[BenMorel]

Another thing that comes to my mind is: if the GeometryCollection contains other collections like MultiPoint, say:

GEOMETRYCOLLECTION(
    MULTIPOINT((1 2), (3 4)),
    MULTIPOINT((3 4), (1 2))
)

Your algorithm (converting to WKB) will not detect that the contained geometries are equal, and will fail to deduplicate them.

Using Geometry::equals() on each pair of geometries would get us closer to the goal, but would be computationally expensive.

All in all, I think this is a problem that's hard to solve well, and it would be maybe better tackled in your own application if you just need a simple binary comparison, or in a third-party library that would extend brick/geo's capabilities if you want a deeper comparison algorithm!

[BenMorel]

As it stands, I don't think this functionality should be implemented in the library, as there are many ways to implement it, each with its own drawbacks (binary or ST_Equals() equality, performance). It's probably better done in your own app, where you can apply your own algo that works for you.

@Kolyunya Please let me know if you're planning to PR other changes that could break BC, otherwise I think we're ready to release version 0.4.0!

[Kolyunya]

@BenMorel exactly the same geometries may occur for example when they are stored in the database, queried by separate routines and then merged together. I don't know, maybe my use case is too specific...

Not planning to make any breaking changes at the moment. Thanks!

[BenMorel]

Thanks for the heads up, @Kolyunya. Version 0.4.0 is released!

I don't know if your use-case is that specific, but it's easy enough to include your deduplication algorithm in a helper class in your app!