fix: nearly identical input should give an intersection

[barendgehrels]

Fixes: #1471

✅ AI assisted

Nearly identical polygons should intersect

Two almost-identical polygons (coordinates differing by ~1 epsilon) produced an empty intersection, a doubled union, and a wrong difference.

Root cause: such input yields zero turns, so select_rings decides containment from a single representative point — but that point is a ring vertex, which lands exactly on the other ring's boundary (range_in_geometry == 0), and an on-boundary point was treated as "not within".

Fix: in select_rings, when the overlay produced no turns, treat an entirely-on-boundary ring as "within" for the binary set operations (union/intersection/difference).

Related to commit 2780c9d (which changed this code, reversal would have fixed this as well - but would have given regression in other cases).

This PR also enhances the test (that initially passed for #1471): if two geometries intersect but don't touch, their intersection must be non-empty — independent of the overlay path, so it catches this class of bug that area-balance checks cannot.

[vissarion]

LGMT thanks!

[barendgehrels]

LGMT thanks!

@vissarion thanks for the quick review. It's a bugfix, 1.92 is still open, I think we can make that. Then I'll also include it in the release notes.
@tinko92 also OK with this?

[vissarion]

@vissarion thanks for the quick review. It's a bugfix, 1.92 is still open, I think we can make that.

I agree, if Tinko is OK, I will merge it to main.

Then I'll also include it in the release notes. @tinko92 also OK with this?

Thanks, I will then update the boost release notes.

[tinko92]

Sorry that I couldn't review this yesterday, there are some lines that were non-obvious to me (I left comments). I am OK with merging this before the release since it fixes cases and does not break anything, so my comments are not blockers.

Most comments are about passing something (constant false or a computed value) for overlay_has_turns being ambiguous. Maybe some argument name that indicates "decide within for the no turns special case" with a default value would be more clear here. But again, I am OK with merging this since it is well-limited to a special case, does not seem to break anything and fixes a real problem.

[barendgehrels]

overlay_has_turns

Maybe include_on_border is clearer (and then it should be reverted). I could give it a default value false (avoiding the change in dissolve - and then it will have turns.empty()).

[barendgehrels]

I'll update the PR later today. Thanks for the approvals!

[barendgehrels]

overlay_has_turns

Maybe include_on_border is clearer (and then it should be reverted). I could give it a default value false (avoiding the change in dissolve - and then it will have turns.empty()).

✅ (include_on_boundary seems the best OGC fitting term)

[tinko92]

Thanks for the quick changes and the fix! In my opinion this can be merged.

[barendgehrels]

Thanks, I will then update the boost release notes.

Merged! ✔️