test: add confetti buffer, a robustness test for buffer and merge_elements

[barendgehrels]

Add a confetti-buffer robustness test 🎉

Reference https://www.boost.org/releases/1.47.0/ mentioning Boost Geometry as new library

Celebrating 15 years of Boost.Geometry in Boost (July 11, 2011)! 🎂 A new robustness test that also happens to produce a festive picture.

What it does

test/robustness/overlay/buffer/confetti_buffer.cpp generates random "confetti" streamers — bottom-to-top snaky lines of varying length and extent — and buffers them all at once as a single multi_linestring. This stresses the buffer with many interacting pieces per run.

Also exercises merge_elements

Each test independently buffers every streamer, collects the results in a geometry_collection, and checks that merge_elements (the divide-and-conquer union of the collection) yields the same area as the buffer-at-once result. So this is a robustness test for both buffer and merge_elements. Sweeps of 1000 tests (various distances) pass with 0 problems.

See also #947

Colorful output

SVG rendering lives in confetti_buffer_svg.hpp: each streamer's buffer is drawn as a translucent colored ribbon on a dark canvas, with the merged buffer outlined on top — a warm, confetti-like palette. Where the birthday picture comes from.

✅ AI-assisted, 🤖 Generated with Claude Code

[barendgehrels]

No problems found sofar

$ test/robustness/overlay/buffer/boost_geometry_robustness_confetti_buffer --count 100000  --distance 3.1415 --ppc 72 --svg 0
Using random base seed 847908


tests: 100000 problems: 0 time: 4047.09

[mloskot]

Congratulations everyone!

[barendgehrels]

Thanks for the approvals. I propose to merge it indeed.

Because it is not only a nice ad hoc piece of art... It is also a proper robustness test.

Testing:

• merge_elements (there was earlier exactly one unit test calling it once...)
• buffer on complex multi-linestring geometries (there are several tests for multi-linestring, but not yet robustness tests - and probably not this complex).

[vissarion]

Congrats!