fix: revise traversal_switch_detector for isolated areas

[barendgehrels]

Not ready for review

Fixes: issue 1349

This is quite a revisal. Issue #1349 was caused by a logic error, and not by a precision error. It was easily fixed by removing the (in hindsight weird) condition (added by me) to reverse the meaning of isolated.

But that caused the original issue again. I made an inverse case of that one as well, and that failed anyway (also with its earlier "fix") for difference.

I decided to go for another way to detect isolation, which is now clearer - but even then not that easy. Below some cases for reference.

Below some pictures of intersections (blue, green: input, turquoise: both (= intersection), dotted red/green: resulting intersection.

Case 869 (only ii turns)

Case 128 (with a separate polygon connected as "multiple" with only ii turns (turn 6,9,18), it should not be isolated)

Case 141 (idem but it should be isolated and the "multiple" region inside (turn 13,14) is a separate polygon)

Case case_recursive_boxes_81 which looks the same, but here it are two outer polygons with a part (at turn 26, 27, 42)

[tinko92]

I think, this creates a regression. Consider

#include <iostream>
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/geometries.hpp>

namespace bg = boost::geometry;
using point = bg::model::point<int, 2, bg::cs::cartesian>;
using polygon = bg::model::polygon<point>;
using multipoly = bg::model::multi_polygon<polygon>;

int main() {
    auto p1 = bg::from_wkt<multipoly>("MULTIPOLYGON(((2 10,2 8,0 8,0 10,2 10)),((10 8,10 2,8 2,8 0,0 0,0 4,2 4,2 8,4 8,4 10,6 10,6 8,6 6,8 6,8 8,10 8),(8 2,8 4,4 4,4 2,8 2)))");
    auto p2 = bg::from_wkt<multipoly>("MULTIPOLYGON(((2 6,2 4,2 2,2 0,0 0,0 6,2 6)),((2 10,2 8,0 8,0 10,2 10)),((6 8,6 6,2 6,2 8,6 8)),((8 4,8 2,6 2,6 4,8 4)),((10 8,10 6,8 6,8 8,10 8)),((8 10,8 8,6 8,6 10,8 10)))");
    multipoly p3;
    bg::sym_difference(p1, p2, p3);
    std::cout << bg::wkt(p3) << "\n";
    return 0;
}

Output on develop: MULTIPOLYGON(((10 6,10 2,8 2,8 0,2 0,2 4,0 4,0 6,10 6),(6 2,6 4,4 4,4 2,6 2)),((4 8,4 10,6 10,8 10,8 8,4 8)))
Output on this branch: MULTIPOLYGON(((10 6,10 2,8 2,8 0,2 0,2 4,0 4,0 6,10 6)),((4 8,4 10,6 10,8 10,8 8,4 8)))

[barendgehrels]

I think, this creates a regression. Consider

👍 I'll check

[barendgehrels]

I think, this creates a regression. Consider

👍 I'll check

Indeed it fails now. I have to revise it.

[barendgehrels]

The region information for #869 is incorrect - there is a fix that fixes this all but leads to yet another situation which goes wrong.
I have to revise the code, probably getting rid of the isolation concept.

[barendgehrels]

@tinko92 we (@vissarion and I) added you as a collaborator, now I can add you as a reviewer, great.

For the rest this PR is still in draft and might be closed by a replacement later this month.

[barendgehrels]

Replaced by #1356