optimal_convex_partition_2 fails with very simple polygon

[mmertama]

Issue Details

CGAL error: precondition violation!
Expression : is_simple_2(first, last, traits)
File : /opt/homebrew/include/CGAL/Polygon_2/Polygon_2_algorithms_impl.h
Line : 542
Explanation:
Refer to the bug-reporting instructions at https://www.cgal.org/bug_report.html
libc++abi: terminating due to uncaught exception of type CGAL::Precondition_exception: CGAL ERROR: precondition violation!
Expr: is_simple_2(first, last, traits)
File: /opt/homebrew/include/CGAL/Polygon_2/Polygon_2_algorithms_impl.h
Line: 542

Source Code

void test(){

using Kern = CGAL::Exact_predicates_inexact_constructions_kernel;
using Tra = CGAL::Partition_traits_2<Kern>;
Tra::Polygon_2  polygon;
std::list<Tra::Polygon_2>   partition_polys;

const std::vector<double> vec = {-640,-160,-640,-384,-448,-384,-448,-448,-320,-448,-320,-384,
-128,-384,-128,-152,-128,-128,-128,0,-128,24,-128,256,-320,256,-448,64,-320,64,-320,-192,-448,-192,-512,-192,-512,-160
};

for (unsigned i = 0; i < vec.size(); i += 2) {
    polygon.push_back(Tra::Point_2{vec[i], vec[i + 1]});
}
   
assert(polygon.orientation() == CGAL::Orientation::COUNTERCLOCKWISE);
assert(polygon.is_simple());

CGAL::optimal_convex_partition_2(polygon.vertices_begin(),
                                polygon.vertices_end(),
                                std::back_inserter(partition_polys));

assert(CGAL::partition_is_valid_2(polygon.vertices_begin(),
                                 polygon.vertices_end(),
                                 partition_polys.begin(),
                                 partition_polys.end()));

}

Environment

• OS: Mac 16.3
• Compiler: Clang (system default C++)
• Release or debug mode: Debug
• Specific flags used (if any):
• CGAL version: 6.0.1
• Boost version: 1.87.0_1
• Other libraries versions if used (Eigen, TBB, etc.):

[mmertama]

TBC if duplicate of #2575

[mmertama]

I tried greene_approx_convex_partition_2, and it worked with that paricular data, but failed with: std::vector vec2 = {256, 3072, 129.41, 3009.41, 448, 3008, 496, 3136, 496, 3160, 496, 3304, 496, 3328,
448, 3456, 129.41, 3454.59, 67.21, 3391.47, 129.85, 3266.89, 160.18, 3268, 256, 3392, 320, 3392, 320,
3328, 256, 3328, 192.25, 3267.99, 224.25, 3267.99, 256.13, 3266, 288, 3264, 320, 3264, 320, 3200, 288,
3200, 256.13, 3198, 224.25, 3196.01, 192.25, 3196.01, 160.18, 3196, 256, 3136, 320, 3136, 320, 3072}; ... that is also simple set.

[mmertama]

This time the failure is
CGAL error: postcondition violation! Expression : convex_partition_is_valid_2(polygon.vertices_begin(), polygon.vertices_end(), res.output_so_far_begin(), res.output_so_far_end(), traits) File : /opt/homebrew/include/CGAL/Partition_2/partition_greene_approx_convex_2.h Line : 829 Explanation:

[mmertama]

There is CGAL_PARTITION_NO_POSTCONDITIONS, but when set, code wont compile - whole CGAL::Partition is fundamentally broken and should not be used?

[sloriot]

-384,-448,-384,-448 is a duplicated point making the is_simple precondition failing.

[mmertama]

Sorry where do you see duplicate points? How much I try I cannot spot them:
-640,-160
-640,-384
-448,-384
-448,-448
-320,-448
-320,-384
-128,-384
-128,-152
-128,-128
-128,0
-128,24
-128,256
-320,256
-448,64
-320,64
-320,-192
-448,-192
-512,-192
-512,-160

[sloriot]

My bad. But there is a missing comma after -384 on the first line of points.

[sloriot]

I still have the assertion after adding the missing comma. Sorry for the fast closing.

[mmertama]

My example code had a missing comma after -384, <-- It was lost when edited here, sorry. Now the example should show the fail condition correctly. @sloriot, Im sorry about confusion