Surface_mesh: Add exact_num_faces(const SM&)

[afabri]

Summary of Changes

Add a partial specialization for CGAL::internal::exact_num_faces(const Surface_mesh&) to have an O(1) time. This was pointed out as important for the Face_count_stop_predicate in Issue #8158

Release Management

• Affected package(s): Surface_mesh
• Issue(s) solved (if any): fix edges(sm).size() takes linear time #5720
• License and copyright ownership: unchanged

[sloriot]

why only one and not the four?

[lrineau]

TODO: that new function overload needs a test.

It is used in Surface_mesh_simplification. How do you want me to check that the specialization is taken? Add an #ifdef CGAL_TESTSUITE with a std::cout ?

[sloriot]

Successfully tested in 6.0-Ic-251