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[Zohar <>]

Hi,

I randomly flip edges in a mesh, and at some point it crashes.
I copied below the cgal source, and I mark (with a comment) the
CGAL_precondition which fails after the filp.


    Halfedge_handle flip_edge( Halfedge_handle h) const {
        // performs an edge flip. It returns h after
        // rotating the edge h one vertex in the direction of the face
        // orientation. Precondition: `h != Halfedge_handle()' and both
        // incident faces of h are triangles.
        CGAL_precondition( h != Halfedge_handle());
        CGAL_precondition( h == h->next()->next()->next());
        CGAL_precondition( h->opposite() ==
                    h->opposite()->next()->next()->next());
        Halfedge_handle hprev = h->next()->next();
        Halfedge_handle gprev = h->opposite()->next()->next();
        remove_tip( hprev);
        remove_tip( gprev);
        set_face_halfedge(  hprev);
        set_face_halfedge(  gprev);
        set_vertex_halfedge( hprev);
        set_vertex_halfedge( gprev);
        set_face( hprev->next(), hprev->face());
        set_face( gprev->next(), gprev->face());
        hprev = hprev->next();
        gprev = gprev->next();
        insert_tip( h, gprev);
        insert_tip( h->opposite(), hprev);
        CGAL_postcondition( h == h->next()->next()->next());
        CGAL_postcondition( h->opposite() ==
                     h->opposite()->next()->next()->next()); // *** Fails -
crash
        return h;
    }




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