Specific return codes for labeling operators

[sebmestrallet]

Currently, labeling operators return either :

• nothing (see e.g. include/labeling_operators_on_distortion.h straighten_boundary_with_GCO()
• a boolean : failure or success (most of them)
• an unsigned integer : the number of processed components (see e.g. include/labeling_operators_on_invalidity.h fix_as_much_invalid_corners_as_possible()

But there are many reasons a labeling operator can fail :

• bad type of input mesh
• no facet attribute linked to the GEO::MeshHalfedges instance, so no facet regions and no halfedges on border
• while tracing a path on the surface, best next edge for the given direction is the last edge (backtracking)
• did not find the optimal label w.r.t. forbidden axes and orthogonality constraints (see include/labeling.h find_optimal_label())
• unexpected feature-edges/boundaries configuration
• etc

Each operator could have a set of return codes :

struct my_operator {
	enum return_code {
		SUCCESS,
		BAD_MESH,
		NO_LABELING,
		TRACE_PATH_BACKTRACKING,
		// etc
	};
	
	return_code operator()(/* args */) const {
		// labeling operator definition here
		return SUCCESS;
	}
};

Even better, a payload could be attached to each return code, e.g. to know how many components were processed, or if the operator had nothing to do :

struct my_operator {

	struct Success {
		unsigned int nb_processed {};
	};

	struct TracePathBacktracking {
		index_t vertex_index {};
		MeshHalfedges::Halfedge new_halfedge {};
	};

	// etc

	typedef std::variant<Success,InvalidLabel,/* etc */> return_code;
	
	return_code operator()(/* args */) const {
		// labeling operator definition here
		// return value:
			return Success{count_chart_processed};
		// or
			return TracePathBacktracking{current_vertex,current_halfedge};
	}
};