collapse use-lane instructions if possible

include/engine/api/route_api.hpp

@@ -151,6 +151,7 @@ class RouteAPI : public BaseAPI
                                                               phantoms.source_phantom,
                                                               phantoms.target_phantom);
                 leg.steps = guidance::anticipateLaneChange(std::move(leg.steps));
+                leg.steps = guidance::collapseUseLane(std::move(leg.steps));
                 leg_geometry = guidance::resyncGeometry(std::move(leg_geometry), leg.steps);
             }
 

include/engine/guidance/post_processing.hpp

@@ -43,6 +43,15 @@ std::vector<RouteStep> buildIntersections(std::vector<RouteStep> steps);
 // remove steps invalidated by post-processing
 std::vector<RouteStep> removeNoTurnInstructions(std::vector<RouteStep> steps);
 
+// remove use lane information that is not actually a turn. For post-processing, we need to
+// associate lanes with every turn. Some of these use-lane instructions are not required after lane
+// anticipation anymore. This function removes all use lane instructions that are not actually used
+// anymore since all lanes going straight are used anyhow.
+// FIXME this is currently only a heuristic. We need knowledge on which lanes actually might become
+// turn lanes. If a straight lane becomes a turn lane, this might be something to consider. Right
+// now we bet on lane-anticipation to catch this.
+std::vector<RouteStep> collapseUseLane(std::vector<RouteStep> steps);
+
 // postProcess will break the connection between the leg geometry
 // for which a segment is supposed to represent exactly the coordinates
 // between routing maneuvers and the route steps itself.