Anticipate Lane Changes

include/engine/api/route_api.hpp

@@ -149,6 +149,7 @@ class RouteAPI : public BaseAPI
                                                               leg_geometry,
                                                               phantoms.source_phantom,
                                                               phantoms.target_phantom);
+                leg.steps = guidance::anticipateLaneChange(std::move(leg.steps));
                 leg_geometry = guidance::resyncGeometry(std::move(leg_geometry), leg.steps);
             }
 

include/engine/guidance/post_processing.hpp

@@ -43,6 +43,11 @@ std::vector<RouteStep> buildIntersections(std::vector<RouteStep> steps);
 // remove steps invalidated by post-processing
 std::vector<RouteStep> removeNoTurnInstructions(std::vector<RouteStep> steps);
 
+// Constrains lanes for multi-hop situations where lane changes depend on earlier ones.
+// Instead of forcing users to change lanes rapidly in a short amount of time,
+// we anticipate lane changes emitting only matching lanes early on.
+std::vector<RouteStep> anticipateLaneChange(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.

include/util/group_by.hpp

@@ -0,0 +1,40 @@
+#ifndef OSRM_GROUP_BY
+#define OSRM_GROUP_BY
+
+#include <algorithm>
+#include <utility>
+
+namespace osrm
+{
+namespace util
+{
+
+// Runs fn on consecutive items in sub-ranges determined by pred.
+//
+// Example:
+//   vector<int> v{1,2,2,2,3,4,4};
+//   group_by(first, last, even, print);
+//   >>> 2,2,2
+//   >>> 4,4
+//
+// Note: this mimics Python's itertools.groupby
+template <typename Iter, typename Pred, typename Fn>
+Fn group_by(Iter first, Iter last, Pred pred, Fn fn)
+{
+    while (first != last)
+    {
+        first = std::find_if(first, last, pred);
+        auto next = std::find_if_not(first, last, pred);
+
+        (void)fn(std::make_pair(first, next));
+
+        first = next;
+    }
+
+    return fn;
+}
+
+} // ns util
+} // ns osrm
+
+#endif