Allow single edge paths in MLD alternatives, #4691

features/testbot/alternative_loop.feature

@@ -31,6 +31,30 @@ Feature: Alternative route
             | 5    | 6  | dc,ca,ab,bd,dc,dc |             |
             | 7    | 8  | ca,ab,bd,dc,ca,ca |             |
 
+
+    Scenario: Alternative Loop Paths with an asymmetric circle
+        Given a grid size of 10 meters
+        Given the node map
+            """
+              a b  c
+            l       d
+            k       e
+            j       f
+              i h g
+            """
+
+        And the ways
+            | nodes         | oneway |
+            | abcdefghijkla | no     |
+
+        And the query options
+            | alternatives | true |
+
+        When I route I should get
+            | from | to | route                       | alternative                 | weight |
+            | e    | k  | abcdefghijkla,abcdefghijkla | abcdefghijkla,abcdefghijkla |    6.8 |
+
+
     # This test case does not work in a platform independent way
     # since it depends on a specific CH structure that is only
     # present on linux it seems.

src/engine/routing_algorithms/alternative_path_mld.cpp

@@ -233,6 +233,12 @@ RandIt filterPackedPathsByCellSharing(RandIt first, RandIt last, const Partition
         cells.insert(get_cell(std::get<1>(edge)));
 
     const auto over_sharing_limit = [&](const auto &packed) {
+
+        if (packed.path.empty())
+        { // don't remove routes with single-node (empty) path
+            return false;
+        }
+
         const auto not_seen = [&](const PackedEdge edge) {
             const auto source_cell = get_cell(std::get<0>(edge));
             const auto target_cell = get_cell(std::get<1>(edge));
@@ -310,7 +316,7 @@ RandIt filterPackedPathsByLocalOptimality(const WeightedViaNodePackedPath &path,
         // Check plateaux edges towards the target. Terminates at the source / target
         // at the latest, since parent(target)==target for the reverse heap and
         // parent(target) != target in the forward heap (and vice versa).
-        while (has_plateaux_at_node(node, fst, snd))
+        while (node != fst.GetData(node).parent && has_plateaux_at_node(node, fst, snd))
             node = fst.GetData(node).parent;
 
         return node;
@@ -320,7 +326,11 @@ RandIt filterPackedPathsByLocalOptimality(const WeightedViaNodePackedPath &path,
         BOOST_ASSERT(packed.via.node != path.via.node);
         BOOST_ASSERT(packed.via.weight != INVALID_EDGE_WEIGHT);
         BOOST_ASSERT(packed.via.node != SPECIAL_NODEID);
-        BOOST_ASSERT(!packed.path.empty());
+
+        if (packed.path.empty())
+        { // the edge case when packed.via.node is both source and target node
+            return false;
+        }
 
         const NodeID via = packed.via.node;
 
@@ -395,6 +405,12 @@ template <typename RandIt> RandIt filterUnpackedPathsBySharing(RandIt first, Ran
     edges.insert(begin(shortest_path.edges), begin(shortest_path.edges));
 
     const auto over_sharing_limit = [&](const auto &unpacked) {
+
+        if (unpacked.edges.empty())
+        { // don't remove routes with single-node (empty) path
+            return false;
+        }
+
         const auto not_seen = [&](const EdgeID edge) { return edges.count(edge) < 1; };
         const auto different = std::count_if(begin(unpacked.edges), end(unpacked.edges), not_seen);