Add routing improvement limits

README.md

@@ -1472,6 +1472,15 @@ routing.solve(
 )
 ```
 
+Native improvement-rate stopping is available through search parameters:
+
+```ruby
+search_parameters.improvement_limit_parameters = {
+  improvement_rate_coefficient: 0.01,
+  improvement_rate_solutions_distance: 5
+}
+```
+
 ## Bin Packing
 
 ### The Knapsack Problem

ext/or-tools/routing.cpp

@@ -22,6 +22,7 @@ using operations_research::RoutingSearchStatus;
 
 using Rice::Array;
 using Rice::Class;
+using Rice::Hash;
 using Rice::Module;
 using Rice::Object;
 using Rice::String;
@@ -182,6 +183,32 @@ void init_routing(Rice::Module& m) {
       "lns_time_limit=",
       [](RoutingSearchParameters& self, int64_t value) {
         self.mutable_lns_time_limit()->set_seconds(value);
+      })
+    .define_method(
+      "improvement_limit_parameters",
+      [](RoutingSearchParameters& self) -> Object {
+        if (!self.has_improvement_limit_parameters()) {
+          return Object(Qnil);
+        }
+
+        const auto& parameters = self.improvement_limit_parameters();
+        Hash result;
+        result[Symbol("improvement_rate_coefficient")] =
+          parameters.improvement_rate_coefficient();
+        result[Symbol("improvement_rate_solutions_distance")] =
+          parameters.improvement_rate_solutions_distance();
+        return result;
+      })
+    .define_method(
+      "improvement_limit_parameters=",
+      [](RoutingSearchParameters& self, Hash value) {
+        const double coefficient =
+          value.get<double>(Symbol("improvement_rate_coefficient"));
+        const int distance =
+          value.get<int>(Symbol("improvement_rate_solutions_distance"));
+        auto* parameters = self.mutable_improvement_limit_parameters();
+        parameters->set_improvement_rate_coefficient(coefficient);
+        parameters->set_improvement_rate_solutions_distance(distance);
       });
 
   Rice::define_class_under<RoutingIndexManager>(m, "RoutingIndexManager")

test/routing_test.rb

@@ -907,6 +907,20 @@ def test_search_parameters
     search_parameters.first_solution_strategy = :path_cheapest_arc
     search_parameters.local_search_metaheuristic = :guided_local_search
     search_parameters.log_search = true
+    assert_nil search_parameters.improvement_limit_parameters
+
+    search_parameters.improvement_limit_parameters = {
+      improvement_rate_coefficient: 0.01,
+      improvement_rate_solutions_distance: 5
+    }
+
+    assert_equal(
+      {
+        improvement_rate_coefficient: 0.01,
+        improvement_rate_solutions_distance: 5
+      },
+      search_parameters.improvement_limit_parameters
+    )
   end
 
   def test_set_allowed_vehicles_for_index