Add `StopAreasToWayCostsConverter`

A new class that converts a list of `StopArea`s to `WayCost`s. This will
be called from `Chouette::Route` to get `WayCost`s from its stops, which
will then be JSON serialised and stored in `Route#costs`.

Update `WayCost`:
* Remove comment about calculating the ID automatically. It actually
  needs to be the same as the `JourneyPattern#cost` ID (`key`), which is
  a string with the IDs of the departure and arrival stops.
* Make `#==` check that `id`s are the same, which necessitates making
  `id` a reader.

Refs #6095

3 files changed, 95 insertions, 3 deletions

diff --git a/lib/stop_areas_to_way_costs_converter.rb b/lib/stop_areas_to_way_costs_converter.rb
new file mode 100644
index 000000000..c6a00d31b
--- /dev/null
+++ b/lib/stop_areas_to_way_costs_converter.rb
@@ -0,0 +1,21 @@
+class StopAreasToWayCostsConverter
+  def initialize(stop_areas)
+    @stop_areas = stop_areas
+  end
+
+  def convert
+    @stop_areas.each_cons(2).map do |stop_area_pair|
+      WayCost.new(
+        departure: Geokit::LatLng.new(
+          stop_area_pair[0].latitude,
+          stop_area_pair[0].longitude
+        ),
+        arrival: Geokit::LatLng.new(
+          stop_area_pair[1].latitude,
+          stop_area_pair[1].longitude
+        ),
+        id: "#{stop_area_pair[0].id}-#{stop_area_pair[1].id}"
+      )
+    end
+  end
+end
diff --git a/lib/way_cost.rb b/lib/way_cost.rb
index 5de64177f..9f860308c 100644
--- a/lib/way_cost.rb
+++ b/lib/way_cost.rb
@@ -1,5 +1,5 @@
 class WayCost
-  attr_reader :departure, :arrival
+  attr_reader :departure, :arrival, :id
   attr_accessor :distance, :time
 
   def initialize(
@@ -7,7 +7,7 @@ class WayCost
     arrival:,
     distance: nil,
     time: nil,
-    id: nil  # TODO: calculate ID automatically
+    id: nil
   )
     @departure = departure
     @arrival = arrival
@@ -21,6 +21,7 @@ class WayCost
       @departure == other.departure &&
       @arrival == other.arrival &&
       @distance == other.distance &&
-      @time == other.time
+      @time == other.time &&
+      @id == other.id
   end
 end
diff --git a/spec/lib/stop_areas_to_way_costs_converter_spec.rb b/spec/lib/stop_areas_to_way_costs_converter_spec.rb
new file mode 100644
index 000000000..7e09ede2e
--- /dev/null
+++ b/spec/lib/stop_areas_to_way_costs_converter_spec.rb
@@ -0,0 +1,70 @@
+RSpec.describe StopAreasToWayCostsConverter do
+  describe "#convert" do
+    it "converts a StopArea collection to WayCosts" do
+      route = create(:route_common)
+      coordinates = [
+        [1.5, 1.8],
+        [2.2, 2.1],
+        [3.0, 3.6],
+        [4.9, 4.3]
+      ]
+
+      stop_areas = coordinates.map do |latlng|
+        stop_area = create(
+          :stop_area,
+          area_type: 'zdep',
+          latitude: latlng[0],
+          longitude: latlng[1]
+        )
+
+        create(
+          :stop_point,
+          route: route,
+          stop_area: stop_area
+        )
+
+        stop_area
+      end
+
+      way_costs = [
+        WayCost.new(
+          departure: Geokit::LatLng.new(
+            stop_areas[0].latitude,
+            stop_areas[0].longitude
+          ),
+          arrival: Geokit::LatLng.new(
+            stop_areas[1].latitude,
+            stop_areas[1].longitude
+          ),
+          id: "#{stop_areas[0].id}-#{stop_areas[1].id}"
+        ),
+        WayCost.new(
+          departure: Geokit::LatLng.new(
+            stop_areas[1].latitude,
+            stop_areas[1].longitude
+          ),
+          arrival: Geokit::LatLng.new(
+            stop_areas[2].latitude,
+            stop_areas[2].longitude
+          ),
+          id: "#{stop_areas[1].id}-#{stop_areas[2].id}"
+        ),
+        WayCost.new(
+          departure: Geokit::LatLng.new(
+            stop_areas[2].latitude,
+            stop_areas[2].longitude
+          ),
+          arrival: Geokit::LatLng.new(
+            stop_areas[3].latitude,
+            stop_areas[3].longitude
+          ),
+          id: "#{stop_areas[2].id}-#{stop_areas[3].id}"
+        )
+      ]
+
+      expect(
+        StopAreasToWayCostsConverter.new(stop_areas).convert
+      ).to eq(way_costs)
+    end
+  end
+end