Connect networks to enable multi-mode transfers.

Removing the per-transit type supply routes allows us to find the best
route from A to B even if the unit needs to switch transit modes along
the way.

The "best" route is the one that will generate better gameplay. That is,
convoys are preferred to ships (use cases for GMT are rare in DCS), and
ships are preferred to airlift (reasons to attack cargo ships are also
rare). Avoiding airlift is also a good strategic choice generally since
it consumes aircraft that could be performing other missions.

The extreme weight against airlift in the pathfinding algorithm could
probably be scaled way down so that airlift would be given preference
over a very long trip, possibly only for urgent transfers.

Later when we add rail that will probably be given the most preference,
but possibly between road and shipping.

https://github.com/Khopa/dcs_liberation/issues/823

game/game.py

@@ -33,6 +33,7 @@ from .navmesh import NavMesh
 from .procurement import AircraftProcurementRequest, ProcurementAi
 from .settings import Settings
 from .theater import ConflictTheater
+from .theater.transitnetwork import TransitNetwork, TransitNetworkBuilder
 from .threatzones import ThreatZones
 from .transfers import PendingTransfers
 from .unitmap import UnitMap
@@ -117,6 +118,9 @@ class Game:
 
         self.conditions = self.generate_conditions()
 
+        self.blue_transit_network = self.compute_transit_network_for(player=True)
+        self.red_transit_network = self.compute_transit_network_for(player=False)
+
         self.blue_procurement_requests: List[AircraftProcurementRequest] = []
         self.red_procurement_requests: List[AircraftProcurementRequest] = []
 
@@ -171,6 +175,11 @@ class Game:
             return self.blue_procurement_requests
         return self.red_procurement_requests
 
+    def transit_network_for(self, player: bool) -> TransitNetwork:
+        if player:
+            return self.blue_transit_network
+        return self.red_transit_network
+
     def generate_conditions(self) -> Conditions:
         return Conditions.generate(
             self.theater, self.current_day, self.current_turn_time_of_day, self.settings
@@ -346,6 +355,7 @@ class Game:
         # Plan flights & combat for next turn
         self.compute_conflicts_position()
         self.compute_threat_zones()
+        self.compute_transit_networks()
         self.ground_planners = {}
 
         self.transfers.order_airlift_assets()
@@ -417,6 +427,13 @@ class Game:
         self.current_group_id += 1
         return self.current_group_id
 
+    def compute_transit_networks(self) -> None:
+        self.blue_transit_network = self.compute_transit_network_for(player=True)
+        self.red_transit_network = self.compute_transit_network_for(player=False)
+
+    def compute_transit_network_for(self, player: bool) -> TransitNetwork:
+        return TransitNetworkBuilder(self.theater, player).build()
+
     def compute_threat_zones(self) -> None:
         self.blue_threat_zone = ThreatZones.for_faction(self, player=True)
         self.red_threat_zone = ThreatZones.for_faction(self, player=False)