Make bridges and tunnels significantly more costly

src/road.cpp

@@ -816,28 +816,54 @@ static void PublicRoad_FoundEndNode(AyStar *aystar, OpenListNode *current)
 	}
 }
 
-static const int32 BASE_COST_PER_TILE = 1;  // Cost for existing road or tunnel/bridge.
-static const int32 COST_FOR_NEW_ROAD = 100; // Cost for building a new road.
-static const int32 COST_FOR_SLOPE = 50;     // Additional cost if the road heads up or down a slope.
+static const int32 BASE_COST_PER_TILE  = 1;      // Cost for existing road or tunnel/bridge.
+static const int32 COST_FOR_NEW_ROAD   = 100;    // Cost for building a new road.
+static const int32 COST_FOR_NEW_BRIDGE = 500;    // Cost for building a new bridge (per tile).
+static const int32 COST_FOR_NEW_TUNNEL = 500;    // Cost for building a new tunnel (per tile, with non-linear increase with length as in CmdBuildTunnel costs).
+static const int32 COST_FOR_SLOPE      = 50;     // Additional cost if the road heads up or down a slope.
 
 /** AyStar callback for getting the cost of the current node. */
 static int32 PublicRoad_CalculateG(AyStar *, AyStarNode *current, OpenListNode *parent)
 {
 	int32 cost = 0;
 
+	int32 distance = DistanceManhattan(parent->path.node.tile, current->tile);
+
 	if (IsTileType(current->tile, MP_ROAD) || IsTileType(current->tile, MP_TUNNELBRIDGE)) {
-		cost += (DistanceManhattan(parent->path.node.tile, current->tile)) * BASE_COST_PER_TILE;
+		cost += distance * BASE_COST_PER_TILE;
 	} else {
-		cost += (DistanceManhattan(parent->path.node.tile, current->tile)) * COST_FOR_NEW_ROAD;
+		if (distance > 1) {
+			/* We are planning to build a bridge or tunnel. Make that much more expensive. */
+			const DiagDirection road_direction = DiagdirBetweenTiles(parent->path.node.tile, current->tile);
+			if (IsUpwardsSlope(parent->path.node.tile, road_direction)) {
+				/* Tunnel */
+				/* Prevent excessively long tunnels */
+				if (distance > 30) return AYSTAR_INVALID_NODE;
+
+				cost += 2 * COST_FOR_NEW_TUNNEL;
+				int32 middle_cost = 0;
+				int tiles_coef = 3;
+				int tiles_bump = 25;
+				for (int i = 1; i <= distance; i++) {
+					if (i == tiles_bump) {
+						tiles_coef++;
+						tiles_bump *= 2;
+					}
+					middle_cost += COST_FOR_NEW_TUNNEL;
+					middle_cost += middle_cost >> tiles_coef;
+				}
+				cost += middle_cost;
+			} else {
+				/* Bridge */
+				cost += distance * COST_FOR_NEW_BRIDGE;
+			}
+		} else {
+			cost += distance * COST_FOR_NEW_ROAD;
+		}
 
 		if (GetTileZ(parent->path.node.tile) != GetTileZ(current->tile)) {
 			cost += COST_FOR_SLOPE;
 		}
-
-		if (DistanceManhattan(parent->path.node.tile, current->tile) > 1) {
-			// We are planning to build a bridge or tunnel. Make that a bit more expensive.
-			cost += 6 * COST_FOR_SLOPE;
-		}
 	}
 
 	if (_settings_game.game_creation.build_public_roads == PRC_AVOID_CURVES &&