Add: Towns can build tunnels (#8473)

src/town_cmd.cpp

@@ -1109,44 +1109,44 @@ static bool GrowTownWithRoad(const Town *t, TileIndex tile, RoadBits rcmd)
 }
 
 /**
- * Checks if a town road can be continued on the other side of a bridge.
+ * Checks if a town road can be continued into the next tile.
+ *  Road vehicle stations, bridges, and tunnels are fine, as long as they are facing the right direction.
  *
- * @param end_tile The end tile of the bridge
- * @param bridge_dir The direction of the bridge
+ * @param tile The tile where the road would be built
+ * @param road_dir The direction of the road
  * @return true if the road can be continued, else false
  */
-static bool CanRoadContinueAfterBridge(const Town* t, const TileIndex end_tile, const DiagDirection bridge_dir)
+static bool CanRoadContinueIntoNextTile(const Town* t, const TileIndex tile, const DiagDirection road_dir)
 {
-	const int delta = TileOffsByDiagDir(bridge_dir); // +1 tile in the direction of the bridge
-	TileIndex next_tile = end_tile + delta; // The tile beyond the bridge
-	RoadBits rcmd = DiagDirToRoadBits(ReverseDiagDir(bridge_dir));
+	const int delta = TileOffsByDiagDir(road_dir); // +1 tile in the direction of the road
+	TileIndex next_tile = tile + delta; // The tile beyond which must be connectable to the target tile
+	RoadBits rcmd = DiagDirToRoadBits(ReverseDiagDir(road_dir));
 	RoadType rt = GetTownRoadType(t);
 
 	/* Before we try anything, make sure the tile is on the map and not the void. */
 	if (!IsValidTile(next_tile)) return false;
 
-	/* If the next tile is a bridge or tunnel, allow if it's a road bridge/tunnel continuing in the same direction. */
+	/* If the next tile is a bridge or tunnel, allow if it's continuing in the same direction. */
 	if (IsTileType(next_tile, MP_TUNNELBRIDGE)) {
-		return GetTunnelBridgeTransportType(next_tile) == TRANSPORT_ROAD && GetTunnelBridgeDirection(next_tile) == bridge_dir;
+		return GetTunnelBridgeTransportType(next_tile) == TRANSPORT_ROAD && GetTunnelBridgeDirection(next_tile) == road_dir;
 	}
 
 	/* If the next tile is a station, allow if it's a road station facing the proper direction. Otherwise return false. */
 	if (IsTileType(next_tile, MP_STATION)) {
 		/* If the next tile is a road station, allow if it's facing the same direction, otherwise disallow. */
-		return IsRoadStop(next_tile) && GetRoadStopDir(next_tile) == ReverseDiagDir(bridge_dir);
+		return IsRoadStop(next_tile) && GetRoadStopDir(next_tile) == ReverseDiagDir(road_dir);
 	}
 
-	/* If the next tile is a road depot, allow if it's facing the new bridge. */
+	/* If the next tile is a road depot, allow if it's facing the right way. */
 	if (IsTileType(next_tile, MP_ROAD)) {
-		return IsRoadDepot(next_tile) && GetRoadDepotDirection(next_tile) == ReverseDiagDir(bridge_dir);
+		return IsRoadDepot(next_tile) && GetRoadDepotDirection(next_tile) == ReverseDiagDir(road_dir);
 	}
 
 	/* If the next tile is a railroad track, check if towns are allowed to build level crossings.
-	 * If level crossing are not allowed, reject the bridge. Else allow DoCommand to determine if the rail track is buildable. */
+	 * If level crossing are not allowed, reject the construction. Else allow DoCommand to determine if the rail track is buildable. */
 	if (IsTileType(next_tile, MP_RAILWAY) && !_settings_game.economy.allow_town_level_crossings) return false;
 
-	/* If a road tile can be built, the bridge is allowed.
-	 * If not, the bridge is rejected. */
+	/* If a road tile can be built, the construction is allowed. */
 	return DoCommand(next_tile, rcmd | (rt << 4), t->index, DC_AUTO | DC_NO_WATER, CMD_BUILD_ROAD).Succeeded();
 }
 
@@ -1208,7 +1208,7 @@ static bool GrowTownWithBridge(const Town *t, const TileIndex tile, const DiagDi
 	if (bridge_length == 1) return false;
 
 	/* Make sure the road can be continued past the bridge. At this point, bridge_tile holds the end tile of the bridge. */
-	if (!CanRoadContinueAfterBridge(t, bridge_tile, bridge_dir)) return false;
+	if (!CanRoadContinueIntoNextTile(t, bridge_tile, bridge_dir)) return false;
 
 	for (uint8 times = 0; times <= 22; times++) {
 		byte bridge_type = RandomRange(MAX_BRIDGES - 1);
@@ -1225,6 +1225,75 @@ static bool GrowTownWithBridge(const Town *t, const TileIndex tile, const DiagDi
 	return false;
 }
 
+/**
+ * Grows the town with a tunnel.
+ *  First we check if a tunnel is reasonable.
+ *  If so we check if we are able to build it.
+ *
+ * @param t The current town
+ * @param tile The current tile
+ * @param tunnel_dir The valid direction in which to grow a tunnel
+ * @return true if a tunnel has been built, else false
+ */
+static bool GrowTownWithTunnel(const Town* t, const TileIndex tile, const DiagDirection tunnel_dir)
+{
+	assert(tunnel_dir < DIAGDIR_END);
+
+	Slope slope = GetTileSlope(tile);
+
+	/* Only consider building a tunnel if the starting tile is sloped properly. */
+	if (slope != InclinedSlope(tunnel_dir)) return false;
+
+	/* Assure that the tunnel is connectable to the start side */
+	if (!(GetTownRoadBits(TileAddByDiagDir(tile, ReverseDiagDir(tunnel_dir))) & DiagDirToRoadBits(tunnel_dir))) return false;
+
+	const int delta = TileOffsByDiagDir(tunnel_dir);
+	int max_tunnel_length = 0;
+
+	/* There are two conditions for building tunnels: Under a mountain and under an obstruction. */
+	if (CanRoadContinueIntoNextTile(t, tile, tunnel_dir)) {
+		/* Only tunnel under a mountain if the slope is continuous for at least 4 tiles. We want tunneling to be a last resort for large hills. */
+		TileIndex slope_tile = tile;
+		for (uint8 tiles = 0; tiles < 4; tiles++) {
+			slope = GetTileSlope(slope_tile);
+			if (slope != InclinedSlope(tunnel_dir) && !IsSteepSlope(slope) && !IsSlopeWithOneCornerRaised(slope)) return false;
+			slope_tile += delta;
+		}
+
+		/* More population means longer tunnels, but make sure we can at least cover the smallest mountain which neccesitates tunneling. */
+		max_tunnel_length = (t->cache.population / 1000) + 7;
+	} else {
+		/* When tunneling under an obstruction, the length limit is 5, enough to tunnel under a four-track railway. */
+		max_tunnel_length = 5;
+	}
+
+	uint8 tunnel_length = 0;
+	TileIndex tunnel_tile = tile; // Iteratator to store the other end tile of the tunnel.
+
+	/* Find the end tile of the tunnel for length and continuation checks. */
+	do {
+		if (tunnel_length++ >= max_tunnel_length) return false;
+		tunnel_tile += delta;
+		/* The tunnel ends when start and end tiles are the same height. */
+	} while (IsValidTile(tunnel_tile) && GetTileZ(tile) != GetTileZ(tunnel_tile));
+
+	/* Don't allow a tunnel where the start and end tiles are adjacent. */
+	if (tunnel_length == 1) return false;
+
+	/* Make sure the road can be continued past the tunnel. At this point, tunnel_tile holds the end tile of the tunnel. */
+	if (!CanRoadContinueIntoNextTile(t, tunnel_tile, tunnel_dir)) return false;
+
+	/* Attempt to build the tunnel. Return false if it fails to let the town build a road instead. */
+	RoadType rt = GetTownRoadType(t);
+	if (DoCommand(tile, rt | (TRANSPORT_ROAD << 8), 0, CommandFlagsToDCFlags(GetCommandFlags(CMD_BUILD_TUNNEL)), CMD_BUILD_TUNNEL).Succeeded()) {
+		DoCommand(tile, rt | (TRANSPORT_ROAD << 8), 0, DC_EXEC | CommandFlagsToDCFlags(GetCommandFlags(CMD_BUILD_TUNNEL)), CMD_BUILD_TUNNEL);
+		_grow_town_result = GROWTH_SUCCEED;
+		return true;
+	}
+
+	return false;
+}
+
 /**
  * Checks whether at least one surrounding roads allows to build a house here
  *
@@ -1460,10 +1529,11 @@ static void GrowTownInTile(TileIndex *tile_ptr, RoadBits cur_rb, DiagDirection t
 	rcmd = CleanUpRoadBits(tile, rcmd);
 	if (rcmd == ROAD_NONE) return;
 
-	/* Only use the target direction for bridges to ensure they're connected.
+	/* Only use the target direction for bridges and tunnels to ensure they're connected.
 	 * The target_dir is as computed previously according to town layout, so
 	 * it will match it perfectly. */
 	if (GrowTownWithBridge(t1, tile, target_dir)) return;
+	if (GrowTownWithTunnel(t1, tile, target_dir)) return;
 
 	GrowTownWithRoad(t1, tile, rcmd);
 }