Function GrowTownWithTunnel

src/town_cmd.cpp:1401–1458

* 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 */

 * @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 TileIndexDiff 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_t tiles = 0; tiles < 4; tiles++) {
			if (!IsValidTile(slope_tile)) return false;
			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 necessitates 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_t tunnel_length = 0;
	TileIndex tunnel_tile = tile; // Iterator 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();
	if (Command<CMD_BUILD_TUNNEL>::Do(CommandFlagsToDCFlags(GetCommandFlags<CMD_BUILD_TUNNEL>()), tile, TRANSPORT_ROAD, rt).Succeeded()) {
		Command<CMD_BUILD_TUNNEL>::Do(CommandFlagsToDCFlags(GetCommandFlags<CMD_BUILD_TUNNEL>()).Set(DoCommandFlag::Execute), tile, TRANSPORT_ROAD, rt);
		return true;
	}

	return false;
}