(svn r16243) -Fix [FS#2875]: CmdBuildTunnel could be called with invalid parameters from the api code, causing crashes later

2 files changed, 20 insertions, 3 deletions

diff --git a/src/ai/api/ai_tunnel.cpp b/src/ai/api/ai_tunnel.cpp
index cc7d95606..ea103ce4e 100644
--- a/src/ai/api/ai_tunnel.cpp
+++ b/src/ai/api/ai_tunnel.cpp
@@ -23,8 +23,21 @@
 	/* If it's a tunnel alread, take the easy way out! */
 	if (IsTunnelTile(tile)) return ::GetOtherTunnelEnd(tile);
 
-	::DoCommand(tile, 0, 0, DC_AUTO, CMD_BUILD_TUNNEL);
-	return _build_tunnel_endtile == 0 ? INVALID_TILE : _build_tunnel_endtile;
+	uint start_z;
+	Slope start_tileh = ::GetTileSlope(tile, &start_z);
+	DiagDirection direction = ::GetInclinedSlopeDirection(start_tileh);
+	if (direction == INVALID_DIAGDIR) return INVALID_TILE;
+
+	TileIndexDiff delta = ::TileOffsByDiagDir(direction);
+	uint end_z;
+	do {
+		tile += delta;
+		if (!::IsValidTile(tile)) return INVALID_TILE;
+
+		::GetTileSlope(tile, &end_z);
+	} while (start_z != end_z);
+
+	return tile;
 }
 
 static void _DoCommandReturnBuildTunnel2(class AIInstance *instance)
diff --git a/src/ai/api/ai_tunnel.hpp b/src/ai/api/ai_tunnel.hpp
index 3e554b68c..de0a7635c 100644
--- a/src/ai/api/ai_tunnel.hpp
+++ b/src/ai/api/ai_tunnel.hpp
@@ -46,11 +46,15 @@ public:
 
 	/**
 	 * Get the tile that exits on the other end of a (would be) tunnel starting
-	 *  at tile.
+	 *  at tile. If there is no 'simple' inclined slope at the start tile,
+	 *  this function will return AIMap::TILE_INVALID.
 	 * @param tile The tile that is an entrance to a tunnel or the tile where you may want to build a tunnel.
 	 * @pre AIMap::IsValidTile(tile).
 	 * @return The TileIndex that is the other end of the (would be) tunnel, or
 	 *  AIMap::TILE_INVALID if no other end was found (can't build tunnel).
+	 * @note Even if this function returns a valid tile, that is no guarantee
+	 *  that building a tunnel will succeed. Use BuildTunnel in AITestMode to
+	 *  check whether a tunnel can actually be build.
 	 */
 	static TileIndex GetOtherTunnelEnd(TileIndex tile);