code: private static Collection<TileRef> getTiles(Map<Circuit, Collection<TileRef>> circuits, Circuit circuit)
      {
          if (!circuits.containsKey(circuit))
          {
              circuits.put(circuit, new HashSet<TileRef>());
          }
          return circuits.get(circuit);
      }
docs: Get the tiles for the circuit. Create empty list if new circuit.
  
  @param circuits The circuits data.
  @param circuit The circuit type.
  @return The associated tiles.