/** Directions to search towards given track bits and the ship's enter direction. */
static const DiagDirection _ship_search_directions[6][4] = {
{ DIAGDIR_NE, INVALID_DIAGDIR, DIAGDIR_SW, INVALID_DIAGDIR },
{ INVALID_DIAGDIR, DIAGDIR_SE, INVALID_DIAGDIR, DIAGDIR_NW },
{ INVALID_DIAGDIR, DIAGDIR_NE, DIAGDIR_NW, INVALID_DIAGDIR },