def available_moves(hex_board):
    """
    Get all empty positions of the HexBoard = all available moves.
    :param hex_board: HexBoard class object
    :return: list of all empty positions on the current HexBoard.
    """
    return [(i, j) for i in range(hex_board.size) for j in range(hex_board.size) if hex_board.is_empty((i, j))]