Add initial notebook data structure (#4)

* Add initial notebook data structure

* Move required frields out of metadata

lib/live_book/notebook.ex

@@ -0,0 +1,39 @@
+defmodule LiveBook.Notebook do
+  @moduledoc """
+  Data structure representing a notebook.
+
+  A notebook it's just the representation and roughly
+  maps to a file that the user can edit.
+  A notebook *session* is a living process that holds a specific
+  notebook instance and allows users to collaboratively apply
+  changes to this notebook.
+
+  A notebook is divided into a set of isolated *sections*.
+  """
+
+  defstruct [:name, :version, :sections, :metadata]
+
+  alias LiveBook.Notebook.Section
+
+  @type t :: %__MODULE__{
+          name: String.t(),
+          version: String.t(),
+          sections: list(Section.t()),
+          metadata: %{atom() => term()}
+        }
+
+  @version "1.0"
+
+  @doc """
+  Returns a blank notebook.
+  """
+  @spec new() :: t()
+  def new() do
+    %__MODULE__{
+      name: "Untitled notebook",
+      version: @version,
+      sections: [],
+      metadata: %{}
+    }
+  end
+end

lib/live_book/notebook/cell.ex

@@ -0,0 +1,39 @@
+defmodule LiveBook.Notebook.Cell do
+  @moduledoc """
+  Data structure representing a single cell in a notebook.
+
+  A cell is the smallest unit of work in a notebook.
+  It primarly consists of text content that the user can edit
+  and may potentially produce some output (e.g. during code execution).
+  """
+
+  defstruct [:id, :type, :source, :outputs, :metadata]
+
+  alias LiveBook.Utils
+
+  @type cell_id :: Utils.id()
+  @type cell_type :: :markdown | :elixir
+
+  @type t :: %__MODULE__{
+          id: cell_id(),
+          type: cell_type(),
+          source: String.t(),
+          # TODO: expand on this
+          outputs: list(),
+          metadata: %{atom() => term()}
+        }
+
+  @doc """
+  Returns an empty cell of the given type.
+  """
+  @spec new(cell_type()) :: t()
+  def new(type) do
+    %__MODULE__{
+      id: Utils.random_id(),
+      type: type,
+      source: "",
+      outputs: [],
+      metadata: %{}
+    }
+  end
+end

lib/live_book/notebook/section.ex

@@ -0,0 +1,35 @@
+defmodule LiveBook.Notebook.Section do
+  @moduledoc """
+  Data structure representing a single section in a notebook.
+
+  Each section contains a number of cells and is isolated
+  in the sense that cells don't interfere with cells in other sections.
+  """
+
+  defstruct [:id, :name, :cells, :metadata]
+
+  alias LiveBook.Notebook.Cell
+  alias LiveBook.Utils
+
+  @type section_id :: Utils.id()
+
+  @type t :: %__MODULE__{
+          id: section_id(),
+          name: String.t(),
+          cells: list(Cell.t()),
+          metadata: %{atom() => term()}
+        }
+
+  @doc """
+  Returns a blank section.
+  """
+  @spec new() :: t()
+  def new() do
+    %__MODULE__{
+      id: Utils.random_id(),
+      name: "Section",
+      cells: [],
+      metadata: %{}
+    }
+  end
+end