defprotocol RDF.Data do
  @moduledoc """
  An abstraction over the different data structures for collections of RDF statements.
  """

  @doc """
  Adds statements to a RDF data structure.

  As opposed to the specific `add` functions on the RDF data structures, which
  always return the same structure type than the first argument, `merge` might
  result in another RDF data structure, eg. merging two `RDF.Description` with
  different subjects results in a `RDF.Graph` or adding a quad to a `RDF.Graph`
  with a different name than the graph context of the quad results in a
  `RDF.Dataset`. But it is always guaranteed that the resulting structure has
  a `RDF.Data` implementation.
  """
  def merge(data, statements)

  @doc """
  Deletes statements from a RDF data structure.

  As opposed to the `delete` functions on RDF data structures directly, this
  function only deletes exactly matching structures.

  TODO: rename this function to make the different semantics explicit
  """
  def delete(data, statements)

  @doc """
  Deletes one statement from a RDF data structure and returns a tuple with deleted statement and the changed data structure.
  """
  def pop(data)

  @doc """
  Checks if the given statement exists within a RDF data structure.
  """
  def include?(data, statements)

  @doc """
  Returns the list of all statements of a RDF data structure.
  """
  def statements(data)

  @doc """
  Returns a `RDF.Description` of the given subject.

  Note: On a `RDF.Dataset` this will return an aggregated `RDF.Description` with
  the statements about this subject from all graphs.
  """
  def description(data, subject)

  @doc """
  Returns the set of all resources which are subject of the statements of a RDF data structure.
  """
  def subjects(data)

  @doc """
  Returns the set of all properties used within the statements of RDF data structure.
  """
  def predicates(data)

  @doc """
  Returns the  set of all resources used in the objects within the statements of a RDF data structure.
  """
  def objects(data)

  @doc """
  Returns the set of all resources used within the statements of a RDF data structure
  """
  def resources(data)

  @doc """
  Returns the count of all resources which are subject of the statements of a RDF data structure.
  """
  def subject_count(data)

  @doc """
  Returns the count of all statements of a RDF data structure.
  """
  def statement_count(data)

end

defimpl RDF.Data, for: RDF.Description do
  def merge(%RDF.Description{subject: subject} = description, {s, _, _} = triple) do
    with ^subject <- RDF.Statement.convert_subject(s) do
      RDF.Description.add(description, triple)
    else
      _ ->
        RDF.Graph.new(description)
        |> RDF.Graph.add(triple)
    end
  end

  def merge(description, {_, _, _, _} = quad),
    do: RDF.Dataset.new(description) |> RDF.Dataset.add(quad)

  def merge(%RDF.Description{subject: subject} = description,
            %RDF.Description{subject: other_subject} = other_description)
    when other_subject == subject,
    do: RDF.Description.add(description, other_description)

  def merge(description, %RDF.Description{} = other_description),
    do: RDF.Graph.new(description) |> RDF.Graph.add(other_description)

  def merge(description, %RDF.Graph{} = graph),
    do: RDF.Data.merge(graph, description)

  def merge(description, %RDF.Dataset{} = dataset),
    do: RDF.Data.merge(dataset, description)


  def delete(%RDF.Description{subject: subject} = description,
             %RDF.Description{subject: other_subject})
    when subject != other_subject,     do: description
  def delete(description, statements), do: RDF.Description.delete(description, statements)


  def pop(description),                do: RDF.Description.pop(description)

  def include?(description, statements),
    do: RDF.Description.include?(description, statements)

  def statements(description), do: RDF.Description.statements(description)

  def description(%RDF.Description{subject: subject} = description, s) do
    with ^subject <- RDF.Statement.convert_subject(s) do
      description
    else
      _ -> RDF.Description.new(s)
    end
  end

  def subjects(%RDF.Description{subject: subject}), do: MapSet.new([subject])
  def predicates(description), do: RDF.Description.predicates(description)
  def objects(description),    do: RDF.Description.objects(description)

  def resources(%RDF.Description{subject: subject} = description),
    do: RDF.Description.resources(description) |> MapSet.put(subject)

  def subject_count(_),             do: 1
  def statement_count(description), do: RDF.Description.count(description)
end


defimpl RDF.Data, for: RDF.Graph do
  def merge(%RDF.Graph{name: name} = graph, {_, _, _, graph_context} = quad) do
    with ^name <- RDF.Statement.convert_graph_name(graph_context) do
      RDF.Graph.add(graph, quad)
    else
      _ ->
        RDF.Dataset.new(graph)
        |> RDF.Dataset.add(quad)
    end
  end

  def merge(graph, {_, _, _} = triple),
    do: RDF.Graph.add(graph, triple)

  def merge(description, {_, _, _, _} = quad),
    do: RDF.Dataset.new(description) |> RDF.Dataset.add(quad)

  def merge(graph, %RDF.Description{} = description),
    do: RDF.Graph.add(graph, description)

  def merge(%RDF.Graph{name: name} = graph,
            %RDF.Graph{name: other_name} = other_graph)
    when other_name == name,
    do: RDF.Graph.add(graph, other_graph)

  def merge(graph, %RDF.Graph{} = other_graph),
    do: RDF.Dataset.new(graph) |> RDF.Dataset.add(other_graph)

  def merge(graph, %RDF.Dataset{} = dataset),
    do: RDF.Data.merge(dataset, graph)


  def delete(%RDF.Graph{name: name} = graph, %RDF.Graph{name: other_name})
    when name != other_name,     do: graph
  def delete(graph, statements), do: RDF.Graph.delete(graph, statements)

  def pop(graph),                do: RDF.Graph.pop(graph)

  def include?(graph, statements), do: RDF.Graph.include?(graph, statements)

  def statements(graph), do: RDF.Graph.statements(graph)

  def description(graph, subject),
    do: RDF.Graph.description(graph, subject) || RDF.Description.new(subject)

  def subjects(graph),   do: RDF.Graph.subjects(graph)
  def predicates(graph), do: RDF.Graph.predicates(graph)
  def objects(graph),    do: RDF.Graph.objects(graph)
  def resources(graph),  do: RDF.Graph.resources(graph)

  def subject_count(graph),   do: RDF.Graph.subject_count(graph)
  def statement_count(graph), do: RDF.Graph.triple_count(graph)
end


defimpl RDF.Data, for: RDF.Dataset do
  def merge(dataset, {_, _, _} = triple),
    do: RDF.Dataset.add(dataset, triple)
  def merge(dataset, {_, _, _, _} = quad),
    do: RDF.Dataset.add(dataset, quad)
  def merge(dataset, %RDF.Description{} = description),
    do: RDF.Dataset.add(dataset, description)
  def merge(dataset, %RDF.Graph{} = graph),
    do: RDF.Dataset.add(dataset, graph)
  def merge(dataset, %RDF.Dataset{} = other_dataset),
    do: RDF.Dataset.add(dataset, other_dataset)


  def delete(%RDF.Dataset{name: name} = dataset, %RDF.Dataset{name: other_name})
    when name != other_name,       do: dataset
  def delete(dataset, statements), do: RDF.Dataset.delete(dataset, statements)

  def pop(dataset),                do: RDF.Dataset.pop(dataset)

  def include?(dataset, statements), do: RDF.Dataset.include?(dataset, statements)

  def statements(dataset), do: RDF.Dataset.statements(dataset)

  def description(dataset, subject) do
    with subject = RDF.Statement.convert_subject(subject) do
      Enum.reduce RDF.Dataset.graphs(dataset), RDF.Description.new(subject), fn
        %RDF.Graph{descriptions: %{^subject => graph_description}}, description ->
          RDF.Description.add(description, graph_description)
        _, description ->
          description
        end
    end
  end

  def subjects(dataset),   do: RDF.Dataset.subjects(dataset)
  def predicates(dataset), do: RDF.Dataset.predicates(dataset)
  def objects(dataset),    do: RDF.Dataset.objects(dataset)
  def resources(dataset),  do: RDF.Dataset.resources(dataset)

  def subject_count(dataset),   do: dataset |> subjects |> Enum.count
  def statement_count(dataset), do: RDF.Dataset.statement_count(dataset)
end