defmodule RDF.Dataset do
  @moduledoc """
  A set of `RDF.Graph`s.

  It may have multiple named graphs and at most one unnamed ("default") graph.

  `RDF.Dataset` implements:

  - Elixir's `Access` behaviour
  - Elixir's `Enumerable` protocol
  - Elixir's `Inspect` protocol
  - the `RDF.Data` protocol

  """

  defstruct name: nil, graphs: %{}

  @behaviour Access

  alias RDF.{Description, Graph, IRI, Statement}
  import RDF.Statement
  import RDF.Utils

  @type graph_name :: IRI.t() | nil

  @type t :: %__MODULE__{
          name: graph_name,
          graphs: %{graph_name => Graph.t()}
        }

  @type input :: Graph.input() | t

  @type update_graph_fun :: (Graph.t() -> {Graph.t(), input} | :pop)

  @doc """
  Creates an empty unnamed `RDF.Dataset`.
  """
  @spec new :: t
  def new, do: %__MODULE__{}

  @doc """
  Creates an `RDF.Dataset`.

  If a keyword list is given an empty dataset is created.
  Otherwise an unnamed dataset initialized with the given data is created.

  See `new/2` for available arguments and the different ways to provide data.

  ## Examples

      RDF.Dataset.new({EX.S, EX.p, EX.O})

      RDF.Dataset.new(name: EX.GraphName)

  """
  @spec new(input | keyword) :: t
  def new(data_or_options)

  def new(data_or_options)
      when is_list(data_or_options) and length(data_or_options) != 0 do
    if Keyword.keyword?(data_or_options) do
      new([], data_or_options)
    else
      new(data_or_options, [])
    end
  end

  def new(data), do: new(data, [])

  @doc """
  Creates an `RDF.Dataset` initialized with data.

  The initial RDF triples can be provided in any form accepted by `add/3`.

  Available options:

  - `name`: the name of the dataset to be created

  """
  @spec new(input, keyword) :: t
  def new(data, options)

  def new(%__MODULE__{} = graph, options) do
    %__MODULE__{graph | name: options |> Keyword.get(:name) |> coerce_graph_name()}
  end

  def new(data, options) do
    %__MODULE__{}
    |> new(options)
    |> add(data)
  end

  @doc """
  Returns the dataset name IRI of `dataset`.
  """
  @spec name(t) :: RDF.Statement.graph_name()
  def name(%__MODULE__{} = dataset), do: dataset.name

  @doc """
  Changes the dataset name of `dataset`.
  """
  @spec change_name(t, RDF.Statement.coercible_graph_name()) :: t
  def change_name(%__MODULE__{} = dataset, new_name) do
    %__MODULE__{dataset | name: coerce_graph_name(new_name)}
  end

  defp destination_graph(opts, default \\ nil) do
    opts
    |> Keyword.get(:graph, default)
    |> coerce_graph_name()
  end

  @doc """
  Adds triples and quads to a `RDF.Dataset`.

  The triples can be provided in any form accepted by `add/2`.

  - as a single statement tuple
  - an `RDF.Description`
  - an `RDF.Graph`
  - an `RDF.Dataset`
  - or a list with any combination of the former

  The `graph` option allows to set a different destination graph to which the
  statements should be added, ignoring the graph context of given quads or the
  name of given graphs.
  """
  @spec add(t, input, keyword) :: t
  def add(dataset, statements, opts \\ [])

  def add(dataset, {_, _, _, graph} = quad, opts),
    do: do_add(dataset, destination_graph(opts, graph), quad)

  def add(dataset, %Graph{} = graph, opts),
    do: do_add(dataset, destination_graph(opts, graph.name), graph)

  def add(%__MODULE__{} = dataset, %__MODULE__{} = other_dataset, opts) do
    other_dataset
    |> graphs()
    |> Enum.reduce(dataset, &add(&2, &1, opts))
  end

  def add(dataset, input, opts) when is_list(input) or is_map(input) do
    Enum.reduce(input, dataset, &add(&2, &1, opts))
  end

  def add(dataset, input, opts), do: do_add(dataset, destination_graph(opts), input)

  defp do_add(dataset, graph_name, input) do
    %__MODULE__{
      dataset
      | graphs:
          lazy_map_update(
            dataset.graphs,
            graph_name,
            # when new:
            fn -> Graph.new(input, name: graph_name) end,
            # when update:
            fn graph -> Graph.add(graph, input) end
          )
    }
  end

  @doc """
  Adds statements to a `RDF.Dataset` and overwrites all existing statements with the same subjects and predicates in the specified graph context.

  ## Examples

      iex> dataset = RDF.Dataset.new({EX.S, EX.P1, EX.O1})
      ...> RDF.Dataset.put(dataset, {EX.S, EX.P1, EX.O2})
      RDF.Dataset.new({EX.S, EX.P1, EX.O2})
      iex> RDF.Dataset.put(dataset, {EX.S, EX.P2, EX.O2})
      RDF.Dataset.new([{EX.S, EX.P1, EX.O1}, {EX.S, EX.P2, EX.O2}])
      iex> RDF.Dataset.new([{EX.S1, EX.P1, EX.O1}, {EX.S2, EX.P2, EX.O2}]) |>
      ...>   RDF.Dataset.put([{EX.S1, EX.P2, EX.O3}, {EX.S2, EX.P2, EX.O3}])
      RDF.Dataset.new([{EX.S1, EX.P1, EX.O1}, {EX.S1, EX.P2, EX.O3}, {EX.S2, EX.P2, EX.O3}])
  """
  @spec put(t, input, Statement.coercible_graph_name() | boolean | nil) :: t
  def put(dataset, statements, graph_context \\ false)

  def put(%__MODULE__{} = dataset, {subject, predicate, objects}, false),
    do: put(dataset, {subject, predicate, objects, nil})

  def put(%__MODULE__{} = dataset, {subject, predicate, objects}, graph_context),
    do: put(dataset, {subject, predicate, objects, graph_context})

  def put(
        %__MODULE__{name: name, graphs: graphs},
        {subject, predicate, objects, graph_context},
        false
      ) do
    with graph_context = coerce_graph_name(graph_context) do
      new_graph =
        case graphs[graph_context] do
          graph = %Graph{} ->
            Graph.put(graph, {subject, predicate, objects})

          nil ->
            Graph.new({subject, predicate, objects}, name: graph_context)
        end

      %__MODULE__{name: name, graphs: Map.put(graphs, graph_context, new_graph)}
    end
  end

  def put(%__MODULE__{} = dataset, {subject, predicate, objects, _}, graph_context),
    do: put(dataset, {subject, predicate, objects, graph_context}, false)

  def put(%__MODULE__{} = dataset, statements, false) when is_list(statements) do
    do_put(
      dataset,
      Enum.group_by(
        statements,
        fn
          {s, _, _} -> {s, nil}
          {s, _, _, nil} -> {s, nil}
          {s, _, _, c} -> {s, coerce_graph_name(c)}
        end,
        fn
          {_, p, o, _} -> {p, o}
          {_, p, o} -> {p, o}
        end
      )
    )
  end

  def put(%__MODULE__{} = dataset, statements, graph_context) when is_list(statements) do
    with graph_context = coerce_graph_name(graph_context) do
      do_put(
        dataset,
        Enum.group_by(
          statements,
          fn
            {s, _, _, _} -> {s, graph_context}
            {s, _, _} -> {s, graph_context}
          end,
          fn
            {_, p, o, _} -> {p, o}
            {_, p, o} -> {p, o}
          end
        )
      )
    end
  end

  def put(%__MODULE__{} = dataset, %Description{} = description, false),
    do: put(dataset, description, nil)

  def put(%__MODULE__{name: name, graphs: graphs}, %Description{} = description, graph_context) do
    with graph_context = coerce_graph_name(graph_context) do
      updated_graph =
        Map.get(graphs, graph_context, Graph.new(name: graph_context))
        |> Graph.put(description)

      %__MODULE__{
        name: name,
        graphs: Map.put(graphs, graph_context, updated_graph)
      }
    end
  end

  def put(%__MODULE__{name: name, graphs: graphs}, %Graph{} = graph, false) do
    %__MODULE__{
      name: name,
      graphs:
        Map.update(graphs, graph.name, graph, fn current ->
          Graph.put(current, graph)
        end)
    }
  end

  def put(%__MODULE__{} = dataset, %Graph{} = graph, graph_context),
    do: put(dataset, %Graph{graph | name: coerce_graph_name(graph_context)}, false)

  def put(%__MODULE__{} = dataset, %__MODULE__{} = other_dataset, graph_context) do
    with graph_context = graph_context && coerce_graph_name(graph_context) do
      Enum.reduce(graphs(other_dataset), dataset, fn graph, dataset ->
        put(dataset, graph, graph_context)
      end)
    end
  end

  defp do_put(%__MODULE__{} = dataset, statements) when is_map(statements) do
    Enum.reduce(statements, dataset, fn {subject_with_context, predications}, dataset ->
      do_put(dataset, subject_with_context, predications)
    end)
  end

  defp do_put(%__MODULE__{name: name, graphs: graphs}, {subject, graph_context}, predications)
       when is_list(predications) do
    with graph_context = coerce_graph_name(graph_context) do
      graph = Map.get(graphs, graph_context, Graph.new(name: graph_context))

      new_graphs =
        graphs
        |> Map.put(graph_context, Graph.put(graph, {subject, predications}))

      %__MODULE__{name: name, graphs: new_graphs}
    end
  end

  @doc """
  Deletes statements from a `RDF.Dataset`.

  The `graph` option allows to set a different destination graph from which the
  statements should be deleted, ignoring the graph context of given quads or the
  name of given graphs.

  Note: When the statements to be deleted are given as another `RDF.Dataset`,
  the dataset name must not match dataset name of the dataset from which the statements
  are deleted. If you want to delete only datasets with matching names, you can
  use `RDF.Data.delete/2`.
  """
  @spec delete(t, input, keyword) :: t
  def delete(dataset, statements, opts \\ [])

  def delete(dataset, {_, _, _, graph} = quad, opts),
    do: do_delete(dataset, destination_graph(opts, graph), quad)

  def delete(dataset, %Graph{} = graph, opts),
    do: do_delete(dataset, destination_graph(opts, graph.name), graph)

  def delete(%__MODULE__{} = dataset, %__MODULE__{} = other_dataset, opts) do
    other_dataset
    |> graphs()
    |> Enum.reduce(dataset, &delete(&2, &1, opts))
  end

  def delete(dataset, input, opts) when is_list(input) or is_map(input) do
    Enum.reduce(input, dataset, &delete(&2, &1, opts))
  end

  def delete(dataset, input, opts), do: do_delete(dataset, destination_graph(opts), input)

  defp do_delete(dataset, graph_name, input) do
    if existing_graph = dataset.graphs[graph_name] do
      new_graph = Graph.delete(existing_graph, input)

      %__MODULE__{
        dataset
        | graphs:
            if Enum.empty?(new_graph) do
              Map.delete(dataset.graphs, graph_name)
            else
              Map.put(dataset.graphs, graph_name, new_graph)
            end
      }
    else
      dataset
    end
  end

  @doc """
  Deletes the given graph.
  """
  @spec delete_graph(t, Statement.graph_name() | [Statement.graph_name()] | nil) :: t
  def delete_graph(graph, graph_names)

  def delete_graph(%__MODULE__{} = dataset, graph_names) when is_list(graph_names) do
    Enum.reduce(graph_names, dataset, fn graph_name, dataset ->
      delete_graph(dataset, graph_name)
    end)
  end

  def delete_graph(%__MODULE__{name: name, graphs: graphs}, graph_name) do
    with graph_name = coerce_graph_name(graph_name) do
      %__MODULE__{name: name, graphs: Map.delete(graphs, graph_name)}
    end
  end

  @doc """
  Deletes the default graph.
  """
  @spec delete_default_graph(t) :: t
  def delete_default_graph(%__MODULE__{} = graph),
    do: delete_graph(graph, nil)

  @doc """
  Fetches the `RDF.Graph` with the given name.

  When a graph with the given name can not be found can not be found `:error` is returned.

  ## Examples

      iex> dataset = RDF.Dataset.new([{EX.S1, EX.P1, EX.O1, EX.Graph}, {EX.S2, EX.P2, EX.O2}])
      ...> RDF.Dataset.fetch(dataset, EX.Graph)
      {:ok, RDF.Graph.new({EX.S1, EX.P1, EX.O1}, name: EX.Graph)}
      iex> RDF.Dataset.fetch(dataset, nil)
      {:ok, RDF.Graph.new({EX.S2, EX.P2, EX.O2})}
      iex> RDF.Dataset.fetch(dataset, EX.Foo)
      :error
  """
  @impl Access
  @spec fetch(t, Statement.graph_name() | nil) :: {:ok, Graph.t()} | :error
  def fetch(%__MODULE__{graphs: graphs}, graph_name) do
    Access.fetch(graphs, coerce_graph_name(graph_name))
  end

  @doc """
  Fetches the `RDF.Graph` with the given name.

  When a graph with the given name can not be found can not be found the optionally
  given default value or `nil` is returned

  ## Examples

      iex> dataset = RDF.Dataset.new([{EX.S1, EX.P1, EX.O1, EX.Graph}, {EX.S2, EX.P2, EX.O2}])
      ...> RDF.Dataset.get(dataset, EX.Graph)
      RDF.Graph.new({EX.S1, EX.P1, EX.O1}, name: EX.Graph)
      iex> RDF.Dataset.get(dataset, nil)
      RDF.Graph.new({EX.S2, EX.P2, EX.O2})
      iex> RDF.Dataset.get(dataset, EX.Foo)
      nil
      iex> RDF.Dataset.get(dataset, EX.Foo, :bar)
      :bar
  """
  @spec get(t, Statement.graph_name() | nil, Graph.t() | nil) :: Graph.t() | nil
  def get(%__MODULE__{} = dataset, graph_name, default \\ nil) do
    case fetch(dataset, graph_name) do
      {:ok, value} -> value
      :error -> default
    end
  end

  @doc """
  The graph with given name.
  """
  @spec graph(t, Statement.graph_name() | nil) :: Graph.t()
  def graph(%__MODULE__{graphs: graphs}, graph_name),
    do: Map.get(graphs, coerce_graph_name(graph_name))

  @doc """
  The default graph of a `RDF.Dataset`.
  """
  @spec default_graph(t) :: Graph.t()
  def default_graph(%__MODULE__{graphs: graphs}),
    do: Map.get(graphs, nil, Graph.new())

  @doc """
  The set of all graphs.
  """
  @spec graphs(t) :: [Graph.t()]
  def graphs(%__MODULE__{graphs: graphs}), do: Map.values(graphs)

  @doc """
  Gets and updates the graph with the given name, in a single pass.

  Invokes the passed function on the `RDF.Graph` with the given name;
  this function should return either `{graph_to_return, new_graph}` or `:pop`.

  If the passed function returns `{graph_to_return, new_graph}`, the
  return value of `get_and_update` is `{graph_to_return, new_dataset}` where
  `new_dataset` is the input `Dataset` updated with `new_graph` for
  the given name.

  If the passed function returns `:pop` the graph with the given name is
  removed and a `{removed_graph, new_dataset}` tuple gets returned.

  ## Examples

      iex> dataset = RDF.Dataset.new({EX.S, EX.P, EX.O, EX.Graph})
      ...> RDF.Dataset.get_and_update(dataset, EX.Graph, fn current_graph ->
      ...>     {current_graph, {EX.S, EX.P, EX.NEW}}
      ...>   end)
      {RDF.Graph.new({EX.S, EX.P, EX.O}, name: EX.Graph), RDF.Dataset.new({EX.S, EX.P, EX.NEW, EX.Graph})}
  """
  @impl Access
  @spec get_and_update(t, Statement.graph_name() | nil, update_graph_fun) :: {Graph.t(), input}
  def get_and_update(%__MODULE__{} = dataset, graph_name, fun) do
    with graph_context = coerce_graph_name(graph_name) do
      case fun.(get(dataset, graph_context)) do
        {old_graph, new_graph} ->
          {old_graph, put(dataset, new_graph, graph_context)}

        :pop ->
          pop(dataset, graph_context)

        other ->
          raise "the given function must return a two-element tuple or :pop, got: #{
                  inspect(other)
                }"
      end
    end
  end

  @doc """
  Pops an arbitrary statement from a `RDF.Dataset`.
  """
  @spec pop(t) :: {Statement.t() | nil, t}
  def pop(dataset)

  def pop(%__MODULE__{graphs: graphs} = dataset)
      when graphs == %{},
      do: {nil, dataset}

  def pop(%__MODULE__{name: name, graphs: graphs}) do
    # TODO: Find a faster way ...
    [{graph_name, graph}] = Enum.take(graphs, 1)
    {{s, p, o}, popped_graph} = Graph.pop(graph)

    popped =
      if Enum.empty?(popped_graph),
        do: graphs |> Map.delete(graph_name),
        else: graphs |> Map.put(graph_name, popped_graph)

    {{s, p, o, graph_name}, %__MODULE__{name: name, graphs: popped}}
  end

  @doc """
  Pops the graph with the given name.

  When a graph with given name can not be found the optionally given default value
  or `nil` is returned.

  ## Examples

      iex> dataset = RDF.Dataset.new([
      ...>   {EX.S1, EX.P1, EX.O1, EX.Graph},
      ...>   {EX.S2, EX.P2, EX.O2}])
      ...> RDF.Dataset.pop(dataset, EX.Graph)
      {RDF.Graph.new({EX.S1, EX.P1, EX.O1}, name: EX.Graph), RDF.Dataset.new({EX.S2, EX.P2, EX.O2})}
      iex> RDF.Dataset.pop(dataset, EX.Foo)
      {nil, dataset}
  """
  @impl Access
  @spec pop(t, Statement.coercible_graph_name()) :: {Statement.t() | nil, t}
  def pop(%__MODULE__{name: name, graphs: graphs} = dataset, graph_name) do
    case Access.pop(graphs, coerce_graph_name(graph_name)) do
      {nil, _} ->
        {nil, dataset}

      {graph, new_graphs} ->
        {graph, %__MODULE__{name: name, graphs: new_graphs}}
    end
  end

  @doc """
  The number of statements within a `RDF.Dataset`.

  ## Examples

      iex> RDF.Dataset.new([
      ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
      ...>   {EX.S2, EX.p2, EX.O2},
      ...>   {EX.S1, EX.p2, EX.O3}]) |>
      ...>   RDF.Dataset.statement_count
      3
  """
  @spec statement_count(t) :: non_neg_integer
  def statement_count(%__MODULE__{graphs: graphs}) do
    Enum.reduce(graphs, 0, fn {_, graph}, count ->
      count + Graph.triple_count(graph)
    end)
  end

  @doc """
  The set of all subjects used in the statement within all graphs of a `RDF.Dataset`.

  ## Examples

      iex> RDF.Dataset.new([
      ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
      ...>   {EX.S2, EX.p2, EX.O2},
      ...>   {EX.S1, EX.p2, EX.O3}]) |>
      ...>   RDF.Dataset.subjects
      MapSet.new([RDF.iri(EX.S1), RDF.iri(EX.S2)])
  """
  def subjects(%__MODULE__{graphs: graphs}) do
    Enum.reduce(graphs, MapSet.new(), fn {_, graph}, subjects ->
      MapSet.union(subjects, Graph.subjects(graph))
    end)
  end

  @doc """
  The set of all properties used in the predicates within all graphs of a `RDF.Dataset`.

  ## Examples

      iex> RDF.Dataset.new([
      ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
      ...>   {EX.S2, EX.p2, EX.O2},
      ...>   {EX.S1, EX.p2, EX.O3}]) |>
      ...>   RDF.Dataset.predicates
      MapSet.new([EX.p1, EX.p2])
  """
  def predicates(%__MODULE__{graphs: graphs}) do
    Enum.reduce(graphs, MapSet.new(), fn {_, graph}, predicates ->
      MapSet.union(predicates, Graph.predicates(graph))
    end)
  end

  @doc """
  The set of all resources used in the objects within a `RDF.Dataset`.

  Note: This function does collect only IRIs and BlankNodes, not Literals.

  ## Examples

      iex> RDF.Dataset.new([
      ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
      ...>   {EX.S2, EX.p2, EX.O2, EX.Graph},
      ...>   {EX.S3, EX.p1, EX.O2},
      ...>   {EX.S4, EX.p2, RDF.bnode(:bnode)},
      ...>   {EX.S5, EX.p3, "foo"}
      ...> ]) |> RDF.Dataset.objects
      MapSet.new([RDF.iri(EX.O1), RDF.iri(EX.O2), RDF.bnode(:bnode)])
  """
  def objects(%__MODULE__{graphs: graphs}) do
    Enum.reduce(graphs, MapSet.new(), fn {_, graph}, objects ->
      MapSet.union(objects, Graph.objects(graph))
    end)
  end

  @doc """
  The set of all resources used within a `RDF.Dataset`.

  ## Examples

    iex> RDF.Dataset.new([
    ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
    ...>   {EX.S2, EX.p1, EX.O2, EX.Graph},
    ...>   {EX.S2, EX.p2, RDF.bnode(:bnode)},
    ...>   {EX.S3, EX.p1, "foo"}
    ...> ]) |> RDF.Dataset.resources
    MapSet.new([RDF.iri(EX.S1), RDF.iri(EX.S2), RDF.iri(EX.S3),
      RDF.iri(EX.O1), RDF.iri(EX.O2), RDF.bnode(:bnode), EX.p1, EX.p2])
  """
  def resources(%__MODULE__{graphs: graphs}) do
    Enum.reduce(graphs, MapSet.new(), fn {_, graph}, resources ->
      MapSet.union(resources, Graph.resources(graph))
    end)
  end

  @doc """
  All statements within all graphs of a `RDF.Dataset`.

  ## Examples

        iex> RDF.Dataset.new([
        ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
        ...>   {EX.S2, EX.p2, EX.O2},
        ...>   {EX.S1, EX.p2, EX.O3}]) |>
        ...>   RDF.Dataset.statements
        [{RDF.iri(EX.S1), RDF.iri(EX.p1), RDF.iri(EX.O1), RDF.iri(EX.Graph)},
         {RDF.iri(EX.S1), RDF.iri(EX.p2), RDF.iri(EX.O3)},
         {RDF.iri(EX.S2), RDF.iri(EX.p2), RDF.iri(EX.O2)}]
  """
  @spec statements(t) :: [Statement.t()]
  def statements(%__MODULE__{graphs: graphs}) do
    Enum.reduce(graphs, [], fn {_, graph}, all_statements ->
      statements = Graph.triples(graph)

      if graph.name do
        Enum.map(statements, fn {s, p, o} -> {s, p, o, graph.name} end)
      else
        statements
      end ++ all_statements
    end)
  end

  @doc """
  Checks if the given `input` statements exist within `dataset`.

  The `graph` option allows to set a different destination graph in which the
  statements should be checked, ignoring the graph context of given quads or the
  name of given graphs.

  ## Examples

        iex> dataset = RDF.Dataset.new([
        ...>   {EX.S1, EX.p1, EX.O1, EX.Graph},
        ...>   {EX.S2, EX.p2, EX.O2},
        ...>   {EX.S1, EX.p2, EX.O3}])
        ...> RDF.Dataset.include?(dataset, {EX.S1, EX.p1, EX.O1, EX.Graph})
        true
  """
  @spec include?(t, input, keyword) :: boolean
  def include?(dataset, input, opts \\ [])

  def include?(dataset, {_, _, _, graph} = quad, opts),
    do: do_include?(dataset, destination_graph(opts, graph), quad)

  def include?(dataset, %Graph{} = graph, opts),
    do: do_include?(dataset, destination_graph(opts, graph.name), graph)

  def include?(%__MODULE__{} = dataset, %__MODULE__{} = other_dataset, opts) do
    other_dataset
    |> graphs()
    |> Enum.all?(&include?(dataset, &1, opts))
  end

  def include?(dataset, input, opts) when is_list(input) or is_map(input) do
    Enum.all?(input, &include?(dataset, &1, opts))
  end

  def include?(dataset, input, opts), do: do_include?(dataset, destination_graph(opts), input)

  defp do_include?(%__MODULE__{graphs: graphs}, graph_name, input) do
    if graph = graphs[graph_name] do
      Graph.include?(graph, input)
    else
      false
    end
  end

  @doc """
  Checks if a graph of a `RDF.Dataset` contains statements about the given resource.

  ## Examples

        iex> RDF.Dataset.new([{EX.S1, EX.p1, EX.O1}]) |> RDF.Dataset.describes?(EX.S1)
        true
        iex> RDF.Dataset.new([{EX.S1, EX.p1, EX.O1}]) |> RDF.Dataset.describes?(EX.S2)
        false
  """
  @spec describes?(t, Statement.t(), Statement.coercible_graph_name() | nil) :: boolean
  def describes?(%__MODULE__{graphs: graphs}, subject, graph_context \\ nil) do
    with graph_context = coerce_graph_name(graph_context) do
      if graph = graphs[graph_context] do
        Graph.describes?(graph, subject)
      else
        false
      end
    end
  end

  @doc """
  Returns the names of all graphs of a `RDF.Dataset` containing statements about the given subject.

  ## Examples
        iex> dataset = RDF.Dataset.new([
        ...>   {EX.S1, EX.p, EX.O},
        ...>   {EX.S2, EX.p, EX.O},
        ...>   {EX.S1, EX.p, EX.O, EX.Graph1},
        ...>   {EX.S2, EX.p, EX.O, EX.Graph2}])
        ...> RDF.Dataset.who_describes(dataset, EX.S1)
        [nil, RDF.iri(EX.Graph1)]
  """
  @spec who_describes(t, Statement.coercible_subject()) :: [Graph.t()]
  def who_describes(%__MODULE__{graphs: graphs}, subject) do
    with subject = coerce_subject(subject) do
      graphs
      |> Map.values()
      |> Stream.filter(&Graph.describes?(&1, subject))
      |> Enum.map(& &1.name)
    end
  end

  @doc """
  Returns a nested map of the native Elixir values of a `RDF.Dataset`.

  The optional second argument allows to specify a custom mapping with a function
  which will receive a tuple `{statement_position, rdf_term}` where
  `statement_position` is one of the atoms `:subject`, `:predicate`, `:object`,
   or `graph_name` while `rdf_term` is the RDF term to be mapped.

  ## Examples

      iex> [
      ...>   {~I<http://example.com/S>, ~I<http://example.com/p>, ~L"Foo", ~I<http://example.com/Graph>},
      ...>   {~I<http://example.com/S>, ~I<http://example.com/p>, RDF.XSD.integer(42), }
      ...> ]
      ...> |> RDF.Dataset.new()
      ...> |> RDF.Dataset.values()
      %{
        "http://example.com/Graph" => %{
          "http://example.com/S" => %{"http://example.com/p" => ["Foo"]}
        },
        nil => %{
          "http://example.com/S" => %{"http://example.com/p" => [42]}
        }
      }

      iex> [
      ...>   {~I<http://example.com/S>, ~I<http://example.com/p>, ~L"Foo", ~I<http://example.com/Graph>},
      ...>   {~I<http://example.com/S>, ~I<http://example.com/p>, RDF.XSD.integer(42), }
      ...> ]
      ...> |> RDF.Dataset.new()
      ...> |> RDF.Dataset.values(fn
      ...>      {:graph_name, graph_name} ->
      ...>        graph_name
      ...>      {:predicate, predicate} ->
      ...>        predicate
      ...>        |> to_string()
      ...>        |> String.split("/")
      ...>        |> List.last()
      ...>        |> String.to_atom()
      ...>    {_, term} ->
      ...>      RDF.Term.value(term)
      ...>    end)
      %{
        ~I<http://example.com/Graph> => %{
          "http://example.com/S" => %{p: ["Foo"]}
        },
        nil => %{
          "http://example.com/S" => %{p: [42]}
        }
      }

  """
  @spec values(t, Statement.term_mapping()) :: map
  def values(dataset, mapping \\ &RDF.Statement.default_term_mapping/1)

  def values(%__MODULE__{graphs: graphs}, mapping) do
    Map.new(graphs, fn {graph_name, graph} ->
      {mapping.({:graph_name, graph_name}), Graph.values(graph, mapping)}
    end)
  end

  @doc """
  Checks if two `RDF.Dataset`s are equal.

  Two `RDF.Dataset`s are considered to be equal if they contain the same triples
  and have the same name.
  """
  @spec equal?(t | any, t | any) :: boolean
  def equal?(dataset1, dataset2)

  def equal?(%__MODULE__{} = dataset1, %__MODULE__{} = dataset2) do
    clear_metadata(dataset1) == clear_metadata(dataset2)
  end

  def equal?(_, _), do: false

  defp clear_metadata(%__MODULE__{graphs: graphs} = dataset) do
    %__MODULE__{
      dataset
      | graphs:
          Map.new(graphs, fn {name, graph} ->
            {name, RDF.Graph.clear_metadata(graph)}
          end)
    }
  end

  defimpl Enumerable do
    alias RDF.Dataset

    def member?(dataset, statement), do: {:ok, Dataset.include?(dataset, statement)}
    def count(dataset), do: {:ok, Dataset.statement_count(dataset)}
    def slice(_dataset), do: {:error, __MODULE__}

    def reduce(%Dataset{graphs: graphs}, {:cont, acc}, _fun)
        when map_size(graphs) == 0,
        do: {:done, acc}

    def reduce(%Dataset{} = dataset, {:cont, acc}, fun) do
      {statement, rest} = Dataset.pop(dataset)
      reduce(rest, fun.(statement, acc), fun)
    end

    def reduce(_, {:halt, acc}, _fun), do: {:halted, acc}

    def reduce(dataset = %Dataset{}, {:suspend, acc}, fun) do
      {:suspended, acc, &reduce(dataset, &1, fun)}
    end
  end

  defimpl Collectable do
    alias RDF.Dataset

    def into(original) do
      collector_fun = fn
        dataset, {:cont, list} when is_list(list) ->
          Dataset.add(dataset, List.to_tuple(list))

        dataset, {:cont, elem} ->
          Dataset.add(dataset, elem)

        dataset, :done ->
          dataset

        _dataset, :halt ->
          :ok
      end

      {original, collector_fun}
    end
  end
end