Add ordering of descriptions to Turtle encoder

lib/rdf/serializations/turtle_encoder.ex

@@ -18,6 +18,10 @@ defmodule RDF.Turtle.Encoder do
   @rdf_type RDF.type
   @rdf_nil  RDF.nil
 
+  # Defines rdf:type of subjects to be serialized at the beginning of the encoded graph
+  @top_classes [RDF.NS.RDFS.Class] |> Enum.map(&RDF.uri/1)
+
+  # Defines order of predicates at the beginning of a resource description
   @predicate_order [RDF.type, RDF.NS.RDFS.label, RDF.uri("http://purl.org/dc/terms/title")]
   @ordered_properties MapSet.new(@predicate_order)
 
@@ -78,11 +82,52 @@ defmodule RDF.Turtle.Encoder do
   defp graph_statements(state) do
     State.data(state)
     |> RDF.Data.descriptions
+    |> order_descriptions(state)
     |> Enum.map(&description_statements(&1, state))
     |> Enum.reject(&is_nil/1)
     |> Enum.join("\n")
   end
 
+  defp order_descriptions(descriptions, state) do
+    base_uri = State.base_uri(state)
+    group =
+      Enum.group_by descriptions, fn
+        %Description{subject: ^base_uri} ->
+          :base
+        description ->
+          with types when not is_nil(types) <- description.predications[@rdf_type] do
+            Enum.find @top_classes, :other, fn top_class ->
+              Map.has_key?(types, top_class)
+            end
+          else
+            _ -> :other
+          end
+      end
+    ordered_descriptions = (
+        @top_classes
+        |> Stream.map(fn top_class -> group[top_class] end)
+        |> Stream.reject(&is_nil/1)
+        |> Stream.map(&sort_description_group/1)
+        |> Enum.reduce([], fn class_group, ordered_descriptions ->
+             ordered_descriptions ++ class_group
+           end)
+      ) ++ (group |> Map.get(:other, []) |> sort_description_group())
+
+    case group[:base] do
+      [base] -> [base | ordered_descriptions]
+      _      -> ordered_descriptions
+    end
+  end
+
+  defp sort_description_group(descriptions) do
+    Enum.sort descriptions, fn
+      %Description{subject: %URI{}}, %Description{subject: %BlankNode{}} -> true
+      %Description{subject: %BlankNode{}}, %Description{subject: %URI{}} -> false
+      %Description{subject: s1}, %Description{subject: s2} ->
+        to_string(s1) < to_string(s2)
+    end
+  end
+
   defp description_statements(description, state, nesting \\ 0) do
     with %BlankNode{} <- description.subject,
          ref_count when ref_count < 2 <-

lib/rdf/serializations/turtle_encoder_state.ex

@@ -22,6 +22,14 @@ defmodule RDF.Turtle.Encoder.State do
     bnode_ref_counter(state) |> Map.get(bnode, 0)
   end
 
+  def base_uri(state) do
+    with {:ok, base} <- base(state) do
+      RDF.uri(base)
+    else
+      _ -> nil
+    end
+  end
+
   def list_values(head, state), do: Agent.get(state, &(&1.list_values[head]))
 
   def preprocess(state) do

test/unit/turtle_encoder_test.exs

@@ -6,7 +6,7 @@ defmodule RDF.Turtle.EncoderTest do
   doctest Turtle.Encoder
 
   alias RDF.Graph
-  alias RDF.NS.XSD
+  alias RDF.NS.{XSD, RDFS, OWL}
 
   import RDF.Sigils
 
@@ -101,6 +101,39 @@ defmodule RDF.Turtle.EncoderTest do
                   <http://example.org/#p1> _:1, _:bar, _:foo .
               """
     end
+
+    test "ordering of descriptions" do
+      assert Turtle.Encoder.encode!(Graph.new([
+                {EX.__base_uri__, RDF.type, OWL.Ontology},
+                {EX.S1, RDF.type, EX.O},
+                {EX.S2, RDF.type, RDFS.Class},
+                {EX.S3, RDF.type, RDF.Property},
+              ]),
+                base: EX.__base_uri__,
+                prefixes: %{
+                  rdf:  RDF.__base_uri__,
+                  rdfs: RDFS.__base_uri__,
+                  owl:  OWL.__base_uri__,
+                }) ==
+              """
+              @base <#{to_string(EX.__base_uri__)}> .
+              @prefix rdf: <#{to_string(RDF.__base_uri__)}> .
+              @prefix rdfs: <#{to_string(RDFS.__base_uri__)}> .
+              @prefix owl: <#{to_string(OWL.__base_uri__)}> .
+
+              <>
+                  a owl:Ontology .
+
+              <S2>
+                  a rdfs:Class .
+
+              <S1>
+                  a <O> .
+
+              <S3>
+                  a rdf:Property .
+              """
+    end
   end