From ec7f4b3e7e9a7991d0b3b5d1e82f262ae56892af Mon Sep 17 00:00:00 2001
From: Marcel Otto <marcelotto@gmx.de>
Date: Fri, 19 May 2017 18:11:58 +0200
Subject: [PATCH] core: NTriples encoder

---
 lib/rdf/serializations/ntriples_encoder.ex | 47 +++++++++++++++
 test/unit/ntriples_encoder_test.exs        | 69 ++++++++++++++++++++++
 2 files changed, 116 insertions(+)
 create mode 100644 lib/rdf/serializations/ntriples_encoder.ex
 create mode 100644 test/unit/ntriples_encoder_test.exs

diff --git a/lib/rdf/serializations/ntriples_encoder.ex b/lib/rdf/serializations/ntriples_encoder.ex
new file mode 100644
index 0000000..e5d9ccc
--- /dev/null
+++ b/lib/rdf/serializations/ntriples_encoder.ex
@@ -0,0 +1,47 @@
+defmodule RDF.NTriples.Encoder do
+  use RDF.Serialization.Encoder
+
+  alias RDF.{Literal, BlankNode}
+
+  @xsd_string RDF.Datatype.NS.XSD.string
+
+  def encode(data, opts \\ []) do
+    result =
+      data
+      |> Enum.reduce([], fn (statement, result) ->
+           [statement(statement) | result]
+         end)
+      |> Enum.reverse
+      |> Enum.join("\n")
+    {:ok, (if result == "", do: result, else: result <> "\n")}
+  end
+
+  def statement({subject, predicate, object}) do
+    "#{term(subject)} #{term(predicate)} #{term(object)} ."
+  end
+
+  def term(%URI{} = uri) do
+    "<#{to_string(uri)}>"
+  end
+
+  def term(%Literal{value: value, language: language}) when not is_nil(language) do
+    ~s["#{value}"@#{language}]
+  end
+
+  def term(%Literal{value: value, language: language}) when not is_nil(language) do
+    ~s["#{value}"@#{language}]
+  end
+
+  def term(%Literal{datatype: @xsd_string} = literal) do
+    ~s["#{Literal.lexical(literal)}"]
+  end
+
+  def term(%Literal{datatype: datatype} = literal) do
+    ~s["#{Literal.lexical(literal)}"^^<#{to_string(datatype)}>]
+  end
+
+  def term(%BlankNode{} = bnode) do
+    to_string(bnode)
+  end
+
+end
diff --git a/test/unit/ntriples_encoder_test.exs b/test/unit/ntriples_encoder_test.exs
new file mode 100644
index 0000000..d2ad61b
--- /dev/null
+++ b/test/unit/ntriples_encoder_test.exs
@@ -0,0 +1,69 @@
+defmodule RDF.NTriples.EncoderTest do
+  use ExUnit.Case, async: false
+
+  alias RDF.NTriples
+
+  doctest NTriples.Encoder
+
+  alias RDF.Graph
+  alias RDF.NS.XSD
+
+  import RDF.Sigils
+
+  use RDF.Vocabulary.Namespace
+
+  defvocab EX,
+    base_uri: "http://example.org/#",
+    terms: [], strict: false
+
+
+  describe "serializing a graph" do
+    test "an empty graph is serialized to an empty string" do
+      assert NTriples.Encoder.encode!(Graph.new) == ""
+    end
+
+    test "statements with URIs only" do
+      assert NTriples.Encoder.encode!(Graph.new [
+                {EX.S1, EX.p1, EX.O1},
+                {EX.S1, EX.p1, EX.O2},
+                {EX.S1, EX.p2, EX.O3},
+                {EX.S2, EX.p3, EX.O4},
+              ]) ==
+              """
+              <http://example.org/#S1> <http://example.org/#p1> <http://example.org/#O1> .
+              <http://example.org/#S1> <http://example.org/#p1> <http://example.org/#O2> .
+              <http://example.org/#S1> <http://example.org/#p2> <http://example.org/#O3> .
+              <http://example.org/#S2> <http://example.org/#p3> <http://example.org/#O4> .
+              """
+    end
+
+    test "statements with literals" do
+      assert NTriples.Encoder.encode!(Graph.new [
+                {EX.S1, EX.p1, ~L"foo"},
+                {EX.S1, EX.p1, ~L"foo"en},
+                {EX.S1, EX.p2, 42},
+                {EX.S2, EX.p3, RDF.literal("strange things", datatype: EX.custom)},
+              ]) ==
+              """
+              <http://example.org/#S1> <http://example.org/#p1> "foo"@en .
+              <http://example.org/#S1> <http://example.org/#p1> "foo" .
+              <http://example.org/#S1> <http://example.org/#p2> "42"^^<#{XSD.integer}> .
+              <http://example.org/#S2> <http://example.org/#p3> "strange things"^^<#{EX.custom}> .
+              """
+    end
+
+    test "statements with blank nodes" do
+      assert NTriples.Encoder.encode!(Graph.new [
+                {EX.S1, EX.p1, RDF.bnode(1)},
+                {EX.S1, EX.p1, RDF.bnode("foo")},
+                {EX.S1, EX.p1, RDF.bnode(:bar)},
+              ]) ==
+              """
+              <http://example.org/#S1> <http://example.org/#p1> _:1 .
+              <http://example.org/#S1> <http://example.org/#p1> _:bar .
+              <http://example.org/#S1> <http://example.org/#p1> _:foo .
+              """
+    end
+  end
+
+end