From a43205deaad8cc5289eed97045314e876565847d Mon Sep 17 00:00:00 2001
From: Marcel Otto <marcelotto@gmx.de>
Date: Sun, 19 Mar 2017 14:51:28 +0100
Subject: [PATCH] json_ld: solve the list reference problem with an agent

---
 lib/json/ld/flattening.ex     | 39 ++++++++++++++++++++++-----
 test/unit/flattening_test.exs | 51 +++++++++++++++++------------------
 2 files changed, 57 insertions(+), 33 deletions(-)

diff --git a/lib/json/ld/flattening.ex b/lib/json/ld/flattening.ex
index 304890e..216f1bb 100644
--- a/lib/json/ld/flattening.ex
+++ b/lib/json/ld/flattening.ex
@@ -132,16 +132,23 @@ defmodule JSON.LD.Flattening do
             node_map
           end
         else
-          # TODO: list a reference! We'll have to rewrite this to work without references
-          list = Map.update(list, "@list", [element], fn l -> l ++ [element] end)
+          append_to_list(list, element)
           node_map
         end
 
       # 5)
       Map.has_key?(element, "@list") ->
-        result = %{"@list" => []}
-        node_map = generate_node_map(element["@list"], node_map, node_id_map,
-                          active_graph, active_subject, active_property, result)
+        {:ok, result_list} = new_list
+        {node_map, result} =
+          try do
+            {
+              generate_node_map(element["@list"], node_map, node_id_map,
+                active_graph, active_subject, active_property, result_list),
+              get_list(result_list)
+            }
+        after
+          terminate_list(result_list)
+        end
         if node do
           update_in(node_map, [active_graph, active_subject, active_property], fn
             nil   -> [result]
@@ -204,8 +211,7 @@ defmodule JSON.LD.Flattening do
                 end)
             # 6.6.3) TODO: Spec fixme: specs says to add ELEMENT to @list member, should be REFERENCE
             else
-            # TODO: list a reference! We'll have to rewrite this to work without references
-              list = Map.update(list, "@list", [reference], fn l -> l ++ [reference] end)
+              append_to_list(list, reference)
             end
           end
         end
@@ -289,4 +295,23 @@ defmodule JSON.LD.Flattening do
   defp deep_compare(v, v), do: true
   defp deep_compare(_, _), do: false
 
+
+  defp new_list do
+    Agent.start_link fn -> %{"@list" => []} end
+  end
+
+  defp terminate_list(pid) do
+    Agent.stop pid
+  end
+
+  defp get_list(pid) do
+    Agent.get pid, fn list_node -> list_node end
+  end
+
+  defp append_to_list(pid, element) do
+    Agent.update pid, fn list_node ->
+      Map.update(list_node, "@list", [element], fn list -> list ++ [element] end)
+    end
+  end
+
 end
diff --git a/test/unit/flattening_test.exs b/test/unit/flattening_test.exs
index be4b26b..2984788 100644
--- a/test/unit/flattening_test.exs
+++ b/test/unit/flattening_test.exs
@@ -211,32 +211,31 @@ defmodule JSON.LD.FlatteningTest do
         }]
       """)
     },
-# TODO: @list don't work yet, since the reference-based implementation in the spec is not possible in Elixir
-#    "Test Manifest (shortened)" => %{
-#      input: Poison.Parser.parse!("""
-#        {
-#          "@id": "",
-#          "http://example/sequence": {"@list": [
-#            {
-#              "@id": "#t0001",
-#              "http://example/name": "Keywords cannot be aliased to other keywords",
-#              "http://example/input": {"@id": "error-expand-0001-in.jsonld"}
-#            }
-#          ]}
-#        }
-#      """),
-#      output: Poison.Parser.parse!("""
-#        [{
-#          "@id": "",
-#          "http://example/sequence": [{"@list": [{"@id": "#t0001"}]}]
-#        }, {
-#          "@id": "#t0001",
-#          "http://example/input": [{"@id": "error-expand-0001-in.jsonld"}],
-#          "http://example/name": [{"@value": "Keywords cannot be aliased to other keywords"}]
-#        }]
-#      """),
-#      options: %{}
-#    },
+    "Test Manifest (shortened)" => %{
+      input: Poison.Parser.parse!("""
+        {
+          "@id": "",
+          "http://example/sequence": {"@list": [
+            {
+              "@id": "#t0001",
+              "http://example/name": "Keywords cannot be aliased to other keywords",
+              "http://example/input": {"@id": "error-expand-0001-in.jsonld"}
+            }
+          ]}
+        }
+      """),
+      output: Poison.Parser.parse!("""
+        [{
+          "@id": "",
+          "http://example/sequence": [{"@list": [{"@id": "#t0001"}]}]
+        }, {
+          "@id": "#t0001",
+          "http://example/input": [{"@id": "error-expand-0001-in.jsonld"}],
+          "http://example/name": [{"@value": "Keywords cannot be aliased to other keywords"}]
+        }]
+      """),
+      options: %{}
+    },
     "@reverse bnode issue (0045)" => %{
       input: Poison.Parser.parse!("""
         {