[tracker/wip/sam/resource: 8/15] libtracker-sparql: Add JSON-LD serialisation to TrackerResource

[Sam Thursfield <sthursfield src gnome org>]

commit 9b1b3cc03bbce195e753c6e005c2ac8b47d24031
Author: Sam Thursfield <sam afuera me uk>
Date:   Thu Apr 7 18:03:00 2016 +0100

    libtracker-sparql: Add JSON-LD serialisation to TrackerResource

 src/libtracker-sparql/tracker-resource.c |  143 ++++++++++++++++++++++++++++++
 src/libtracker-sparql/tracker-resource.h |    2 +
 2 files changed, 145 insertions(+), 0 deletions(-)
---
diff --git a/src/libtracker-sparql/tracker-resource.c b/src/libtracker-sparql/tracker-resource.c
index 191811f..289e15e 100644
--- a/src/libtracker-sparql/tracker-resource.c
+++ b/src/libtracker-sparql/tracker-resource.c
@@ -18,6 +18,7 @@
  */
 
 #include <glib.h>
+#include <json-glib/json-glib.h>
 
 #include <string.h>
 
@@ -561,6 +562,148 @@ tracker_resource_compare (TrackerResource *a,
 };
 
 
+static void generate_jsonld_foreach (gpointer key, gpointer value_ptr, gpointer user_data);
+
+/* FIXME: this could hit an infinite loop if there are circular resource
+ * relationships, make sure those are tested & detected.
+ */
+/* This is not exposed publically right now because then everything including
+ * tracker-resource.h would need to pull in the json-glib dependency ...
+ */
+static JsonNode *
+tracker_resource_generate_jsonld (TrackerResource *self,
+                                   GError **error)
+{
+       /* FIXME: generate a JSON-LD context ! */
+
+       TrackerResourcePrivate *priv = GET_PRIVATE (self);
+       JsonBuilder *builder;
+       JsonNode *result;
+
+       builder = json_builder_new ();
+       json_builder_begin_object (builder);
+
+       /* The JSON-LD spec says it is "important that nodes have an identifier", but
+        * doesn't mandate one. I think it's better to omit the ID for blank nodes
+        * (where the caller passed NULL as an identifier) than to emit something
+        * SPARQL-specific like '_:123'.
+        */
+       if (strncmp (priv->identifier, "_:", 2) != 0) {
+               json_builder_set_member_name (builder, "@id");
+               json_builder_add_string_value (builder, priv->identifier);
+       }
+
+       g_hash_table_foreach (priv->properties, generate_jsonld_foreach, builder);
+
+       json_builder_end_object (builder);
+
+       result = json_builder_get_root (builder);
+       g_object_unref (builder);
+       return result;
+};
+
+static void
+append_value_to_json_builder (const GValue *value,
+                              JsonBuilder *builder)
+{
+       JsonNode *node;
+
+       if (G_VALUE_HOLDS (value, TRACKER_TYPE_RESOURCE)) {
+               TrackerResource *resource;
+               GError *error = NULL;
+
+               resource = TRACKER_RESOURCE (g_value_get_object (value));
+               node = tracker_resource_generate_jsonld (resource, &error);
+
+               if (node) {
+                       json_builder_add_value (builder, node);
+               } else {
+                       g_warning ("Unable to serialize value: %s", error->message);
+                       g_error_free (error);
+               }
+       } else if (G_VALUE_HOLDS (value, TRACKER_TYPE_URI)) {
+               /* URIs can be treated the same as strings in JSON-LD provided the @context
+                * sets the type of that property correctly. However, json_node_set_value()
+                * will reject a GValue holding TRACKER_TYPE_URI, so we have to extract the
+                * string manually here.
+                */
+               const char *uri = g_value_get_string (value);
+               node = json_node_new (JSON_NODE_VALUE);
+               json_node_set_string (node, uri);
+               json_builder_add_value (builder, node);
+       } else {
+               node = json_node_new (JSON_NODE_VALUE);
+               json_node_set_value (node, value);
+               json_builder_add_value (builder, node);
+       }
+}
+
+static void
+generate_jsonld_foreach (gpointer key,
+                         gpointer value_ptr,
+                         gpointer user_data)
+{
+       const char *property = key;
+       const GValue *value = value_ptr;
+       JsonBuilder *builder = JSON_BUILDER (user_data);
+
+       /* FIXME: shouldn't hardcode the unexpanded prefix here!!! */
+       if (strcmp (property, "rdf:type") == 0) {
+               property = "@type";
+       }
+
+       json_builder_set_member_name (builder, property);
+       if (G_VALUE_HOLDS (value, G_TYPE_PTR_ARRAY)) {
+               json_builder_begin_array (builder);
+               g_ptr_array_foreach (g_value_get_boxed (value), (GFunc) append_value_to_json_builder, 
builder);
+               json_builder_end_array (builder);
+       } else {
+               append_value_to_json_builder (value, builder);
+       }
+}
+
+/**
+ * tracker_resource_print_jsonld:
+ * @resource: a #TrackerResource
+ * @error: address where an error can be returned
+ *
+ * Serialize all the information in @resource as a JSON-LD document.
+ *
+ * See <http://www.jsonld.org/> for more information on the JSON-LD
+ * serialization format.
+ *
+ * Returns: a newly-allocated string
+ *
+ * Since: 1.10
+ */
+char *
+tracker_resource_print_jsonld (TrackerResource *resource,
+                               GError **error)
+{
+       GError *sub_error = NULL;
+       JsonNode *json_root_node;
+       JsonGenerator *generator;
+       char *result;
+
+       json_root_node = tracker_resource_generate_jsonld (resource, &sub_error);
+
+       if (json_root_node == NULL) {
+               g_propagate_error (error, sub_error);
+               return NULL;
+       }
+
+       generator = json_generator_new ();
+       json_generator_set_root (generator, json_root_node);
+       json_generator_set_pretty (generator, TRUE);
+
+       result = json_generator_to_data (generator, NULL);
+
+       json_node_free (json_root_node);
+       g_object_unref (generator);
+
+       return result;
+}
+
 typedef struct {
        TrackerNamespaceManager *namespaces;
        GString *string;
diff --git a/src/libtracker-sparql/tracker-resource.h b/src/libtracker-sparql/tracker-resource.h
index fe67b57..f225a64 100644
--- a/src/libtracker-sparql/tracker-resource.h
+++ b/src/libtracker-sparql/tracker-resource.h
@@ -75,6 +75,8 @@ gint tracker_resource_identifier_compare_func (TrackerResource *resource, const
 
 char *tracker_resource_print_turtle(TrackerResource *self, TrackerNamespaceManager *namespaces);
 
+char *tracker_resource_print_jsonld (TrackerResource *self, GError **error);
+
 void tracker_resource_generate_sparql_update (TrackerResource *self, TrackerSparqlBuilder *builder, 
TrackerNamespaceManager *namespaces, const char *graph_id, GError **error);
 
 G_END_DECLS