[subset] Implement Kahn's algo for topological sorting instead of BFS.

src/hb-repacker.hh

@@ -35,10 +35,13 @@
 
 struct graph_t
 {
+  // TODO(garretrieger): add an error tracking system similar to what serialize_context_t
+  //                     does.
+
   /*
    * A topological sorting of an object graph. Ordered
    * in reverse serialization order (first object in the
-   * serialization is at the end of the graph). This matches
+   * serialization is at the end of the list). This matches
    * the 'packed' object stack used internally in the
    * serializer
    */
@@ -92,14 +95,11 @@ struct graph_t
   }
 
   /*
-   * Generates a new topological sorting of graph using BFS.
+   * Generates a new topological sorting of graph using Kahn's
+   * algorithm: https://en.wikipedia.org/wiki/Topological_sorting#Algorithms
    */
-  void sort_bfs ()
+  void sort_kahn ()
   {
-    // BFS doesn't always produce a topological sort so this is just
-    // for testing re-ordering capabilities for now.
-    // Will need to use a more advanced topological sorting algorithm
-
     if (objects_.length <= 1) {
       // Graph of 1 or less doesn't need sorting.
       return;
@@ -108,25 +108,28 @@ struct graph_t
     hb_vector_t<unsigned> queue;
     hb_vector_t<hb_serialize_context_t::object_t> sorted_graph;
     hb_map_t id_map;
+    hb_map_t edge_count;
+    incoming_edge_count (&edge_count);
 
     // Object graphs are in reverse order, the first object is at the end
-    // of the vector.
+    // of the vector. Since the graph is topologically sorted it's safe to
+    // assume the first object has no incoming edges.
     queue.push (objects_.length - 1);
     int new_id = objects_.length - 1;
 
-    hb_set_t visited;
     while (queue.length)
     {
       unsigned next_id = queue[0];
       queue.remove(0);
-      visited.add(next_id);
 
       hb_serialize_context_t::object_t& next = objects_[next_id];
       sorted_graph.push (next);
       id_map.set (next_id, new_id--);
 
       for (const auto& link : next.links) {
-        if (!visited.has (link.objidx))
+        // TODO(garretrieger): sort children from smallest to largest
+        edge_count.set (link.objidx, edge_count.get (link.objidx) - 1);
+        if (!edge_count.get (link.objidx))
           queue.push (link.objidx);
       }
     }
@@ -138,19 +141,7 @@ struct graph_t
       assert (false);
     }
 
-    // Apply objidx remapping.
-    // TODO(garretrieger): extract this to a helper.
-    for (unsigned i = 0; i < sorted_graph.length; i++)
-    {
-      for (unsigned j = 0; j < sorted_graph[i].links.length; j++)
-      {
-        auto& link = sorted_graph[i].links[j];
-        if (!id_map.has (link.objidx))
-          // TODO(garretrieger): handle this.
-          assert (false);
-        link.objidx = id_map.get (link.objidx);
-      }
-    }
+    remap_obj_indices (id_map, &sorted_graph);
 
     sorted_graph.as_array ().reverse ();
     objects_ = sorted_graph;
@@ -163,13 +154,53 @@ struct graph_t
   bool will_overflow()
   {
     // TODO(garretrieger): implement me.
-    // Check for offsets that exceed their width or are negative if
-    // using a non-signed link.
+    // - Check for offsets that exceed their width or;
+    // - are negative if using a non-signed link.
     return false;
   }
 
  private:
 
+  /*
+   * Updates all objidx's in all links using the provided mapping.
+   */
+  void remap_obj_indices (const hb_map_t& id_map,
+                          hb_vector_t<hb_serialize_context_t::object_t>* sorted_graph)
+  {
+    for (unsigned i = 0; i < sorted_graph->length; i++)
+    {
+      for (unsigned j = 0; j < (*sorted_graph)[i].links.length; j++)
+      {
+        auto& link = (*sorted_graph)[i].links[j];
+        if (!id_map.has (link.objidx))
+          // TODO(garretrieger): handle this.
+          assert (false);
+        link.objidx = id_map.get (link.objidx);
+      }
+    }
+  }
+
+  /*
+   * Creates a map from objid to # of incoming edges.
+   */
+  void incoming_edge_count (hb_map_t* out)
+  {
+    for (unsigned i = 0; i < objects_.length; i++)
+    {
+      if (!out->has (i))
+        out->set (i, 0);
+
+      for (const auto& l : objects_[i].links)
+      {
+        unsigned id = l.objidx;
+        if (out->has (id))
+          out->set (id, out->get (id) + 1);
+        else
+          out->set (id, 1);
+      }
+    }
+  }
+
   template <typename O> void
   serialize_link_of_type (const hb_serialize_context_t::object_t::link_t& link,
                           char* head,
@@ -220,9 +251,10 @@ inline void
 hb_resolve_overflows (const hb_vector_t<hb_serialize_context_t::object_t *>& packed,
                       hb_serialize_context_t* c) {
   graph_t sorted_graph (packed);
-  sorted_graph.sort_bfs ();
+  sorted_graph.sort_kahn ();
   if (sorted_graph.will_overflow ()) {
-    // TODO(garretrieger): additional offset resolution strategies
+    // TODO(garretrieger): try additional offset resolution strategies
+    // - Dijkstra sort of weighted graph.
     // - Promotion to extension lookups.
     // - Table duplication.
     // - Table splitting.

src/test-repacker.cc

@@ -71,7 +71,7 @@ populate_serializer_simple (hb_serialize_context_t* c)
 }
 
 static void
-populate_serializer_complex (hb_serialize_context_t* c)
+populate_serializer_complex_1 (hb_serialize_context_t* c)
 {
   c->start_serialize<char> ();
 
@@ -90,15 +90,41 @@ populate_serializer_complex (hb_serialize_context_t* c)
   c->end_serialize();
 }
 
-static void test_sort_bfs ()
+static void
+populate_serializer_complex_2 (hb_serialize_context_t* c)
+{
+  c->start_serialize<char> ();
+
+  unsigned obj_5 = add_object ("mno", 3, c);
+
+  unsigned obj_4 = add_object ("jkl", 3, c);
+
+  start_object ("ghi", 3, c);
+  add_offset (obj_4, c);
+  unsigned obj_3 = c->pop_pack (false);
+
+  start_object ("def", 3, c);
+  add_offset (obj_3, c);
+  unsigned obj_2 = c->pop_pack (false);
+
+  start_object ("abc", 3, c);
+  add_offset (obj_2, c);
+  add_offset (obj_4, c);
+  add_offset (obj_5, c);
+  c->pop_pack ();
+
+  c->end_serialize();
+}
+
+static void test_sort_kahn_1 ()
 {
   size_t buffer_size = 100;
   void* buffer = malloc (buffer_size);
   hb_serialize_context_t c (buffer, buffer_size);
-  populate_serializer_complex (&c);
+  populate_serializer_complex_1 (&c);
 
   graph_t graph (c.object_graph ());
-  graph.sort_bfs ();
+  graph.sort_kahn ();
 
   assert(strncmp (graph.objects_[3].head, "abc", 3) == 0);
   assert(graph.objects_[3].links.length == 2);
@@ -116,6 +142,38 @@ static void test_sort_bfs ()
   assert(graph.objects_[0].links.length == 0);
 }
 
+static void test_sort_kahn_2 ()
+{
+  size_t buffer_size = 100;
+  void* buffer = malloc (buffer_size);
+  hb_serialize_context_t c (buffer, buffer_size);
+  populate_serializer_complex_2 (&c);
+
+  graph_t graph (c.object_graph ());
+  graph.sort_kahn ();
+
+
+  assert(strncmp (graph.objects_[4].head, "abc", 3) == 0);
+  assert(graph.objects_[4].links.length == 3);
+  assert(graph.objects_[4].links[0].objidx == 3);
+    assert(graph.objects_[4].links[1].objidx == 0);
+  assert(graph.objects_[4].links[2].objidx == 2);
+
+  assert(strncmp (graph.objects_[3].head, "def", 3) == 0);
+  assert(graph.objects_[3].links.length == 1);
+  assert(graph.objects_[3].links[0].objidx == 1);
+
+  assert(strncmp (graph.objects_[2].head, "mno", 3) == 0);
+  assert(graph.objects_[2].links.length == 0);
+
+  assert(strncmp (graph.objects_[1].head, "ghi", 3) == 0);
+  assert(graph.objects_[1].links.length == 1);
+  assert(graph.objects_[1].links[0].objidx == 0);
+
+  assert(strncmp (graph.objects_[0].head, "jkl", 3) == 0);
+  assert(graph.objects_[0].links.length == 0);
+}
+
 static void
 test_serialize ()
 {
@@ -142,5 +200,6 @@ int
 main (int argc, char **argv)
 {
   test_serialize ();
-  test_sort_bfs ();
+  test_sort_kahn_1 ();
+  test_sort_kahn_2 ();
 }