[subset] Prepare the repacker for handling 24bit offsets in GSUB/GPOS.

The boring expansion (https://github.com/be-fonts/boring-expansion-spec) plans to introduce 24bit offsets into GSUB/GPOS. This changes the repacker to treat 24 bit offsets similar to 32 bit offsets and assign the top level 24 bit offsets into spaces to improve packing.

src/graph/graph.hh

@@ -264,29 +264,58 @@ struct graph_t
     hb_swap (vertices_, sorted_graph);
   }
 
-  /*
-   * Assign unique space numbers to each connected subgraph of 32 bit offset(s).
-   */
-  bool assign_32bit_spaces ()
+  void find_space_roots (hb_set_t& visited, hb_set_t& roots)
   {
-    unsigned root_index = root_idx ();
-    hb_set_t visited;
-    hb_set_t roots;
-    for (unsigned i = 0; i <= root_index; i++)
+    int root_index = (int) root_idx ();
+    for (int i = root_index; i >= 0; i--)
     {
+      if (visited.has (i)) continue;
+
       // Only real links can form 32 bit spaces
       for (auto& l : vertices_[i].obj.real_links)
       {
-        if (l.width == 4 && !l.is_signed)
+        if (l.is_signed || l.width < 3)
+          continue;
+
+        if (i == root_index && l.width == 3)
+          // Ignore 24bit links from the root node, this skips past the single 24bit
+          // pointer to the lookup list.
+          continue;
+
+        if (l.width == 3)
         {
-          roots.add (l.objidx);
-          find_subgraph (l.objidx, visited);
+          // A 24bit offset forms a root, unless there is 32bit offsets somewhere
+          // in it's subgraph, then those become the roots instead.
+          hb_set_t sub_roots;
+          find_32bit_roots (l.objidx, sub_roots);
+          if (sub_roots) {
+            for (unsigned sub_root_idx : sub_roots) {
+              roots.add (sub_root_idx);
+              find_subgraph (sub_root_idx, visited);
+            }
+            continue;
+          }
         }
+
+        roots.add (l.objidx);
+        find_subgraph (l.objidx, visited);
       }
     }
+  }
 
-    // Mark everything not in the subgraphs of 32 bit roots as visited.
-    // This prevents 32 bit subgraphs from being connected via nodes not in the 32 bit subgraphs.
+  /*
+   * Assign unique space numbers to each connected subgraph of 24 bit and/or 32 bit offset(s).
+   * Currently, this is implemented specifically tailored to the structure of a GPOS/GSUB
+   * (including with 24bit offsets) table.
+   */
+  bool assign_spaces ()
+  {
+    hb_set_t visited;
+    hb_set_t roots;
+    find_space_roots (visited, roots);
+
+    // Mark everything not in the subgraphs of the roots as visited. This prevents
+    // subgraphs from being connected via nodes not in those subgraphs.
     visited.invert ();
 
     if (!roots) return false;
@@ -422,6 +451,18 @@ struct graph_t
       find_subgraph (link.objidx, subgraph);
   }
 
+  void find_32bit_roots (unsigned node_idx, hb_set_t& found)
+  {
+    for (const auto& link : vertices_[node_idx].obj.all_links ())
+    {
+      if (!link.is_signed && link.width == 4) {
+        found.add (link.objidx);
+        continue;
+      }
+      find_32bit_roots (link.objidx, found);
+    }
+  }
+
   /*
    * duplicates all nodes in the subgraph reachable from node_idx. Does not re-assign
    * links. index_map is updated with mappings from old id to new id. If a duplication has already
@@ -622,7 +663,7 @@ struct graph_t
  private:
 
   /*
-   * Returns the numbers of incoming edges that are 32bits wide.
+   * Returns the numbers of incoming edges that are 24 or 32 bits wide.
    */
   unsigned wide_parents (unsigned node_idx, hb_set_t& parents) const
   {
@@ -636,7 +677,9 @@ struct graph_t
       // Only real links can be wide
       for (const auto& l : vertices_[p].obj.real_links)
       {
-        if (l.objidx == node_idx && l.width == 4 && !l.is_signed)
+        if (l.objidx == node_idx
+            && (l.width == 3 || l.width == 4)
+            && !l.is_signed)
         {
           count++;
           parents.add (p);

src/hb-repacker.hh

@@ -172,7 +172,7 @@ hb_resolve_overflows (const T& packed,
       && will_overflow)
   {
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Assigning spaces to 32 bit subgraphs.");
-    if (sorted_graph.assign_32bit_spaces ())
+    if (sorted_graph.assign_spaces ())
       sorted_graph.sort_shortest_distance ();
   }