kc3-lang/harfbuzz/src/hb-repacker.hh

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• Hash : 5914acb3
  Author :
  Date : 2021-12-10T10:05:47
  

  [repacker] Clear distance and position cache when assigning a new space.
  
  A change in space will effect the distance assigned to the node and any of it's children so clear the distance cache.
  

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• src/hb-repacker.hh

• /*
   * Copyright © 2020  Google, Inc.
   *
   *  This is part of HarfBuzz, a text shaping library.
   *
   * Permission is hereby granted, without written agreement and without
   * license or royalty fees, to use, copy, modify, and distribute this
   * software and its documentation for any purpose, provided that the
   * above copyright notice and the following two paragraphs appear in
   * all copies of this software.
   *
   * IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
   * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
   * ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
   * IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
   * DAMAGE.
   *
   * THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
   * BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
   * FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
   * ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
   * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
   *
   * Google Author(s): Garret Rieger
   */
  
  #ifndef HB_REPACKER_HH
  #define HB_REPACKER_HH
  
  #include "hb-open-type.hh"
  #include "hb-map.hh"
  #include "hb-priority-queue.hh"
  #include "hb-serialize.hh"
  #include "hb-vector.hh"
  
  /*
   * For a detailed writeup on the overflow resolution algorithm see:
   * docs/repacker.md
   */
  
  struct graph_t
  {
    struct vertex_t
    {
      vertex_t () :
          distance (0),
          space (0),
          parents (),
          start (0),
          end (0),
          priority(0) {}
  
      void fini () {
        obj.fini ();
        parents.fini ();
      }
  
      hb_serialize_context_t::object_t obj;
      int64_t distance;
      int64_t space;
      hb_vector_t<unsigned> parents;
      unsigned start;
      unsigned end;
      unsigned priority;
  
      bool is_shared () const
      {
        return parents.length > 1;
      }
  
      unsigned incoming_edges () const
      {
        return parents.length;
      }
  
      void remove_parent (unsigned parent_index)
      {
        for (unsigned i = 0; i < parents.length; i++)
        {
          if (parents[i] != parent_index) continue;
          parents.remove (i);
          break;
        }
      }
  
      void remap_parents (const hb_vector_t<unsigned>& id_map)
      {
        for (unsigned i = 0; i < parents.length; i++)
          parents[i] = id_map[parents[i]];
      }
  
      void remap_parent (unsigned old_index, unsigned new_index)
      {
        for (unsigned i = 0; i < parents.length; i++)
        {
          if (parents[i] == old_index)
            parents[i] = new_index;
        }
      }
  
      bool is_leaf () const
      {
        return !obj.real_links.length && !obj.virtual_links.length;
      }
  
      bool raise_priority ()
      {
        if (has_max_priority ()) return false;
        priority++;
        return true;
      }
  
      bool has_max_priority () const {
        return priority >= 3;
      }
  
      int64_t modified_distance (unsigned order) const
      {
        // TODO(garretrieger): once priority is high enough, should try
        // setting distance = 0 which will force to sort immediately after
        // it's parent where possible.
  
        int64_t modified_distance =
            hb_min (hb_max(distance + distance_modifier (), 0), 0x7FFFFFFFFFF);
        if (has_max_priority ()) {
          modified_distance = 0;
        }
        return (modified_distance << 18) | (0x003FFFF & order);
      }
  
      int64_t distance_modifier () const
      {
        if (!priority) return 0;
        int64_t table_size = obj.tail - obj.head;
  
        if (priority == 1)
          return -table_size / 2;
  
        return -table_size;
      }
    };
  
    struct overflow_record_t
    {
      unsigned parent;
      unsigned child;
    };
  
    /*
     * A topological sorting of an object graph. Ordered
     * in reverse serialization order (first object in the
     * serialization is at the end of the list). This matches
     * the 'packed' object stack used internally in the
     * serializer
     */
    graph_t (const hb_vector_t<hb_serialize_context_t::object_t *>& objects)
        : parents_invalid (true),
          distance_invalid (true),
          positions_invalid (true),
          successful (true)
    {
      num_roots_for_space_.push (1);
      bool removed_nil = false;
      for (unsigned i = 0; i < objects.length; i++)
      {
        // TODO(grieger): check all links point to valid objects.
  
        // If this graph came from a serialization buffer object 0 is the
        // nil object. We don't need it for our purposes here so drop it.
        if (i == 0 && !objects[i])
        {
          removed_nil = true;
          continue;
        }
  
        vertex_t* v = vertices_.push ();
        if (check_success (!vertices_.in_error ()))
          v->obj = *objects[i];
        if (!removed_nil) continue;
        // Fix indices to account for removed nil object.
        for (auto& l : v->obj.all_links_writer ()) {
          l.objidx--;
        }
      }
    }
  
    ~graph_t ()
    {
      vertices_.fini_deep ();
    }
  
    bool in_error () const
    {
      return !successful ||
          vertices_.in_error () ||
          num_roots_for_space_.in_error ();
    }
  
    const vertex_t& root () const
    {
      return vertices_[root_idx ()];
    }
  
    unsigned root_idx () const
    {
      // Object graphs are in reverse order, the first object is at the end
      // of the vector. Since the graph is topologically sorted it's safe to
      // assume the first object has no incoming edges.
      return vertices_.length - 1;
    }
  
    const hb_serialize_context_t::object_t& object(unsigned i) const
    {
      return vertices_[i].obj;
    }
  
    /*
     * serialize graph into the provided serialization buffer.
     */
    hb_blob_t* serialize () const
    {
      hb_vector_t<char> buffer;
      size_t size = serialized_length ();
      if (!buffer.alloc (size)) {
        DEBUG_MSG (SUBSET_REPACK, nullptr, "Unable to allocate output buffer.");
        return nullptr;
      }
      hb_serialize_context_t c((void *) buffer, size);
  
      c.start_serialize<void> ();
      for (unsigned i = 0; i < vertices_.length; i++) {
        c.push ();
  
        size_t size = vertices_[i].obj.tail - vertices_[i].obj.head;
        char* start = c.allocate_size <char> (size);
        if (!start) {
          DEBUG_MSG (SUBSET_REPACK, nullptr, "Buffer out of space.");
          return nullptr;
        }
  
        memcpy (start, vertices_[i].obj.head, size);
  
        // Only real links needs to be serialized.
        for (const auto& link : vertices_[i].obj.real_links)
          serialize_link (link, start, &c);
  
        // All duplications are already encoded in the graph, so don't
        // enable sharing during packing.
        c.pop_pack (false);
      }
      c.end_serialize ();
  
      if (c.in_error ()) {
        DEBUG_MSG (SUBSET_REPACK, nullptr, "Error during serialization. Err flag: %d",
                   c.errors);
        return nullptr;
      }
  
      return c.copy_blob ();
    }
  
    /*
     * Generates a new topological sorting of graph using Kahn's
     * algorithm: https://en.wikipedia.org/wiki/Topological_sorting#Algorithms
     */
    void sort_kahn ()
    {
      positions_invalid = true;
  
      if (vertices_.length <= 1) {
        // Graph of 1 or less doesn't need sorting.
        return;
      }
  
      hb_vector_t<unsigned> queue;
      hb_vector_t<vertex_t> sorted_graph;
      if (unlikely (!check_success (sorted_graph.resize (vertices_.length)))) return;
      hb_vector_t<unsigned> id_map;
      if (unlikely (!check_success (id_map.resize (vertices_.length)))) return;
  
      hb_vector_t<unsigned> removed_edges;
      if (unlikely (!check_success (removed_edges.resize (vertices_.length)))) return;
      update_parents ();
  
      queue.push (root_idx ());
      int new_id = vertices_.length - 1;
  
      while (!queue.in_error () && queue.length)
      {
        unsigned next_id = queue[0];
        queue.remove (0);
  
        vertex_t& next = vertices_[next_id];
        sorted_graph[new_id] = next;
        id_map[next_id] = new_id--;
  
        for (const auto& link : next.obj.all_links ()) {
          removed_edges[link.objidx]++;
          if (!(vertices_[link.objidx].incoming_edges () - removed_edges[link.objidx]))
            queue.push (link.objidx);
        }
      }
  
      check_success (!queue.in_error ());
      check_success (!sorted_graph.in_error ());
      if (!check_success (new_id == -1))
        print_orphaned_nodes ();
  
      remap_all_obj_indices (id_map, &sorted_graph);
  
      hb_swap (vertices_, sorted_graph);
      sorted_graph.fini_deep ();
    }
  
    /*
     * Generates a new topological sorting of graph ordered by the shortest
     * distance to each node.
     */
    void sort_shortest_distance ()
    {
      positions_invalid = true;
  
      if (vertices_.length <= 1) {
        // Graph of 1 or less doesn't need sorting.
        return;
      }
  
      update_distances ();
  
      hb_priority_queue_t queue;
      hb_vector_t<vertex_t> sorted_graph;
      if (unlikely (!check_success (sorted_graph.resize (vertices_.length)))) return;
      hb_vector_t<unsigned> id_map;
      if (unlikely (!check_success (id_map.resize (vertices_.length)))) return;
  
      hb_vector_t<unsigned> removed_edges;
      if (unlikely (!check_success (removed_edges.resize (vertices_.length)))) return;
      update_parents ();
  
      queue.insert (root ().modified_distance (0), root_idx ());
      int new_id = root_idx ();
      unsigned order = 1;
      while (!queue.in_error () && !queue.is_empty ())
      {
        unsigned next_id = queue.pop_minimum().second;
  
        vertex_t& next = vertices_[next_id];
        sorted_graph[new_id] = next;
        id_map[next_id] = new_id--;
  
        for (const auto& link : next.obj.all_links ()) {
          removed_edges[link.objidx]++;
          if (!(vertices_[link.objidx].incoming_edges () - removed_edges[link.objidx]))
            // Add the order that the links were encountered to the priority.
            // This ensures that ties between priorities objects are broken in a consistent
            // way. More specifically this is set up so that if a set of objects have the same
            // distance they'll be added to the topological order in the order that they are
            // referenced from the parent object.
            queue.insert (vertices_[link.objidx].modified_distance (order++),
                          link.objidx);
        }
      }
  
      check_success (!queue.in_error ());
      check_success (!sorted_graph.in_error ());
      if (!check_success (new_id == -1))
        print_orphaned_nodes ();
  
      remap_all_obj_indices (id_map, &sorted_graph);
  
      hb_swap (vertices_, sorted_graph);
      sorted_graph.fini_deep ();
    }
  
    /*
     * Assign unique space numbers to each connected subgraph of 32 bit offset(s).
     */
    bool assign_32bit_spaces ()
    {
      unsigned root_index = root_idx ();
      hb_set_t visited;
      hb_set_t roots;
      for (unsigned i = 0; i <= root_index; i++)
      {
        // Only real links can form 32 bit spaces
        for (auto& l : vertices_[i].obj.real_links)
        {
          if (l.width == 4 && !l.is_signed)
          {
            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.
      visited.invert ();
  
      if (!roots) return false;
  
      while (roots)
      {
        unsigned next = HB_SET_VALUE_INVALID;
        if (!roots.next (&next)) break;
  
        hb_set_t connected_roots;
        find_connected_nodes (next, roots, visited, connected_roots);
        isolate_subgraph (connected_roots);
  
        unsigned next_space = this->next_space ();
        num_roots_for_space_.push (0);
        for (unsigned root : connected_roots)
        {
          DEBUG_MSG (SUBSET_REPACK, nullptr, "Subgraph %u gets space %u", root, next_space);
          vertices_[root].space = next_space;
          num_roots_for_space_[next_space] = num_roots_for_space_[next_space] + 1;
          distance_invalid = true;
          positions_invalid = true;
        }
  
        // TODO(grieger): special case for GSUB/GPOS use extension promotions to move 16 bit space
        //                into the 32 bit space as needed, instead of using isolation.
      }
  
      return true;
    }
  
    /*
     * Isolates the subgraph of nodes reachable from root. Any links to nodes in the subgraph
     * that originate from outside of the subgraph will be removed by duplicating the linked to
     * object.
     *
     * Indices stored in roots will be updated if any of the roots are duplicated to new indices.
     */
    bool isolate_subgraph (hb_set_t& roots)
    {
      update_parents ();
      hb_hashmap_t<unsigned, unsigned> subgraph;
  
      // incoming edges to root_idx should be all 32 bit in length so we don't need to de-dup these
      // set the subgraph incoming edge count to match all of root_idx's incoming edges
      hb_set_t parents;
      for (unsigned root_idx : roots)
      {
        subgraph.set (root_idx, wide_parents (root_idx, parents));
        find_subgraph (root_idx, subgraph);
      }
  
      unsigned original_root_idx = root_idx ();
      hb_hashmap_t<unsigned, unsigned> index_map;
      bool made_changes = false;
      for (auto entry : subgraph.iter ())
      {
        const auto& node = vertices_[entry.first];
        unsigned subgraph_incoming_edges = entry.second;
  
        if (subgraph_incoming_edges < node.incoming_edges ())
        {
          // Only  de-dup objects with incoming links from outside the subgraph.
          made_changes = true;
          duplicate_subgraph (entry.first, index_map);
        }
      }
  
      if (!made_changes)
        return false;
  
      if (original_root_idx != root_idx ()
          && parents.has (original_root_idx))
      {
        // If the root idx has changed since parents was determined, update root idx in parents
        parents.add (root_idx ());
        parents.del (original_root_idx);
      }
  
      auto new_subgraph =
          + subgraph.keys ()
          | hb_map([&] (unsigned node_idx) {
            if (index_map.has (node_idx)) return index_map[node_idx];
            return node_idx;
          })
          ;
  
      remap_obj_indices (index_map, new_subgraph);
      remap_obj_indices (index_map, parents.iter (), true);
  
      // Update roots set with new indices as needed.
      unsigned next = HB_SET_VALUE_INVALID;
      while (roots.next (&next))
      {
        if (index_map.has (next))
        {
          roots.del (next);
          roots.add (index_map[next]);
        }
      }
  
      return true;
    }
  
    void find_subgraph (unsigned node_idx, hb_hashmap_t<unsigned, unsigned>& subgraph)
    {
      for (const auto& link : vertices_[node_idx].obj.all_links ())
      {
        if (subgraph.has (link.objidx))
        {
          subgraph.set (link.objidx, subgraph[link.objidx] + 1);
          continue;
        }
        subgraph.set (link.objidx, 1);
        find_subgraph (link.objidx, subgraph);
      }
    }
  
    void find_subgraph (unsigned node_idx, hb_set_t& subgraph)
    {
      if (subgraph.has (node_idx)) return;
      subgraph.add (node_idx);
      for (const auto& link : vertices_[node_idx].obj.all_links ())
        find_subgraph (link.objidx, subgraph);
    }
  
    /*
     * 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
     * been performed for a given index, then it will be skipped.
     */
    void duplicate_subgraph (unsigned node_idx, hb_hashmap_t<unsigned, unsigned>& index_map)
    {
      if (index_map.has (node_idx))
        return;
  
      index_map.set (node_idx, duplicate (node_idx));
      for (const auto& l : object (node_idx).all_links ()) {
        duplicate_subgraph (l.objidx, index_map);
      }
    }
  
    /*
     * Creates a copy of node_idx and returns it's new index.
     */
    unsigned duplicate (unsigned node_idx)
    {
      positions_invalid = true;
      distance_invalid = true;
  
      auto* clone = vertices_.push ();
      auto& child = vertices_[node_idx];
      if (vertices_.in_error ()) {
        return -1;
      }
  
      clone->obj.head = child.obj.head;
      clone->obj.tail = child.obj.tail;
      clone->distance = child.distance;
      clone->space = child.space;
      clone->parents.reset ();
  
      unsigned clone_idx = vertices_.length - 2;
      for (const auto& l : child.obj.real_links)
      {
        clone->obj.real_links.push (l);
        vertices_[l.objidx].parents.push (clone_idx);
      }
      for (const auto& l : child.obj.virtual_links)
      {
        clone->obj.virtual_links.push (l);
        vertices_[l.objidx].parents.push (clone_idx);
      }
  
      check_success (!clone->obj.real_links.in_error ());
      check_success (!clone->obj.virtual_links.in_error ());
  
      // The last object is the root of the graph, so swap back the root to the end.
      // The root's obj idx does change, however since it's root nothing else refers to it.
      // all other obj idx's will be unaffected.
      vertex_t root = vertices_[vertices_.length - 2];
      vertices_[clone_idx] = *clone;
      vertices_[vertices_.length - 1] = root;
  
      // Since the root moved, update the parents arrays of all children on the root.
      for (const auto& l : root.obj.all_links ())
        vertices_[l.objidx].remap_parent (root_idx () - 1, root_idx ());
  
      return clone_idx;
    }
  
    /*
     * Creates a copy of child and re-assigns the link from
     * parent to the clone. The copy is a shallow copy, objects
     * linked from child are not duplicated.
     */
    bool duplicate (unsigned parent_idx, unsigned child_idx)
    {
      update_parents ();
  
      unsigned links_to_child = 0;
      for (const auto& l : vertices_[parent_idx].obj.all_links ())
      {
        if (l.objidx == child_idx) links_to_child++;
      }
  
      if (vertices_[child_idx].incoming_edges () <= links_to_child)
      {
        // Can't duplicate this node, doing so would orphan the original one as all remaining links
        // to child are from parent.
        DEBUG_MSG (SUBSET_REPACK, nullptr, "  Not duplicating %d => %d",
                   parent_idx, child_idx);
        return false;
      }
  
      DEBUG_MSG (SUBSET_REPACK, nullptr, "  Duplicating %d => %d",
                 parent_idx, child_idx);
  
      unsigned clone_idx = duplicate (child_idx);
      if (clone_idx == (unsigned) -1) return false;
      // duplicate shifts the root node idx, so if parent_idx was root update it.
      if (parent_idx == clone_idx) parent_idx++;
  
      auto& parent = vertices_[parent_idx];
      for (auto& l : parent.obj.all_links_writer ())
      {
        if (l.objidx != child_idx)
          continue;
  
        reassign_link (l, parent_idx, clone_idx);
      }
  
      return true;
    }
  
    /*
     * Raises the sorting priority of all children.
     */
    bool raise_childrens_priority (unsigned parent_idx)
    {
      DEBUG_MSG (SUBSET_REPACK, nullptr, "  Raising priority of all children of %d",
                 parent_idx);
      // This operation doesn't change ordering until a sort is run, so no need
      // to invalidate positions. It does not change graph structure so no need
      // to update distances or edge counts.
      auto& parent = vertices_[parent_idx].obj;
      bool made_change = false;
      for (auto& l : parent.all_links_writer ())
        made_change |= vertices_[l.objidx].raise_priority ();
      return made_change;
    }
  
    /*
     * Will any offsets overflow on graph when it's serialized?
     */
    bool will_overflow (hb_vector_t<overflow_record_t>* overflows = nullptr)
    {
      if (overflows) overflows->resize (0);
      update_positions ();
  
      for (int parent_idx = vertices_.length - 1; parent_idx >= 0; parent_idx--)
      {
        // Don't need to check virtual links for overflow
        for (const auto& link : vertices_[parent_idx].obj.real_links)
        {
          int64_t offset = compute_offset (parent_idx, link);
          if (is_valid_offset (offset, link))
            continue;
  
          if (!overflows) return true;
  
          overflow_record_t r;
          r.parent = parent_idx;
          r.child = link.objidx;
          overflows->push (r);
        }
      }
  
      if (!overflows) return false;
      return overflows->length;
    }
  
    void print_orphaned_nodes ()
    {
      if (!DEBUG_ENABLED(SUBSET_REPACK)) return;
  
      DEBUG_MSG (SUBSET_REPACK, nullptr, "Graph is not fully connected.");
      parents_invalid = true;
      update_parents();
  
      for (unsigned i = 0; i < root_idx (); i++)
      {
        const auto& v = vertices_[i];
        if (!v.parents)
          DEBUG_MSG (SUBSET_REPACK, nullptr, "Node %u is orphaned.", i);
      }
    }
  
    void print_overflows (const hb_vector_t<overflow_record_t>& overflows)
    {
      if (!DEBUG_ENABLED(SUBSET_REPACK)) return;
  
      update_parents ();
      int limit = 10;
      for (const auto& o : overflows)
      {
        if (!limit--) break;
        const auto& parent = vertices_[o.parent];
        const auto& child = vertices_[o.child];
        DEBUG_MSG (SUBSET_REPACK, nullptr,
                   "  overflow from "
                   "%4d (%4d in, %4d out, space %2d) => "
                   "%4d (%4d in, %4d out, space %2d)",
                   o.parent,
                   parent.incoming_edges (),
                   parent.obj.real_links.length + parent.obj.virtual_links.length,
                   space_for (o.parent),
                   o.child,
                   child.incoming_edges (),
                   child.obj.real_links.length + child.obj.virtual_links.length,
                   space_for (o.child));
      }
      if (overflows.length > 10) {
        DEBUG_MSG (SUBSET_REPACK, nullptr, "  ... plus %d more overflows.", overflows.length - 10);
      }
    }
  
    unsigned num_roots_for_space (unsigned space) const
    {
      return num_roots_for_space_[space];
    }
  
    unsigned next_space () const
    {
      return num_roots_for_space_.length;
    }
  
    void move_to_new_space (const hb_set_t& indices)
    {
      num_roots_for_space_.push (0);
      unsigned new_space = num_roots_for_space_.length - 1;
  
      for (unsigned index : indices) {
        auto& node = vertices_[index];
        num_roots_for_space_[node.space] = num_roots_for_space_[node.space] - 1;
        num_roots_for_space_[new_space] = num_roots_for_space_[new_space] + 1;
        node.space = new_space;
        distance_invalid = true;
        positions_invalid = true;
      }
    }
  
    unsigned space_for (unsigned index, unsigned* root = nullptr) const
    {
      const auto& node = vertices_[index];
      if (node.space)
      {
        if (root != nullptr)
          *root = index;
        return node.space;
      }
  
      if (!node.parents)
      {
        if (root)
          *root = index;
        return 0;
      }
  
      return space_for (node.parents[0], root);
    }
  
    void err_other_error () { this->successful = false; }
  
   private:
  
    size_t serialized_length () const {
      size_t total_size = 0;
      for (unsigned i = 0; i < vertices_.length; i++) {
        size_t size = vertices_[i].obj.tail - vertices_[i].obj.head;
        total_size += size;
      }
      return total_size;
    }
  
    /*
     * Returns the numbers of incoming edges that are 32bits wide.
     */
    unsigned wide_parents (unsigned node_idx, hb_set_t& parents) const
    {
      unsigned count = 0;
      hb_set_t visited;
      for (unsigned p : vertices_[node_idx].parents)
      {
        if (visited.has (p)) continue;
        visited.add (p);
  
        // 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)
          {
            count++;
            parents.add (p);
          }
        }
      }
      return count;
    }
  
    bool check_success (bool success)
    { return this->successful && (success || (err_other_error (), false)); }
  
    /*
     * Creates a map from objid to # of incoming edges.
     */
    void update_parents ()
    {
      if (!parents_invalid) return;
  
      for (unsigned i = 0; i < vertices_.length; i++)
        vertices_[i].parents.reset ();
  
      for (unsigned p = 0; p < vertices_.length; p++)
      {
        for (auto& l : vertices_[p].obj.all_links ())
        {
          vertices_[l.objidx].parents.push (p);
        }
      }
  
      parents_invalid = false;
    }
  
    /*
     * compute the serialized start and end positions for each vertex.
     */
    void update_positions ()
    {
      if (!positions_invalid) return;
  
      unsigned current_pos = 0;
      for (int i = root_idx (); i >= 0; i--)
      {
        auto& v = vertices_[i];
        v.start = current_pos;
        current_pos += v.obj.tail - v.obj.head;
        v.end = current_pos;
      }
  
      positions_invalid = false;
    }
  
    /*
     * Finds the distance to each object in the graph
     * from the initial node.
     */
    void update_distances ()
    {
      if (!distance_invalid) return;
  
      // Uses Dijkstra's algorithm to find all of the shortest distances.
      // https://en.wikipedia.org/wiki/Dijkstra%27s_algorithm
      //
      // Implementation Note:
      // Since our priority queue doesn't support fast priority decreases
      // we instead just add new entries into the queue when a priority changes.
      // Redundant ones are filtered out later on by the visited set.
      // According to https://www3.cs.stonybrook.edu/~rezaul/papers/TR-07-54.pdf
      // for practical performance this is faster then using a more advanced queue
      // (such as a fibonaacci queue) with a fast decrease priority.
      for (unsigned i = 0; i < vertices_.length; i++)
      {
        if (i == vertices_.length - 1)
          vertices_[i].distance = 0;
        else
          vertices_[i].distance = hb_int_max (int64_t);
      }
  
      hb_priority_queue_t queue;
      queue.insert (0, vertices_.length - 1);
  
      hb_vector_t<bool> visited;
      visited.resize (vertices_.length);
  
      while (!queue.in_error () && !queue.is_empty ())
      {
        unsigned next_idx = queue.pop_minimum ().second;
        if (visited[next_idx]) continue;
        const auto& next = vertices_[next_idx];
        int64_t next_distance = vertices_[next_idx].distance;
        visited[next_idx] = true;
  
        for (const auto& link : next.obj.all_links ())
        {
          if (visited[link.objidx]) continue;
  
          const auto& child = vertices_[link.objidx].obj;
          unsigned link_width = link.width ? link.width : 4; // treat virtual offsets as 32 bits wide
          int64_t child_weight = (child.tail - child.head) +
                                 ((int64_t) 1 << (link_width * 8)) * (vertices_[link.objidx].space + 1);
          int64_t child_distance = next_distance + child_weight;
  
          if (child_distance < vertices_[link.objidx].distance)
          {
            vertices_[link.objidx].distance = child_distance;
            queue.insert (child_distance, link.objidx);
          }
        }
      }
  
      check_success (!queue.in_error ());
      if (!check_success (queue.is_empty ()))
      {
        print_orphaned_nodes ();
        return;
      }
  
      distance_invalid = false;
    }
  
    int64_t compute_offset (
        unsigned parent_idx,
        const hb_serialize_context_t::object_t::link_t& link) const
    {
      const auto& parent = vertices_[parent_idx];
      const auto& child = vertices_[link.objidx];
      int64_t offset = 0;
      switch ((hb_serialize_context_t::whence_t) link.whence) {
        case hb_serialize_context_t::whence_t::Head:
          offset = child.start - parent.start; break;
        case hb_serialize_context_t::whence_t::Tail:
          offset = child.start - parent.end; break;
        case hb_serialize_context_t::whence_t::Absolute:
          offset = child.start; break;
      }
  
      assert (offset >= link.bias);
      offset -= link.bias;
      return offset;
    }
  
    bool is_valid_offset (int64_t offset,
                          const hb_serialize_context_t::object_t::link_t& link) const
    {
      if (unlikely (!link.width))
        // Virtual links can't overflow.
        return link.is_signed || offset >= 0;
  
      if (link.is_signed)
      {
        if (link.width == 4)
          return offset >= -((int64_t) 1 << 31) && offset < ((int64_t) 1 << 31);
        else
          return offset >= -(1 << 15) && offset < (1 << 15);
      }
      else
      {
        if (link.width == 4)
          return offset >= 0 && offset < ((int64_t) 1 << 32);
        else if (link.width == 3)
          return offset >= 0 && offset < ((int32_t) 1 << 24);
        else
          return offset >= 0 && offset < (1 << 16);
      }
    }
  
    /*
     * Updates a link in the graph to point to a different object. Corrects the
     * parents vector on the previous and new child nodes.
     */
    void reassign_link (hb_serialize_context_t::object_t::link_t& link,
                        unsigned parent_idx,
                        unsigned new_idx)
    {
      unsigned old_idx = link.objidx;
      link.objidx = new_idx;
      vertices_[old_idx].remove_parent (parent_idx);
      vertices_[new_idx].parents.push (parent_idx);
    }
  
    /*
     * Updates all objidx's in all links using the provided mapping. Corrects incoming edge counts.
     */
    template<typename Iterator, hb_requires (hb_is_iterator (Iterator))>
    void remap_obj_indices (const hb_hashmap_t<unsigned, unsigned>& id_map,
                            Iterator subgraph,
                            bool only_wide = false)
    {
      if (!id_map) return;
      for (unsigned i : subgraph)
      {
        for (auto& link : vertices_[i].obj.all_links_writer ())
        {
          if (!id_map.has (link.objidx)) continue;
          if (only_wide && !(link.width == 4 && !link.is_signed)) continue;
  
          reassign_link (link, i, id_map[link.objidx]);
        }
      }
    }
  
    /*
     * Updates all objidx's in all links using the provided mapping.
     */
    void remap_all_obj_indices (const hb_vector_t<unsigned>& id_map,
                                hb_vector_t<vertex_t>* sorted_graph) const
    {
      for (unsigned i = 0; i < sorted_graph->length; i++)
      {
        (*sorted_graph)[i].remap_parents (id_map);
        for (auto& link : (*sorted_graph)[i].obj.all_links_writer ())
        {
          link.objidx = id_map[link.objidx];
        }
      }
    }
  
    template <typename O> void
    serialize_link_of_type (const hb_serialize_context_t::object_t::link_t& link,
                            char* head,
                            hb_serialize_context_t* c) const
    {
      OT::Offset<O>* offset = reinterpret_cast<OT::Offset<O>*> (head + link.position);
      *offset = 0;
      c->add_link (*offset,
                   // serializer has an extra nil object at the start of the
                   // object array. So all id's are +1 of what our id's are.
                   link.objidx + 1,
                   (hb_serialize_context_t::whence_t) link.whence,
                   link.bias);
    }
  
    void serialize_link (const hb_serialize_context_t::object_t::link_t& link,
                   char* head,
                   hb_serialize_context_t* c) const
    {
      switch (link.width)
      {
      case 0:
        // Virtual links aren't serialized.
        return;
      case 4:
        if (link.is_signed)
        {
          serialize_link_of_type<OT::HBINT32> (link, head, c);
        } else {
          serialize_link_of_type<OT::HBUINT32> (link, head, c);
        }
        return;
      case 2:
        if (link.is_signed)
        {
          serialize_link_of_type<OT::HBINT16> (link, head, c);
        } else {
          serialize_link_of_type<OT::HBUINT16> (link, head, c);
        }
        return;
      case 3:
        serialize_link_of_type<OT::HBUINT24> (link, head, c);
        return;
      default:
        // Unexpected link width.
        assert (0);
      }
    }
  
    /*
     * Finds all nodes in targets that are reachable from start_idx, nodes in visited will be skipped.
     * For this search the graph is treated as being undirected.
     *
     * Connected targets will be added to connected and removed from targets. All visited nodes
     * will be added to visited.
     */
    void find_connected_nodes (unsigned start_idx,
                               hb_set_t& targets,
                               hb_set_t& visited,
                               hb_set_t& connected)
    {
      if (visited.has (start_idx)) return;
      visited.add (start_idx);
  
      if (targets.has (start_idx))
      {
        targets.del (start_idx);
        connected.add (start_idx);
      }
  
      const auto& v = vertices_[start_idx];
  
      // Graph is treated as undirected so search children and parents of start_idx
      for (const auto& l : v.obj.all_links ())
        find_connected_nodes (l.objidx, targets, visited, connected);
  
      for (unsigned p : v.parents)
        find_connected_nodes (p, targets, visited, connected);
    }
  
   public:
    // TODO(garretrieger): make private, will need to move most of offset overflow code into graph.
    hb_vector_t<vertex_t> vertices_;
   private:
    bool parents_invalid;
    bool distance_invalid;
    bool positions_invalid;
    bool successful;
    hb_vector_t<unsigned> num_roots_for_space_;
  };
  
  static bool _try_isolating_subgraphs (const hb_vector_t<graph_t::overflow_record_t>& overflows,
                                        graph_t& sorted_graph)
  {
    unsigned space = 0;
    hb_set_t roots_to_isolate;
  
    for (int i = overflows.length - 1; i >= 0; i--)
    {
      const graph_t::overflow_record_t& r = overflows[i];
  
      unsigned root;
      unsigned overflow_space = sorted_graph.space_for (r.parent, &root);
      if (!overflow_space) continue;
      if (sorted_graph.num_roots_for_space (overflow_space) <= 1) continue;
  
      if (!space) {
        space = overflow_space;
      }
  
      if (space == overflow_space)
        roots_to_isolate.add(root);
    }
  
    if (!roots_to_isolate) return false;
  
    unsigned maximum_to_move = hb_max ((sorted_graph.num_roots_for_space (space) / 2u), 1u);
    if (roots_to_isolate.get_population () > maximum_to_move) {
      // Only move at most half of the roots in a space at a time.
      unsigned extra = roots_to_isolate.get_population () - maximum_to_move;
      while (extra--) {
        unsigned root = HB_SET_VALUE_INVALID;
        roots_to_isolate.previous (&root);
        roots_to_isolate.del (root);
      }
    }
  
    DEBUG_MSG (SUBSET_REPACK, nullptr,
               "Overflow in space %d (%d roots). Moving %d roots to space %d.",
               space,
               sorted_graph.num_roots_for_space (space),
               roots_to_isolate.get_population (),
               sorted_graph.next_space ());
  
    sorted_graph.isolate_subgraph (roots_to_isolate);
    sorted_graph.move_to_new_space (roots_to_isolate);
  
    return true;
  }
  
  static bool _process_overflows (const hb_vector_t<graph_t::overflow_record_t>& overflows,
                                  hb_set_t& priority_bumped_parents,
                                  graph_t& sorted_graph)
  {
    bool resolution_attempted = false;
  
    // Try resolving the furthest overflows first.
    for (int i = overflows.length - 1; i >= 0; i--)
    {
      const graph_t::overflow_record_t& r = overflows[i];
      const auto& child = sorted_graph.vertices_[r.child];
      if (child.is_shared ())
      {
        // The child object is shared, we may be able to eliminate the overflow
        // by duplicating it.
        if (!sorted_graph.duplicate (r.parent, r.child)) continue;
        return true;
      }
  
      if (child.is_leaf () && !priority_bumped_parents.has (r.parent))
      {
        // This object is too far from it's parent, attempt to move it closer.
        //
        // TODO(garretrieger): initially limiting this to leaf's since they can be
        //                     moved closer with fewer consequences. However, this can
        //                     likely can be used for non-leafs as well.
        // TODO(garretrieger): also try lowering priority of the parent. Make it
        //                     get placed further up in the ordering, closer to it's children.
        //                     this is probably preferable if the total size of the parent object
        //                     is < then the total size of the children (and the parent can be moved).
        //                     Since in that case moving the parent will cause a smaller increase in
        //                     the length of other offsets.
        if (sorted_graph.raise_childrens_priority (r.parent)) {
          priority_bumped_parents.add (r.parent);
          resolution_attempted = true;
        }
        continue;
      }
  
      // TODO(garretrieger): add additional offset resolution strategies
      // - Promotion to extension lookups.
      // - Table splitting.
    }
  
    return resolution_attempted;
  }
  
  /*
   * Attempts to modify the topological sorting of the provided object graph to
   * eliminate offset overflows in the links between objects of the graph. If a
   * non-overflowing ordering is found the updated graph is serialized it into the
   * provided serialization context.
   *
   * If necessary the structure of the graph may be modified in ways that do not
   * affect the functionality of the graph. For example shared objects may be
   * duplicated.
   *
   * For a detailed writeup describing how the algorithm operates see:
   * docs/repacker.md
   */
  inline hb_blob_t*
  hb_resolve_overflows (const hb_vector_t<hb_serialize_context_t::object_t *>& packed,
                        hb_tag_t table_tag,
                        unsigned max_rounds = 20) {
    // Kahn sort is ~twice as fast as shortest distance sort and works for many fonts
    // so try it first to save time.
    graph_t sorted_graph (packed);
    sorted_graph.sort_kahn ();
    if (!sorted_graph.will_overflow ())
    {
      return sorted_graph.serialize ();
    }
  
    sorted_graph.sort_shortest_distance ();
  
    if ((table_tag == HB_OT_TAG_GPOS
         ||  table_tag == HB_OT_TAG_GSUB)
        && sorted_graph.will_overflow ())
    {
      DEBUG_MSG (SUBSET_REPACK, nullptr, "Assigning spaces to 32 bit subgraphs.");
      if (sorted_graph.assign_32bit_spaces ())
        sorted_graph.sort_shortest_distance ();
    }
  
    unsigned round = 0;
    hb_vector_t<graph_t::overflow_record_t> overflows;
    // TODO(garretrieger): select a good limit for max rounds.
    while (!sorted_graph.in_error ()
           && sorted_graph.will_overflow (&overflows)
           && round++ < max_rounds) {
      DEBUG_MSG (SUBSET_REPACK, nullptr, "=== Overflow resolution round %d ===", round);
      sorted_graph.print_overflows (overflows);
  
      hb_set_t priority_bumped_parents;
  
      if (!_try_isolating_subgraphs (overflows, sorted_graph))
      {
        if (!_process_overflows (overflows, priority_bumped_parents, sorted_graph))
        {
          DEBUG_MSG (SUBSET_REPACK, nullptr, "No resolution available :(");
          break;
        }
      }
  
      sorted_graph.sort_shortest_distance ();
    }
  
    if (sorted_graph.in_error ())
    {
      DEBUG_MSG (SUBSET_REPACK, nullptr, "Sorted graph in error state.");
      return nullptr;
    }
  
    if (sorted_graph.will_overflow ())
    {
      DEBUG_MSG (SUBSET_REPACK, nullptr, "Offset overflow resolution failed.");
      return nullptr;
    }
  
    return sorted_graph.serialize ();
  }
  
  #endif /* HB_REPACKER_HH */
  

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