Github user hakanmemisoglu commented on a diff in the pull request:

    https://github.com/apache/incubator-quickstep/pull/233#discussion_r112067899
  
    --- Diff: utility/DAG.hpp ---
    @@ -489,6 +495,40 @@ bool DAG<T, LinkMetadataT>::hasCycleHelper(const 
typename DAG<T, LinkMetadataT>:
       return false;
     }
     
    +template <class T, class LinkMetadataT>
    +std::vector<typename DAG<T, LinkMetadataT>::size_type_nodes> DAG<T, 
LinkMetadataT>::getTopologicalSorting() const {
    +  // We implement "Kahn's algorithm" for the sorting.
    +  DCHECK(!hasCycle());
    +  std::unique_ptr<std::vector<typename DAG<T, 
LinkMetadataT>::size_type_nodes>>
    +      sorted_list(new std::vector<size_type_nodes>());
    +  sorted_list->reserve(this->size());
    +  // Key = node ID, value = # incoming edges for this node.
    +  // NOTE(harshad) - We modify the "values" in this map as we go along.
    +  std::unordered_map<typename DAG<T, LinkMetadataT>::size_type_nodes, 
std::size_t> num_incoming_edges;
    +  std::queue<typename DAG<T, LinkMetadataT>::size_type_nodes> 
nodes_with_no_children;
    +  for (auto node_id = 0u; node_id < this->size(); ++node_id) {
    +    if (nodes_[node_id].getDependencies().empty()) {
    +      nodes_with_no_children.emplace(node_id);
    +    }
    +    num_incoming_edges[node_id] = nodes_[node_id].getDependencies().size();
    +  }
    +  while (!nodes_with_no_children.empty()) {
    --- End diff --
    
    I think this is the place where Kahn's algorithm starts to work. Can you 
separate methods into logical pieces with comments or by refactoring?


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