void prepare_frontier(frontier_t* f,
                      gcuda::multi_context_t& context) override {
  auto P = this->get_problem();
  f->push_back(P->param.single_source);
}

void loop(gcuda::multi_context_t& context) override {
  auto E = this->get_enactor();   // Pointer to enactor interface.
  auto P = this->get_problem();   // Pointer to problem (data) interface.
  auto G = P->get_graph();        // Graph that we are processing.

  auto single_source = P->param.single_source;  // Initial source node.
  auto distances = P->result.distances;         // Distances array for BFS.
  auto visited = P->visited.data().get();       // Visited map.
  auto iteration = this->iteration;             // Iteration we are on.

  // Following lambda expression is applied on every source,
  // neighbor, edge, weight tuple during the traversal.
  // Our intent here is to find and update the minimum distance when found.
  // And return which neighbor goes in the output frontier after traversal.
  auto search = [=] __host__ __device__(
                      vertex_t const& source,    // ... source
                      vertex_t const& neighbor,  // neighbor
                      edge_t const& edge,        // edge
                      weight_t const& weight     // weight (tuple).
                      ) -> bool {
    auto old_distance =
      math::atomic::min(&distances[neighbor], iteration   1);
    return (iteration   1 < old_distance);
  };

  // Execute advance operator on the search lambda expression.
  // Uses load_balance_t::block_mapped algorithm (try others for perf. tuning.)
  operators::advance::execute<operators::load_balance_t::block_mapped>(
    G, E, search, context);
}