sortedTopologicalOrdering property

Set<T>? sortedTopologicalOrdering

Returns an ordered set of all vertices in topological order.

• For every directed edge: (vertex1 -> vertex2), vertex1 is listed before vertex2.

• If a vertex comparator is specified, the sorting will be applied in addition to the topological order.

• Note: There is no topological ordering if the graph is cyclic. In that case the function returns null.

• Any self-loop renders a directed graph cyclic.

• Based on Kahn's algorithm.

Implementation

Set<T>? get sortedTopologicalOrdering {
  if (_comparator == null) return topologicalOrdering;

  final result = <T>{};

  // Get modifiable in-degree map.
  final localInDegreeMap = HashMap<T, int>.of(inDegreeMap);

  // Add all sources (vertices with inDegree == 0) to queue.
  // Using a list instead of a queue to enable sorting of [sources].
  //
  // Note: In an acyclic directed graph there is at least
  //       one vertex with inDegree equal to zero.
  final sources = PriorityQueue<T>(comparator);
  for (final vertex in vertices) {
    if (localInDegreeMap[vertex] == 0) {
      sources.add(vertex);
    }
  }
  // Initialize count of visited vertices.
  var count = 0;

  // Note: In an acyclic directed graph at least
  // one vertex has outDegree zero.
  while (sources.isNotEmpty) {
    result.add(sources.removeFirst());

    // Simulate removing the current vertex from the
    //   graph. => Connected vertices will have inDegree = inDegree - 1.
    for (final vertex in edges(result.last)) {
      var inDegree = localInDegreeMap[vertex]!;
      inDegree--;
      localInDegreeMap[vertex] = inDegree;

      // Add new local source vertices of the remaining graph to the queue.
      if (inDegree == 0) {
        sources.add(vertex);
      }
    }
    // Increment count of visited vertices.
    count++;
  }
  return (count == length) ? result : null;
}