LLDependenciesBase::VertexList LLDependenciesBase::topo_sort(int vertices, const EdgeList& edges) const
{
// Construct a Boost Graph Library graph according to the constraints
// we've collected. It seems as though we ought to be able to capture
// the uniqueness of vertex keys using a setS of vertices with a
// string property -- but I don't yet understand adjacency_list well
// enough to get there. All the examples I've seen so far use integers
// for vertices.
// Define the Graph type. Use a vector for vertices so we can use the
// default topological_sort vertex lookup by int index. Use a set for
// edges because the same dependency may be stated twice: Node "a" may
// specify that it must precede "b", while "b" may also state that it
// must follow "a".
typedef boost::adjacency_list<boost::setS, boost::vecS, boost::directedS,
boost::no_property> Graph;
// Instantiate the graph. Without vertex properties, we need say no
// more about vertices than the total number.
Graph g(edges.begin(), edges.end(), vertices);
// topo sort
typedef boost::graph_traits<Graph>::vertex_descriptor VertexDesc;
typedef std::vector<VertexDesc> SortedList;
SortedList sorted;
// note that it throws not_a_dag if it finds a cycle
try
{
boost::topological_sort(g, std::back_inserter(sorted));
}
catch (const boost::not_a_dag& e)
{
// translate to the exception we define
std::ostringstream out;
out << "LLDependencies cycle: " << e.what() << '\n';
// Omit independent nodes: display only those that might contribute to
// the cycle.
describe(out, false);
throw Cycle(out.str());
}
// A peculiarity of boost::topological_sort() is that it emits results in
// REVERSE topological order: to get the result you want, you must
// traverse the SortedList using reverse iterators.
return VertexList(sorted.rbegin(), sorted.rend());
}
