private static IEnumerable <V> ReversePostOrder <V>(DiGraph <V> graph)
{
var visited = new HashSet <V>();
var stack = new LinkedList <V>();
void Go(V v)
{
visited.Add(v);
var neighbours = graph.Edges(v).Select(e => e.to);
foreach (var w in neighbours)
{
if (!visited.Contains(w))
{
Go(w);
}
}
stack.Push(v);
}
// Account for the fact that:
// - Not all vertices are reachable from one another
// - The vertex we call Go on first might have incoming links
foreach (var v in graph.Vertices())
{
if (!visited.Contains(v))
{
Go(v);
}
}
return(stack);
}
/// <summary>
/// Solves the single-source shortest path problem in O(E*ln V) time for a graph with none-negative edge weights.
/// </summary>
public static Paths <V> Dijkstra <V>(DiGraph <V> graph, V source) where V : class
{
if (graph == null || source == null)
{
throw new ArgumentNullException();
}
if (!graph.Contains(source))
{
throw new ArgumentException();
}
if (graph.Edges().Any(e => e.weight < 0))
{
throw new ArgumentException();
}
var queue = new BinaryHeap <Double, V>();
var distTo = new HashMap <V, Double>();
var edgeTo = new HashMap <V, Edge <V> >();
void Relax(Edge <V> e)
{
var(v, w) = (e.from, e.to);
var weight = new Double(e.weight);
if (distTo[v] > distTo[w] + weight)
{
distTo[w] = distTo[v] + weight;
edgeTo[w] = e;
if (queue.Contains(w))
{
queue.Update(w, distTo[w]);
}
else
{
queue.Push(w, distTo[w]);
}
}
}
foreach (var v in graph.Vertices())
{
distTo[v] = Double.MaxValue;
}
distTo[source] = new Double(0);
queue.Push(source, new Double(0));
while (!queue.IsEmpty())
{
var(_, v) = queue.Pop();
foreach (var e in graph.Edges(v))
{
Relax(e);
}
}
return(new Paths <V>(source, edgeTo));
}
/// <summary>
/// Solves the single-source shortest path problem in O(E*V) time for a graph without any negative cycles, or in O(V+E) time if it is a DAG.
/// </summary>
public static Paths <V> BellmanFord <V>(DiGraph <V> graph, V source)
{
if (graph == null || source == null)
{
throw new ArgumentNullException();
}
if (!graph.Contains(source))
{
throw new ArgumentException();
}
var distTo = new HashMap <V, Double>();
var edgeTo = new HashMap <V, Edge <V> >();
bool Relax(Edge <V> e)
{
var(v, w) = (e.from, e.to);
var weight = new Double(e.weight);
if (distTo[v] > distTo[w] + weight)
{
distTo[w] = distTo[v] + weight;
edgeTo[w] = e;
return(true);
}
return(false);
}
foreach (var v in graph.Vertices())
{
distTo[v] = Double.MaxValue;
}
distTo[source] = new Double(0);
// The Bellman Ford procedure involves relaxing every edge V times.
// However, for a DAG we can delegate to a simpler algorithm that runs in linear time
try
{
var vertices = TopologicalSort(graph);
foreach (var v in vertices)
{
foreach (var e in graph.Edges(v))
{
Relax(e);
}
}
}
catch (InvalidOperationException)
{
// Graph is not a DAG
var N = graph.Size();
foreach (var i in Enumerable.Range(1, N))
{
foreach (var e in graph.Edges())
{
var relaxed = Relax(e);
if (i == N && relaxed)
{
throw new InvalidOperationException("Negative cycle detected");
}
}
}
}
return(new Paths <V>(source, edgeTo));
}
