/// <summary>
/// Computes a shortest paths tree from <tt>s</tt> to every other vertex in the edge-weighted digraph <tt>G</tt>.
/// </summary>
/// <param name="graph">the edge-weighted digraph</param>
/// <param name="source">the source vertex</param>
public DijkstraDirectedSearch(EdgeWeightedDirectedGraph graph, int source)
{
for (int i = graph.NumberOfEdges - 1; i >= 0; i--)
{
WeightedGraphEdge e = graph.GetEdge(i);
if (e.Cost < 0)
{
throw new RPGException(ErrorMessage.INTERNAL_BAD_ARGUMENT, "edge " + e + " has negative weight");
}
}
distTo = new double[graph.NumberOfVertices];
edgeTo = new WeightedGraphEdge[graph.NumberOfVertices];
for (int v = 0; v < graph.NumberOfVertices; v++)
{
distTo[v] = Double.PositiveInfinity;
}
distTo[source] = 0.0;
// relax vertices in order of distance from s
pq = new IndexMinPQ <Double>(graph.NumberOfVertices);
pq.Insert(source, distTo[source]);
while (!pq.IsEmpty)
{
int v = pq.DelMin();
WeightedGraphEdge[] adj = graph.GetVertexAdjacencies(v);
for (int i = adj.Length - 1; i >= 0; i--)
{
Relax(adj[i], v);
}
}
// check optimality conditions
Debug.Assert(Check(graph, source));
}
// check optimality conditions:
// (i) for all edges e: distTo[e.to()] <= distTo[e.from()] + e.weight()
// (ii) for all edge e on the SPT: distTo[e.to()] == distTo[e.from()] +
// e.weight()
private bool Check(EdgeWeightedDirectedGraph graph, int s)
{
// check that edge weights are nonnegative
for (int i = graph.NumberOfEdges - 1; i >= 0; i--)
{
WeightedGraphEdge e = graph.GetEdge(i);
if (e.Cost < 0)
{
Console.WriteLine("negative edge weight detected");
return(false);
}
}
// check that distTo[v] and edgeTo[v] are consistent
if (distTo[s] != 0.0 || edgeTo[s] != null)
{
Console.WriteLine("distTo[s] and edgeTo[s] inconsistent");
return(false);
}
for (int v = graph.NumberOfVertices - 1; v >= 0; v--)
{
if (v == s)
{
continue;
}
if (edgeTo[v] == null && distTo[v] != Double.PositiveInfinity)
{
Console.WriteLine("distTo[] and edgeTo[] inconsistent");
return(false);
}
}
// check that all edges e = v->w satisfy distTo[w] <= distTo[v] +
// e.weight()
for (int v = graph.NumberOfVertices - 1; v >= 0; v--)
{
WeightedGraphEdge[] adj = graph.GetVertexAdjacencies(v);
for (int i = adj.Length - 1; i >= 0; i--)
{
WeightedGraphEdge e = adj[i];
int w;
if (v == e.To)
{
w = e.From;
}
else
{
w = e.To;
}
if (distTo[v] + e.Cost < distTo[w])
{
Console.WriteLine("edge " + e + " not relaxed");
return(false);
}
}
}
// check that all edges e = v->w on SPT satisfy distTo[w] == distTo[v] +
// e.weight()
for (int w = 0; w < graph.NumberOfVertices; w++)
{
if (edgeTo[w] == null)
{
continue;
}
WeightedGraphEdge e = edgeTo[w];
int v = e.From;
if (w != e.To)
{
return(false);
}
if (distTo[v] + e.Cost != distTo[w])
{
Console.WriteLine("edge " + e + " on shortest path not tight");
return(false);
}
}
return(true);
}