public static IWeightedGraph <TV, TW> FindShortestPath <TV, TW>(IWeightedGraph <TV, TW> graph,
IVertex <TV> source,
IVertex <TV> target,
TW zeroValue,
TW maxValue,
Func <TW, TW, TW> combineCosts) where TV : IEquatable <TV> where TW : IComparable
{
HashSet <IVertex <TV> > visitedVerteces = new HashSet <IVertex <TV> >();
IPriorityQueue <IVertex <TV>, TW> vertecesToVisit = new NaivePriorityQueue <IVertex <TV>, TW>();
Dictionary <IVertex <TV>, IWeightedEdge <TV, TW> > predecessor = new Dictionary <IVertex <TV>, IWeightedEdge <TV, TW> >();
// Initialize queue and predecessor map
foreach (IVertex <TV> vertex in graph.GetAllVerteces())
{
vertecesToVisit.Enqueue(vertex, maxValue);
predecessor.Add(vertex, null);
}
vertecesToVisit.UpdatePriority(source, zeroValue);
// Continue as long as there are verteces to visit
IVertex <TV> currentVertex = null;
TW currentCosts = zeroValue;
while (!vertecesToVisit.Empty)
{
// Get the closest next vertex
(currentVertex, currentCosts) = vertecesToVisit.Dequeue();
// Check whether we reached our target
if (currentVertex == target)
{
break;
}
visitedVerteces.Add(currentVertex);
// If not, discover the next verteces that can be visited
foreach (IWeightedEdge <TV, TW> edge in graph.GetEdgesOfVertex(currentVertex))
{
if (edge.Weight.CompareTo(zeroValue) < 0)
{
throw new NegativeEdgeWeightException();
}
IVertex <TV> connectedVertex = edge.ConnectedVertex(currentVertex);
if (!visitedVerteces.Contains(connectedVertex))
{
TW newCosts = combineCosts(currentCosts, edge.Weight);
if (newCosts.CompareTo(vertecesToVisit.GetPriorityOf(connectedVertex)) < 0)
{
vertecesToVisit.UpdatePriority(connectedVertex, newCosts);
predecessor[connectedVertex] = edge;
}
}
}
}
// If we reached the target vertex, build result graph
if (currentVertex == target)
{
GraphBuilder <TV, TW> builder = new GraphBuilder <TV, TW>(true).AddVertex(target.Value);
while (currentVertex != source)
{
IWeightedEdge <TV, TW> pathEdge = predecessor[currentVertex];
IVertex <TV> previousVertex = pathEdge.ConnectedVertex(currentVertex);
builder
.AddVertex(previousVertex.Value)
.AddEdge(previousVertex.Value, currentVertex.Value, pathEdge.Weight);
currentVertex = previousVertex;
}
return(builder.Build());
}
else
{
throw new VertexNotReachableException <TV>(target);
}
}
/// <summary>
/// Builds the minimum spanning tree of a given graph.
/// </summary>
/// <typeparam name="TV">Type of the graph vertex values.</typeparam>
/// <typeparam name="TW">Type of the graph edge weights.</typeparam>
/// <param name="graph">The graph got which to build the minimum spanning tree.</param>
/// <returns>Returns the minimum spanning tree of the given graph.</returns>
public static IWeightedGraph <TV, TW> FindMinimumSpanningTree <TV, TW>(IWeightedGraph <TV, TW> graph, TW min, TW max)
where TV : IEquatable <TV>
where TW : IComparable
{
// Builder used to incrementally build the target minimum spanning tree
GraphBuilder <TV, TW> builder = new GraphBuilder <TV, TW>();
// Get all verteces and edges of graph
IEnumerable <IVertex <TV> > verteces = graph.GetAllVerteces();
IEnumerable <IWeightedEdge <TV, TW> > edges = graph.GetAllEdges();
// TODO: replace with more performant priority queue implementation
// Track minimum known cost to connect to target vertex
IPriorityQueue <TV, TW> vertexCosts = new NaivePriorityQueue <TV, TW>();
// Track target vertex and edge connecting to target vertex with minimum known cost
Dictionary <TV, IVertex <TV> > vertexValue = new Dictionary <TV, IVertex <TV> >();
Dictionary <TV, IWeightedEdge <TV, TW> > vertexEdges = new Dictionary <TV, IWeightedEdge <TV, TW> >();
// Initialize vertex costs with all costs set to max
foreach (IVertex <TV> vertex in verteces)
{
vertexCosts.Enqueue(vertex.Value, max);
vertexValue.Add(vertex.Value, vertex);
vertexEdges.Add(vertex.Value, null);
}
// Set starting vertex
vertexCosts.UpdatePriority(verteces.First().Value, min);
// While there are verteces left to add, select vertex with smallest cost
while (!vertexCosts.Empty)
{
// Get and remove vertex with smallest cost
TV minCost = vertexCosts.Dequeue().Item1;
// For easier handling, get vertex and edge
IVertex <TV> vertex = vertexValue[minCost];
IWeightedEdge <TV, TW> edge = vertexEdges[minCost];
// Add vertex and edge to target MST
builder.AddVertex(vertex.Value);
if (edge != null)
{
builder.AddEdge(vertex.Value, edge.ConnectedVertex(vertex.Value).Value, edge.Weight);
}
// Update vertex cost for adjacent verteces and store edges
foreach (IWeightedEdge <TV, TW> connectedEdge in graph.GetEdgesOfVertex(vertex))
{
// Ignore edges leading to verteces already added to the MST
IVertex <TV> targetVertex = connectedEdge.ConnectedVertex(vertex);
if (vertexCosts.Contains(targetVertex.Value) &&
connectedEdge.Weight.CompareTo(vertexCosts.GetPriorityOf(targetVertex.Value)) < 0)
{
vertexCosts.UpdatePriority(targetVertex.Value, connectedEdge.Weight);
vertexEdges[targetVertex.Value] = connectedEdge;
}
}
}
// All verteces added to MST - done!
return(builder.Build());
}
