/// <summary>
/// Prim's algorithm to find the minimum span tree on graph
/// </summary>
/// <param name="graph"> <see cref="Graph"/> instance. </param>
/// <returns> Minimum span tree containing edges and minimum distance. </returns>
public static MinimumSpanTree <T> PrimsMinimumSpanningTree <T>(this DirectedGraph <T> graph) where T : IEquatable <T>
{
if (graph.Vertices.Count == 0 || graph.Edges.Count == 0)
{
return(new MinimumSpanTree <T>(Enumerable.Empty <Edge <T> >().ToList(), 0));
}
var currentVertex = graph.Vertices.First();
int minimumDistance = 0;
var minimumSpanTree = new List <Edge <T> >();
var edgesToVisit = new BinaryHeap <Edge <T> >(BinaryHeapType.MinimumHeap, currentVertex.OutboundEdges.Count, new EdgeComparer <T>());
var verticesVisited = new HashSet <Vertex <T> >();
while (minimumSpanTree.Count < graph.Vertices.Count - 1)
{
foreach (var edge in currentVertex.OutboundEdges)
{
edgesToVisit.Add(edge);
}
verticesVisited.Add(currentVertex);
Edge <T> minimumEdge = null;
while (edgesToVisit.Count > 0)
{
minimumEdge = edgesToVisit.Remove();
if (verticesVisited.Contains(minimumEdge.StartVertex) != verticesVisited.Contains(minimumEdge.EndVertex))
{
break;
}
}
if (minimumEdge == null)
{
throw new MultipleMinimumSpanningTreesException();
}
minimumSpanTree.Add(minimumEdge);
minimumDistance += minimumEdge.Weight;
currentVertex = verticesVisited.Contains(minimumEdge.EndVertex)
? minimumEdge.StartVertex
: minimumEdge.EndVertex;
}
return(new MinimumSpanTree <T>(minimumSpanTree, minimumDistance));
}
/// <summary>
/// Kruskal's algorithm to find the minimum span tree on graph
/// </summary>
/// <param name="graph"> <see cref="Graph"/> instance. </param>
/// <returns> Minimum span tree containing edges and minimum distance. </returns>
public static MinimumSpanTree <T> KruskalsMinimumSpanningTree <T>(this DirectedGraph <T> graph) where T : IEquatable <T>
{
if (graph.Vertices.Count == 0 || graph.Edges.Count == 0)
{
return(new MinimumSpanTree <T>(Enumerable.Empty <Edge <T> >().ToList(), 0));
}
var minimumSpanTree = new List <Edge <T> >();
var minimumDistance = 0;
var unionFind = new DisjointSet <Vertex <T> >(graph.Vertices);
var edgesToVisit = new BinaryHeap <Edge <T> >(BinaryHeapType.MinimumHeap, graph.Edges.Count, new EdgeComparer <T>());
foreach (var edge in graph.Edges)
{
edgesToVisit.Add(edge);
}
while (minimumSpanTree.Count < graph.Vertices.Count - 1)
{
Edge <T> minimumEdge = null;
while (edgesToVisit.Count > 0)
{
minimumEdge = edgesToVisit.Remove();
if (unionFind.Find(minimumEdge.StartVertex) != unionFind.Find(minimumEdge.EndVertex))
{
break;
}
}
if (minimumEdge == null)
{
throw new MultipleMinimumSpanningTreesException();
}
minimumSpanTree.Add(minimumEdge);
minimumDistance += minimumEdge.Weight;
unionFind.Union(minimumEdge.StartVertex, minimumEdge.EndVertex);
}
return(new MinimumSpanTree <T>(minimumSpanTree, minimumDistance));
}
