/// <summary> /// Uses Prim's algorithm to build an MST spanning the mstNodes. /// O(|mstNodes|^2) runtime. /// </summary> /// <param name="startFrom">A GraphNode to start from.</param> public void Span(GraphNode startFrom) { var adjacentEdgeQueue = new LinkedListPriorityQueue <LinkedGraphEdge>(100); var startIndex = startFrom.DistancesIndex; // All nodes that are not yet included. var toAdd = new List <int>(_mstNodes.Count); // If the index node is already included. var inMst = new bool[_distances.CacheSize]; // The spanning edges. var mstEdges = new List <GraphEdge>(_mstNodes.Count); for (var i = 0; i < _mstNodes.Count; i++) { var index = _mstNodes[i].DistancesIndex; if (index != startIndex) { toAdd.Add(index); var adjacentEdge = new LinkedGraphEdge(startIndex, index); adjacentEdgeQueue.Enqueue(adjacentEdge, _distances[startIndex, index]); } } inMst[startIndex] = true; while (toAdd.Count > 0 && adjacentEdgeQueue.Count > 0) { int newIn; LinkedGraphEdge shortestEdge; // Dequeue and ignore edges that are already inside the MST. // Add the first one that is not. do { shortestEdge = adjacentEdgeQueue.Dequeue(); newIn = shortestEdge.Outside; } while (inMst[newIn]); mstEdges.Add(new GraphEdge( _distances.IndexToNode(shortestEdge.Inside), _distances.IndexToNode(shortestEdge.Outside))); inMst[newIn] = true; // Find all newly adjacent edges and enqueue them. for (var i = 0; i < toAdd.Count; i++) { var otherNode = toAdd[i]; if (otherNode == newIn) { toAdd.RemoveAt(i--); } else { var edge = new LinkedGraphEdge(newIn, otherNode); adjacentEdgeQueue.Enqueue(edge, _distances[newIn, otherNode]); } } } SpanningEdges = mstEdges; IsSpanned = true; }
/// <summary> /// Uses Prim's algorithm to build an MST spanning the mstNodes. /// O(|mstNodes|^2) runtime. /// </summary> /// <param name="startIndex">The node index to start from.</param> public void Span(int startIndex) { // All nodes that are not yet included. var toAdd = new List <int>(_mstNodes.Count); // If the index node is already included. var inMst = new bool[_distances.CacheSize]; // The spanning edges. var mstEdges = new List <DirectedGraphEdge>(_mstNodes.Count); using (var adjacentEdgeQueue = new LinkedListPriorityQueue <DirectedGraphEdge>(100, _mstNodes.Count * _mstNodes.Count)) { foreach (var t in _mstNodes) { if (t != startIndex) { toAdd.Add(t); adjacentEdgeQueue.Enqueue(new DirectedGraphEdge(startIndex, t), _distances[startIndex, t]); } } inMst[startIndex] = true; while (toAdd.Count > 0 && !adjacentEdgeQueue.IsEmpty) { int newIn; DirectedGraphEdge shortestEdge; // Dequeue and ignore edges that are already inside the MST. // Add the first one that is not. do { shortestEdge = adjacentEdgeQueue.Dequeue(); newIn = shortestEdge.Outside; } while (inMst[newIn]); mstEdges.Add(shortestEdge); inMst[newIn] = true; // Find all newly adjacent edges and enqueue them. for (var i = 0; i < toAdd.Count; i++) { var otherNode = toAdd[i]; if (otherNode == newIn) { toAdd.RemoveAt(i--); } else { adjacentEdgeQueue.Enqueue(new DirectedGraphEdge(newIn, otherNode), _distances[newIn, otherNode]); } } } } SpanningEdges = mstEdges; }