/// <summary>
/// Finds the shortest path with specified start and end using breath first search.
/// </summary>
/// <typeparam name="TNode">The value contained in the node.</typeparam>
/// <param name="start">The start node.</param>
/// <param name="end">The end node.</param>
public static IEnumerable <Node <TNode> > FindShortestPath <TNode>(Node <TNode> start, Node <TNode> end)
{
var frontier = new Queue <Node <TNode> >();
frontier.Enqueue(start);
var visitedNodes = new NodeDictionary <TNode>();
visitedNodes.Add(start);
while (frontier.Count > 0)
{
var currentNode = frontier.Dequeue();
if (currentNode.Equals(end))
{
break;
}
foreach (var edge in currentNode.Edges)
{
if (!visitedNodes.Contains(edge.Target))
{
frontier.Enqueue(edge.Target);
visitedNodes.Add(edge.Target, currentNode);
}
}
}
var path = BacktrackPathTo(end, visitedNodes);
return(path);
}
/// <summary>
/// Finds the cheapest path with specified start and end using the Dijkstra algorithm (AKA Uniform Cost Search).
/// </summary>
/// <typeparam name="TNode">The value contained in the node.</typeparam>
/// <param name="start">The start node.</param>
/// <param name="end">The end node.</param>
/// <param name="graph">The graph to operate on.</param>
public static IEnumerable <Node <TNode> > FindCheapestPath <TNode>(Node <TNode> start, Node <TNode> end, UndirectedGraph <TNode> graph)
{
var frontier = new Queue <Node <TNode> >();
frontier.Enqueue(start);
var visitedNodes = new NodeDictionary <TNode>();
visitedNodes.Add(start);
var costs = InitializeCosts(graph);
costs[start] = 0d;
while (frontier.Count > 0)
{
var currentNode = frontier.Dequeue();
if (currentNode.Equals(end))
{
break;
}
var prioritizedEdges = new PriorityQueue <Edge <TNode> >(currentNode.Edges);
while (prioritizedEdges.Count > 0)
{
var edge = prioritizedEdges.Dequeue();
var currentCost = costs[edge.Target];
var newCost = costs[currentNode] + edge.Weight;
if (newCost < currentCost)
{
costs[edge.Target] = newCost;
// A cheaper path is found, so the target node predecesor must be replaced with the current node.
if (visitedNodes.Contains(edge.Target))
{
visitedNodes[edge.Target] = currentNode;
}
}
if (!visitedNodes.Contains(edge.Target))
{
frontier.Enqueue(edge.Target);
visitedNodes.Add(edge.Target, currentNode);
}
}
}
var path = BacktrackPathTo(end, visitedNodes);
return(path);
}
private static IEnumerable <Node <TNode> > BacktrackPathTo <TNode>(Node <TNode> end, NodeDictionary <TNode> visitedNodes)
{
LinkedList <Node <TNode> > path = new LinkedList <Node <TNode> >();
Node <TNode> current = end;
Node <TNode> previous = null;
if (visitedNodes.Contains(end))
{
current = end;
previous = visitedNodes.GetPrevious(current);
}
while (previous != null)
{
path.AddFirst(current);
current = previous;
previous = visitedNodes.GetPrevious(current);
}
path.AddFirst(current);
return(path);
}