public override List<Square> GetShortestPathTo(Square destination)
{
//Nodes are initialized with node.distance=MAX_DISTANCE and node.Previous as null by constructor
var sourceNode = new Node() {Square = Source}; // wrapping source Square into a graph Node
sourceNode.Distance = 0; // Initialize source Node distance=0, source to source distance is zero
var q = new Dictionary<Square, Node> { { Source, sourceNode } }; // initialise nodes(vertexes collection)
// using Square keyed Dictionary for low-cost lookup with Square
// instead of a custom-implemented priority Queue
Graph.Vertexes.Where(sq => sq != Source).Select(sq => new Node() {Square = sq}).ToList().ForEach(
n => q.Add(n.Square, n));
bool unreachable = false; // flag to detect if destination unreachable although for knight graph its not possible
while (q.Count != 0) // The main loop
{
var p = q.OrderBy(x => x.Value.Distance).First(); // popping keyvalue pair with Node having minimum distance, Start node in first case
var u = p.Value; // Getting the actual Node
if(u.Square==destination) // If the Square(wrapped by the current node) is destination, return with shortest path, terminate search
{
return GenPath(u);
}
if(u.Distance==INFINITE_DISTANCE) // rest of Nodes unreachable, so the destination
{
unreachable = true;
break;
}
q.Remove(p.Key); // remove entry from Nodes queue(here the dictionary)
foreach (var square in Graph.EdgesAdjacencyList[u.Square]) // for each neighbor of u
{
if(!q.ContainsKey(square)) // where square is not removed from q yet
continue;
int alt = u.Distance + 1; // u has 1 distance to all its neighbors
var v = q[square]; // get the Node from q
if (alt < v.Distance) // Relax (u,v,a)
{
v.Distance = alt;
v.Previous = u;
}
}
}
return unreachable ? null : new List<Square>(); // return null if unreachable, empty list means source and destination are same
}
public abstract List<Square> GetShortestPathTo(Square destination);
private const int INFINITE_DISTANCE = 256; // simulate infinity assuming this will be greater than any Nodes's distance from any source
#endregion Fields
#region Constructors
public DijkstraAlgorithm(Square source, ChessBoardPieceGraph graph)
: base(source, graph)
{
//might want to initialize shortest paths to all destinations
}
protected SingleSourceShortestPathStrategy(Square source, ChessBoardPieceGraph graph)
{
Source = source;
Graph = graph;
}
