/// <summary>
/// Calculates a path from start to end. When no path can be found in
/// reasonable time the search is aborted and an incomplete path is returned.
/// When refresh is not set to true a cached path is returned where possible.
/// </summary>
/// <param name="start">start position in 2d map space</param>
/// <param name="end">end position in 2d map space</param>
/// <param name="refresh">force to recalculate the path</param>
/// <returns></returns>
public TilePath CalculatePath(Point2D start, Point2D end, bool refresh)
{
// swap points to calculate the path backwards (from end to start)
Point2D temp = end;
end = start;
start = temp;
// Check whether the requested path is already known
var request = new TilePathRequest(start, end);
if (!refresh && mPathCache.ContainsKey(request))
{
return(mPathCache[request].Copy());
}
// priority queue of nodes that yet have to be explored sorted in
// ascending order by node costs (F)
var open = new AutoPriorityQueue <TilePathNode>();
// List of nodes that have already been explored
var closed = new LinkedList <ITileCell>();
// Start is to be explored first
var startNode = new TilePathNode(null, mLayer.GetCell(start), end);
open.Enqueue(startNode);
var steps = 0;
do
{
// Examine the cheapest node among the yet to be explored
var current = open.Dequeue();
// Finish?
if (current.Cell.Matches(end) || ++steps > mStepLimit)
{
// Paths which lead to the requested goal are cached for reuse
var path = new TilePath(current);
if (mPathCache.ContainsKey(request))
{
mPathCache[request] = path.Copy();
}
else
{
mPathCache.Add(request, path.Copy());
}
return(path);
}
// Explore all neighbours of the current cell
var neighbours = mLayer.GetNeighbourCells(current.Cell);
foreach (var cell in neighbours)
{
// Discard nodes that are not of interest
var flag = mCollisionLayer[cell.X, cell.Y];
if (closed.Contains(cell) || (cell.Matches(end) == false && (flag & ECollisionType.NotMoveable) != ECollisionType.NotMoveable))
{
continue;
}
// Successor is one of current's neighbours
var successor = new TilePathNode(current, cell, end);
var contained = open.Find(successor);
if (contained != null && successor.CostTotal >= contained.CostTotal)
{
// This cell is already in the open list represented by
// another node that is cheaper
continue;
}
if (contained != null && successor.CostTotal < contained.CostTotal)
{
// This cell is already in the open list but on a more expensive
// path -> "integrate" the node into the current path
contained.Predecessor = current;
contained.Update();
open.Update(contained);
}
else
{
// The cell is not in the open list and therefore still has to
// be explored
open.Enqueue(successor);
}
}
// Add current to the list of the already explored nodes
closed.AddLast(current.Cell);
} while (open.Peek() != null);
return(null);
}