/// <param name="candidateNode">The node to be checked for duplication</param>
/// <returns>True if the candidate node already exists in either the open or closed sets</returns>
private bool IsDuplicate(Node candidateNode)
{
return OpenSet.Concat(ClosedSet)
.Where(node => node.FValue == candidateNode.FValue)
.Any(node => node.State.SequenceEqual(candidateNode.State));
}
/// <param name="goal">The goal node that will be used to trace back up to the initial node</param>
/// <returns>The list of nodes from goal node to initial node</returns>
private LinkedList<Node> GetSolutionPath(Node goal)
{
if (goal == null) return null;
var solution = new LinkedList<Node>();
var current = goal;
while (current != null)
{
_lengthOfSolutionPath++;
solution.AddFirst(current);
current = current.BackPtr;
}
return solution;
}
/// <param name="parent"> The parent node of successor</param>
/// <param name="prev"> The position of space in the parent </param>
/// <param name="next"> The position the space is to be moved to</param>
/// <returns>The successor node resulting from moving the space in the parent state from prev to next </returns>
private Node CreateSuccessorNode(Node parent, int prev, int next)
{
var child = new Node();
child.State = new List<int>(parent.State);
child.State[prev] = parent.State[next];
child.State[next] = parent.State[prev];
child.FValue = Heuristic(child.State);
child.BackPtr = parent;
return child;
}
/// <param name="parent">The parent node whose successors will be calculated</param>
/// <returns>The successor nodes for the given parent</returns>
private LinkedList<Node> GenerateSuccessors(Node parent)
{
var successors = new LinkedList<Node>();
var sideLength = (int)Math.Sqrt(parent.State.Count);
var i = parent.State.TakeWhile(value => value != Node.SPACE).Count();
if (IsMovePossible(i, i - 1, sideLength)) // move space left
successors.AddFirst(CreateSuccessorNode(parent, i, i - 1));
if (IsMovePossible(i, i + 1, sideLength)) // move space right
successors.AddFirst(CreateSuccessorNode(parent, i, i + 1));
if (IsMovePossible(i, i - sideLength, sideLength)) // move space up
successors.AddFirst(CreateSuccessorNode(parent, i, i - sideLength));
if (IsMovePossible(i, i + sideLength, sideLength)) // move space down
successors.AddFirst(CreateSuccessorNode(parent, i, i + sideLength));
return successors;
}
/// <summary>
/// Best first search implementation
/// </summary>
/// <param name="initialNode">The initial node, representing the puzzle to be solved</param>
/// <returns>The solution path from inital node to goal node</returns>
public LinkedList<Node> Search(Node initialNode)
{
// Check for valid params
if (initialNode == null) throw new ArgumentNullException("initialNode");
if (Heuristic == null)
throw new NullReferenceException("Heuristic function not specified, this search requires a heuristic");
var watch = new Stopwatch();
watch.Start();
Node goalNode = null;
var depth = 0;
// Calculate h(start)
initialNode.FValue = Heuristic(initialNode.State);
// Put IS on open
OpenSet.AddFirst(initialNode);
// Loop until |open| = 0 or goalState found
while (OpenSet.Count > 0 && goalNode == null)
{
var currentNode = OpenSet.First; // First is the lowest F value, because OpenSet is sorted by F Value
OpenSet.Remove(currentNode);
ClosedSet.AddFirst(currentNode);
var successors = GenerateSuccessors(currentNode.Value);
_numNodesGenerated += successors.Count;
AddToOpenSet(successors);
goalNode = FindGoal(successors);
depth++;
}
_branchingFactor = _numNodesGenerated/depth;
watch.Stop();
_executionTime = watch.Elapsed;
// If goal found then return the solution path, else return null
return goalNode == null ? null : GetSolutionPath(goalNode);
}