org.geotools.graph.traverse.standard

Class AStarIterator

Object

AbstractGraphIterator

SourceGraphIterator

AStarIterator

All Implemented Interfaces:

GraphIterator

public class AStarIterator
extends SourceGraphIterator

A path iterator that uses a function (usually denoted f(x)) to determine the order in which the algorithm visits nodes, f(x) is a sum of two functions:

• The path-cost function (usually denoted g(x), which may or may not be a heuristic)
• An admissible "heuristic estimate" (usually denoted h(x)).

The iterator proceeds as follows (pseudo-code):

 
     // COST(n,n') : the real cost to go from n to n'
     OPEN = [Source]
     CLOSE = []

     while ( |OPEN| > 0 )
         n = a node in OPEN with less f
         remove n from OPEN
         add n to CLOSE
         if ( n == target ) {
            return  // path find

         // if n != target
         for each node n' that relates to n do
             if n' in OPEN
                 if (g(n) + COST(n,n')) < g(n')
                     g(n') = g(n) + COST(n,n')
                     parent(n') = n
             else
                 g(n') = g(n) + COST(n,n')
                 parent(n') = n
                 add n' to OPEN
     // end while
 
 

For more details see http://en.wikipedia.org/wiki/A_star

Author:

Germán E. Trouillet, Francisco G. Malbrán. Universidad Nacional de Córdoba (UNC)

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	AStarIterator.AStarFunctions Defines the functions needed by A Star.
static class	AStarIterator.AStarNode Internal data structure used to track node costs, and parent nodes.

Constructor Summary

Constructors

Constructor and Description
AStarIterator(Node source, AStarIterator.AStarFunctions afuncs)

Method Summary

Modifier and Type	Method and Description
void	cont(Graphable current, GraphTraversal traversal) Makes a step of the A* algorithm.
Node	getParent(Node n)
protected Iterator<?>	getRelated(Graphable current)
void	init(Graph graph, GraphTraversal traversal) Does Nothing.
void	killBranch(Graphable current, GraphTraversal traversal) Kills the branch of the traversal
Graphable	next(GraphTraversal traversal) Returns the next node in the priority queue. if the queue is empty then there is no path from the source to the destiny in this graph.

Methods inherited from class SourceGraphIterator

getSource, setSource

Methods inherited from class AbstractGraphIterator

getGraph, getTraversal, getWalker, setTraversal

Methods inherited from class Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

AStarIterator

public AStarIterator(Node source,
                     AStarIterator.AStarFunctions afuncs)

Method Detail

init

public void init(Graph graph,
                 GraphTraversal traversal)

Does Nothing. All the work was already done by the constructor.

Parameters:

graph - The graph being whose components are being iterated over.

next

public Graphable next(GraphTraversal traversal)

Returns the next node in the priority queue. if the queue is empty then there is no path from the source to the destiny in this graph.

Returns:

The next component in the iteration, or null if iteration is complete.

See Also:

org.geotools.graph.traverse.GraphIterator#next()

cont

public void cont(Graphable current,
                 GraphTraversal traversal)

Makes a step of the A* algorithm. Takes the current node, looks for its neighbours. The ones which are closed are discarted. The ones "in" the opened queue are checked to see if its necessary to update them. The rest of the nodes are initialized as AStarNodes and inserted in the opened queue.

Parameters:

current - The current component of the traversal.

See Also:

org.geotools.graph.traverse.GraphIterator#cont(Graphable)

killBranch

public void killBranch(Graphable current,
                       GraphTraversal traversal)

Kills the branch of the traversal

Parameters:

current - The current component of the traversal.

See Also:

org.geotools.graph.traverse.GraphIterator#killBranch(Graphable)

getParent

public Node getParent(Node n)

getRelated

protected Iterator<?> getRelated(Graphable current)