org.geotools.graph.traverse.standard

Class DijkstraIterator

Object

AbstractGraphIterator

SourceGraphIterator

DijkstraIterator

All Implemented Interfaces:

GraphIterator

Direct Known Subclasses:

DirectedDijkstraIterator

public class DijkstraIterator
extends SourceGraphIterator

Iterates over the nodes of a graph in pattern using Dijkstra's Shortest Path Algorithm. A Dijkstra iteration returns nodes in an order of increasing cost relative to a specified node (the source node of the iteration).

In a Dijsktra iteration, a weight is associated with each edge and a cost with each node. The iteration operates by maintaining two sets of nodes. The first the set of nodes whose final cost is known, and the second is the set of nodes whose final cost is unknown. Initially, every node except for the source node has a cost of infinity, and resides in the unkown set. The source node has a cost of zero, and is is a member of the known set.

The iteration operatates as follows:

   sn = source node of iteration
   N = set of all nodes
   K = set of nodes with known cost = {sn} 
   U = set of nodes with unknown cost = N - K
 
   cost(sn) = 0
   for each node $un in U
      cost(un) = infinity
 
   while(|U| > 0)
      for each node n in K
        find a node un in U that relates to n
        if cost(n) + weight(n,un) < cost(un)
          cost(un) = cost(n) + weight(n,un)
          
      ln = node with least cost in U
      remove ln from U
      add ln to K
 
      return ln as next node in iteration   
 

The following is an illustration of the algorithm. Edge weights are labelled in blue and the final node costs are labelled in red.

The nodes are returned in order of increasing cost which yields the sequence A,C,B,D,E,F,G,H,I.

Author:

Justin Deoliveira, Refractions Research Inc, jdeolive@refractions.net

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	DijkstraIterator.DijkstraNode Internal data structure used to track node costs, and parent nodes.
static interface	DijkstraIterator.EdgeWeighter Supplies a weight for each edge in the graph to be used by the iteration when calculating node costs.
static interface	DijkstraIterator.NodeWeighter Supplies a weight for each pair of adjacent edges.

Field Summary

Fields

Modifier and Type	Field and Description
protected HashMap<Graphable,DijkstraIterator.DijkstraNode>	nodemap map of graph node to internal dijkstra node
protected DijkstraIterator.NodeWeighter	nweighter provides weights for nodes in the graph
protected PriorityQueue	queue priority queue to manage active nodes
protected DijkstraIterator.EdgeWeighter	weighter provides weights for edges in the graph

Constructor Summary

Constructors

Constructor and Description
DijkstraIterator(DijkstraIterator.EdgeWeighter weighter) Constructs a new Dijkstra iterator which uses the specided EdgeWeighter.
DijkstraIterator(DijkstraIterator.EdgeWeighter weighter, DijkstraIterator.NodeWeighter nweighter) Constructs a new Dijkstra iterator which uses the specided EdgeWeighter and NodeWeighter

Method Summary

Modifier and Type	Method and Description
void	cont(Graphable current, GraphTraversal traversal) Looks for adjacent nodes to the current node which are in the adjacent node and updates costs.
double	getCost(Graphable component) Returns the internal cost of a node which has been calculated by the iterator.
Graphable	getParent(Graphable component) Returns the last node in the known set to update the node.
protected PriorityQueue	getQueue()
protected Iterator	getRelated(Graphable current)
void	init(Graph graph, GraphTraversal traversal) Builds internal priority queue to manage node costs.
void	killBranch(Graphable current, GraphTraversal traversal) Kills the branch of the traversal by not updating the cost of any adjacent nodes.
Graphable	next(GraphTraversal traversal) Returns the next node in the priority queue.

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

Field Detail

weighter

protected DijkstraIterator.EdgeWeighter weighter

provides weights for edges in the graph

nweighter

protected DijkstraIterator.NodeWeighter nweighter

provides weights for nodes in the graph

queue

protected PriorityQueue queue

priority queue to manage active nodes

nodemap

protected HashMap<Graphable,DijkstraIterator.DijkstraNode> nodemap

map of graph node to internal dijkstra node

Constructor Detail

DijkstraIterator

public DijkstraIterator(DijkstraIterator.EdgeWeighter weighter)

Constructs a new Dijkstra iterator which uses the specided EdgeWeighter.

Parameters:

weighter - Calculates weights for edges in the graph being iterated over.

DijkstraIterator

public DijkstraIterator(DijkstraIterator.EdgeWeighter weighter,
                        DijkstraIterator.NodeWeighter nweighter)

Constructs a new Dijkstra iterator which uses the specided EdgeWeighter and NodeWeighter

Parameters:

weighter - Calculates weights for edges in the graph being iterated over.

nweighter - Calculates weights for nodes in the graph being iterated over.

Method Detail

init

public void init(Graph graph,
                 GraphTraversal traversal)

Builds internal priority queue to manage node costs.

Parameters:

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

See Also:

org.geotools.graph.traverse.GraphIterator#init(Graph)

next

public Graphable next(GraphTraversal traversal)

Returns the next node in the priority queue. If the next node coming out of the queue has infinite cost, then the node is not adjacent to any nodes in the set of nodes with known costs. This situation will end the traversal every other node will also have infinite cost. This usally is the result of a disconnected 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)

Looks for adjacent nodes to the current node which are in the adjacent node and updates costs.

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 by not updating the cost of any adjacent nodes.

Parameters:

current - The current component of the traversal.

See Also:

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

getCost

public double getCost(Graphable component)

Returns the internal cost of a node which has been calculated by the iterator.

Parameters:

component - The component whose cost to return.

Returns:

The cost associated with the component.

getParent

public Graphable getParent(Graphable component)

Returns the last node in the known set to update the node. The iteration operates by nodes in the known set updating the cost of nodes in the unknown set. Each time an update occurs, the known node is set as the parent of the unkown node.

Parameters:

component - The node whose parent to return (child)

Returns:

The parent, or null if the method is supplied the source of the iteration.

getQueue

protected PriorityQueue getQueue()

getRelated

protected Iterator getRelated(Graphable current)