org.geotools.graph.traverse
Interface GraphIterator

All Known Implementing Classes:

AbstractGraphIterator

public interface GraphIterator

Defines an algorithm in which to traverse the components of a graph. A graph iterator operates by repeatedly returing graph components to the caller. The order in which to return the components is specific to the iterator. However, most iterators follow the following conventions:

• Components are visited only once
• The next component to be returned is determined by the components that have been previously visited

The following is an example of a GraphIterator. It returns nodes of a graph in a standard Depth First Search order, starting from a specified node. The nodes have been numbered to correspond to the iteration pattern.

* indicates source of traversal


In order to analyze the traversal, the following terms are defined:

The Next Set of a traversal is the set of components that will be visited in a later stage of the traversal.
The Branch Set of an component e is defined as the set of components that can be visited in a later stage of the traversal as a direct result of visiting e.

In most traversals, the two sets are related. The Next Set is built by analyzing the Branch Set of the component being visited in the current stage of the traversal. Revisiting the above example, a Depth First Search Traversal operates as follows:

• Each node is visited only once.
• The Next Set is organized as a Last In First Out Queue (Stack).
• At each stage, every node in the Branch Set that has not yet been visited is added to the Next Set.

As well it is assumed that nodes related to a node are sorted in alphabetic order.

The following table summarizes the stages of the traversal:

Stage	Visited Node	Branch Set	Next Set	Comment
0			{A}	Initial stage, iteration starts explicitly from A
1	A	{B,F}	{F,B}	Related nodes added to Next Set in LIFO order.
2	F	{A,B}	{B,B}	A already visited so not added to Next Set B not yet visited so added to queue.
3	B	{A,C,D,E,F}	{B,E,D,C}	A,F already visited so not added to Next Set
4	B		{E,D,C}	B already visited so ignore and move to next stage
5	E	{B}	{D,C}
6	D	{B,C}	{C,C}
7	C	{B,D}	{C}
8	C		{ }	C already visited so ignore and move to next stage
9			{ }	Next set empty, iteration complete.

At any stage of a travesal a branch may be killed.When a branch is killed at a stage of an iteration, no elements in the current Branch Set are added to the Next Set. This is illustrated by revisiting the Depth First Search Iteration, but this time killing the branch at node B. The following table summarizes the stages of the traversal:

Stage	Visited Node	Branch Set	Next Set	Comment
0			{A}	Initial stage, iteration starts explicitly from A
1	A	{B,F}	{F,B}	Related nodes added to Next Set in LIFO order.
2	F	{A,B}	{B,B}	A already visited so not added to Next Set B not yet visited so added to queue.
3	B	{A,C,D,E,F}	{B}	Branch Killed. No nodes added to Next Set
4	B		{ }	B already visited so ignore and move to next stage
9			{ }	Next set empty, iteration complete.

In this example, killing the branch at node B results in nodes C, D, and E never being visited.

Author:

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

See Also:

GraphWalker, GraphTraversal

Method Summary

void	cont(Graphable current, GraphTraversal traversal) Signals to the iterator that iteration should continue from the current component in the traversal.
GraphTraversal	getTraversal() Returns the traversal for the iterator.
void	init(Graph graph, GraphTraversal traversal) Signals to the itereator that iteration is about to begin.
void	killBranch(Graphable current, GraphTraversal traversal) Signals the iterator to kill the branch at the current component.
Graphable	next(GraphTraversal traversal) Returns the next graph component in the iteration.
void	setTraversal(GraphTraversal traversal) Sets the traversal for the iterator.

Method Detail

setTraversal

public void setTraversal(GraphTraversal traversal)

Sets the traversal for the iterator.

Parameters:

traversal - The traversal requesting components from the iterator.

getTraversal

public GraphTraversal getTraversal()

Returns the traversal for the iterator.

Returns:

The traversal requesting components from the iterator.

init

public void init(Graph graph,
                 GraphTraversal traversal)

Signals to the itereator that iteration is about to begin. This often results in the creation/initialization of any internal data structures used by the iterator.

Parameters:

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

next

public Graphable next(GraphTraversal traversal)

Returns the next graph component in the iteration. To signal to the caller that the iteration is complete, null should be returned.

Returns:

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

cont

public void cont(Graphable current,
                 GraphTraversal traversal)

Signals to the iterator that iteration should continue from the current component in the traversal.

Parameters:

current - The current component of the traversal.

killBranch

public void killBranch(Graphable current,
                       GraphTraversal traversal)

Signals the iterator to kill the branch at the current component.

Parameters:

current - The current component of the traversal.