Package edu.uky.ai.planning.pg

Class PlanGraph

java.lang.Object

edu.uky.ai.planning.pg.PlanGraph

public class PlanGraph
extends Object

plan graph is a data structure that compresses the state space of a planning problem into something that represents possible future states. It is a directed, leveled graph with two kinds of nodes:

A LiteralNode represents a fact:

• A literal node exists at level 0 iff it is true in the initial state.
• A literal node exists at level n > 0 if it exists at level n.
• A literal node exists at level n > 0 if there exists a step node a level n - 1 which has that literal as an effect.

A StepNode represents a step:

• No step nodes exist at level 0.
• A step node exists at level n > 0 if all its preconditions appear at level n - 1.

Node that a plan graph must be initialized to some state before it can be used. This method resets the plan graph and determines which literals are true at level 0.

In order to save a significant amount of memory, there is only 1 object for each literal node and each step node (rather than 1 per level at which that literal or step appears). A node is simply marked with the earliest level at which it appears, since it will continue to appear at all future levels. If a literal or step has not appeared yet, it's level is set to -1 and it is said not to exist (even though it still resides in memory for possible later use).

Note that a plan graph can be built with or without persistence steps and with or without mutexes.

Note that this implementation only computes static mutexes (ones that must always exist). It does not compute mytexes for competing needs or inconsistent support.

Author:

Stephen G. Ware

Field Summary

Fields

Modifier and Type	Field	Description
final Iterable<LiteralNode>	goals	The literal nodes for the problem's goals
protected final LinkedHashMap<Literal,LiteralNode>	literalMap	A mapping of literal objects to their corresponding nodes
final StateSpaceProblem	problem	The planning problem represented by this plan graph
protected final LinkedHashMap<Step,StepNode>	stepMap	A mapping of step objects to their corresponding nodes

Constructor Summary

Constructors

Constructor	Description
PlanGraph(Problem problem, boolean persistence, boolean mutexes)	Constructs a plan graph using all possible steps that could occur as the step nodes.
PlanGraph(StateSpaceProblem problem, boolean persistence, boolean mutexes)	Constructs a new plan graph using all possible steps for step nodes.

Method Summary

Modifier and Type	Method	Description
void	extend()	Adds one new level to the graph.
LiteralNode	get(Literal literal)	Returns a literal object's node in this graph.
StepNode	get(Step step)	Returns a step object's node in this graph.
Level	getLevel(int number)	Returns a level object for the level of the given index.
boolean	goalAchieved()	Tests whether all the problem's goal literals exist at the highest level of the graph.
boolean	hasLeveledOff()	Tests whether or not the graph has leveled off (meaning that no new literals or steps will appear if a new level is added).
void	initialize(State initial)	Resets the graph to have only level 0, which will include exactly those literals which are true in the given state.
int	size()	Returns the number of levels in this graph.
String	toString()

Methods inherited from class java.lang.Object

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

Field Details

problem

public final StateSpaceProblem problem

The planning problem represented by this plan graph

goals

public final Iterable<LiteralNode> goals

The literal nodes for the problem's goals

literalMap

protected final LinkedHashMap<Literal,LiteralNode> literalMap

A mapping of literal objects to their corresponding nodes

stepMap

protected final LinkedHashMap<Step,StepNode> stepMap

A mapping of step objects to their corresponding nodes

Constructor Details

PlanGraph

public PlanGraph(StateSpaceProblem problem,
 boolean persistence,
 boolean mutexes)

Constructs a new plan graph using all possible steps for step nodes.

Parameters:

problem - the problem whose steps should be used as step nodes

persistence - whether or not persistence steps should be generated

mutexes - whether or not mutexes should be calculated

PlanGraph

public PlanGraph(Problem problem,
 boolean persistence,
 boolean mutexes)

Constructs a plan graph using all possible steps that could occur as the step nodes.

Parameters:

problem - the problem

persistence - whether or not persistence steps should be generated

mutexes - whether or not mutexes should be calculated

Method Details

toString

public String toString()

Overrides:

toString in class Object

get

public LiteralNode get(Literal literal)

Returns a literal object's node in this graph.

Parameters:

literal - the literal

Returns:

the literal's corresponding node

get

public StepNode get(Step step)

Returns a step object's node in this graph.

Parameters:

step - the step

Returns:

the step's corresponding node

initialize

public void initialize(State initial)

Resets the graph to have only level 0, which will include exactly those literals which are true in the given state.

Parameters:

initial - any state

extend

public void extend()

Adds one new level to the graph.

size

public int size()

Returns the number of levels in this graph.

Returns:

the number of levels

getLevel

public Level getLevel(int number)

Returns a level object for the level of the given index.

Parameters:

number - the index of the requested level

Returns:

the level

Throws:

IndexOutOfBoundsException - if the requested level does not exist in this graph

goalAchieved

public boolean goalAchieved()

Tests whether all the problem's goal literals exist at the highest level of the graph.

Returns:

true if all goals exist, false otherwise

hasLeveledOff

public boolean hasLeveledOff()

Tests whether or not the graph has leveled off (meaning that no new literals or steps will appear if a new level is added).

Returns:

true if the graph has leveled off, false otherwise