ai.search.informed
Class IDAStarSearcherForIntCostFn<S extends SearchStateForIntCostFn>

java.lang.Object
  ai.search.SearchEngine
      ai.search.SearchEngineForIntCostFn<S>
          ai.search.informed.BestFirstSearcherForIntCostFn<S>
              ai.search.informed.IDAStarSearcherForIntCostFn<S>

public class IDAStarSearcherForIntCostFn<S extends SearchStateForIntCostFn>
extends BestFirstSearcherForIntCostFn<S>

This class represents a search engine that traverses a search space using the IDA* algorithm from a given initial state. The initial state and the heuristic function must be given to this search engine at construction time. Note that the test for repeated states will here only be performed on the predecessors of a newly generated state.

Now the search can be started using the function doSearch(). Successors of the initial state will then be explored using a depth-first search up to a current f-value limit. Whenever the search to the current limit has exhausted its search space the limit is increased and the search is started from scratch. The search will halt when a goal state has been expanded. The goal state and the path to it can then be inspected using the appropriate functions described in the SearchEngineForIntCostFn class.

Thus, a typical invocation of an IDAStarSearcherForIntCostFn would look something like this:

     SearchEngineForIntCostFn s = new IDAStarSearcherForIntCostFn(
             initialState, heuristic, 1000, GRAPH);
     s.doSearch();
     if (s.foundGoalState()) { ... }
 

To continue the search call doSearch() again and the search will be continued as far as possible. Note that this type of search engine will discover the same path to the same goal state multiple times when the depth limit is increased. Paths through a goal state will never be considered.

Author:

Gerhard Wickler

Nested Class Summary

Nested classes/interfaces inherited from class ai.search.SearchEngineForIntCostFn

SearchEngineForIntCostFn.SearchNodeForIntCostFn<S extends SearchStateForIntCostFn>

Nested classes/interfaces inherited from class ai.search.SearchEngine

SearchEngine.GraphType

Field Summary

protected int	currentFLimit the current f-value limit
protected java.util.List<SearchEngineForIntCostFn.SearchNodeForIntCostFn<S>>	searchStack the stack for unexplored (open) nodes

Fields inherited from class ai.search.informed.BestFirstSearcherForIntCostFn

dfTendency, h, hweight

Fields inherited from class ai.search.SearchEngineForIntCostFn

foundGoalNode, initialState

Fields inherited from class ai.search.SearchEngine

doInterrupt, doRepeatTest, searchLimit, searchStarted, traceStream, yieldFrequency

Constructor Summary

IDAStarSearcherForIntCostFn(S state, IntCostHeuristic<S> heuristic, long limit, SearchEngine.GraphType structure)
This constructor creates a new IDAStarSearcherForIntCostFn for the given initial state, heuristic, search limit, and type of search space.

Method Summary

boolean	continuable() This function tests whether the search may be continued.
void	doSearch() This function must be called to start or continue the search.
long	getNrOfExploredStates() This function returns the number of states that have been explored during this search.
long	getNrOfGeneratedStates() This function returns the number of states that have been generated during this search.
void	setBreadthFirstTendency() This function causes an Exception to be thrown as this type of SearchEngine can only search in a depth-first manner.

Methods inherited from class ai.search.informed.BestFirstSearcherForIntCostFn

setDepthFirstTendency, setHeuristicWeight

Methods inherited from class ai.search.SearchEngineForIntCostFn

foundGoalState, getGoalState, getInitialState, getSolutionDepth, getSolutionPath, getSolutionPathCost

Methods inherited from class ai.search.SearchEngine

clone, equals, getSearchLimit, hashCode, interrupt, printTrace, setSearchLimit, setTraceStream, setYieldFrequency, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Field Detail

searchStack

protected java.util.List<SearchEngineForIntCostFn.SearchNodeForIntCostFn<S extends SearchStateForIntCostFn>> searchStack

the stack for unexplored (open) nodes

currentFLimit

protected int currentFLimit

the current f-value limit

Constructor Detail

IDAStarSearcherForIntCostFn

public IDAStarSearcherForIntCostFn(S state,
                                   IntCostHeuristic<S> heuristic,
                                   long limit,
                                   SearchEngine.GraphType structure)

This constructor creates a new IDAStarSearcherForIntCostFn for the given initial state, heuristic, search limit, and type of search space. The search does not start immediately but waits for the function doSearch() to be called.

Parameters:

state - the initial state from which the search space is generated

heuristic - the heuristic function that guides the search

limit - the initial value for the search limit

structure - TREE or GRAPH, depending on what is to be be searched

Method Detail

setBreadthFirstTendency

public void setBreadthFirstTendency()

This function causes an Exception to be thrown as this type of SearchEngine can only search in a depth-first manner.

Overrides:

setBreadthFirstTendency in class BestFirstSearcherForIntCostFn<S extends SearchStateForIntCostFn>

doSearch

public void doSearch()

This function must be called to start or continue the search. It will only return when one of the following conditions becomes true:

• a goal state has been found: in this case SearchEngineForIntCostFn.foundGoalState() will return true, the search will be continuable (continuable() usually will return true), and SearchEngineForIntCostFn.getGoalState() will return a goal state, not null
• the search limit has been reached: in this case SearchEngineForIntCostFn.foundGoalState() will return false, the search will not be continuable (continuable() will return false), and SearchEngineForIntCostFn.getGoalState() will return null; to make the search continuable the search limit has to be increased using SearchEngine.setSearchLimit(long)
• the search space has been exhausted: in this case SearchEngineForIntCostFn.foundGoalState() will return false, the search will not be continuable (continuable() will return false), and SearchEngineForIntCostFn.getGoalState() will return null

Specified by:

doSearch in class SearchEngine

continuable

public boolean continuable()

This function tests whether the search may be continued. Usually this is the case if there are more search states to be explored and the search limit has not yet been exceeded.

Specified by:

continuable in class SearchEngine

Returns:

true if and only if there are more search states that can be explored

getNrOfExploredStates

public long getNrOfExploredStates()

This function returns the number of states that have been explored during this search. A state counts as explored when the goal test has been performed on the state and its successors have been generated.

Specified by:

getNrOfExploredStates in class SearchEngine

Returns:

the number of nodes that have been closed during this search

getNrOfGeneratedStates

public long getNrOfGeneratedStates()

This function returns the number of states that have been generated during this search. Note that these states may not all be different.

Specified by:

getNrOfGeneratedStates in class SearchEngine

Returns:

the number of states that have been generated during this search