package aima.core.environment.nqueens;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Random;
import java.util.Set;
import aima.core.search.framework.GoalTest;
import aima.core.search.local.FitnessFunction;
import aima.core.search.local.Individual;
import aima.core.util.datastructure.XYLocation;
/**
* A class whose purpose is to evaluate the fitness of NQueen individuals and to
* provide utility methods for translating between an NQueensBoard
* representation and the String representation used by the GeneticAlgorithm.
*
* @author Ciaran O'Reilly
*
*/
public class NQueensFitnessFunction implements FitnessFunction<Integer>, GoalTest {
private final NQueensGoalTest goalTest = new NQueensGoalTest();
public NQueensFitnessFunction() {
}
//
// START - Interface FitnessFunction
public double getValue(Individual<Integer> individual) {
double fitness = 0;
NQueensBoard board = getBoardForIndividual(individual);
int boardSize = board.getSize();
// Calculate the number of non-attacking pairs of queens (refer to AIMA
// page 117).
List<XYLocation> qPositions = board.getQueenPositions();
for (int fromX = 0; fromX < (boardSize - 1); fromX++) {
for (int toX = fromX + 1; toX < boardSize; toX++) {
int fromY = qPositions.get(fromX).getYCoOrdinate();
boolean nonAttackingPair = true;
// Check right beside
int toY = fromY;
if (board.queenExistsAt(new XYLocation(toX, toY))) {
nonAttackingPair = false;
}
// Check right and above
toY = fromY - (toX - fromX);
if (toY >= 0) {
if (board.queenExistsAt(new XYLocation(toX, toY))) {
nonAttackingPair = false;
}
}
// Check right and below
toY = fromY + (toX - fromX);
if (toY < boardSize) {
if (board.queenExistsAt(new XYLocation(toX, toY))) {
nonAttackingPair = false;
}
}
if (nonAttackingPair) {
fitness += 1.0;
}
}
}
return fitness;
}
// END - Interface FitnessFunction
//
//
// START - Interface GoalTest
@SuppressWarnings("unchecked")
public boolean isGoalState(Object state) {
return goalTest.isGoalState(getBoardForIndividual((Individual<Integer>) state));
}
// END - Interface GoalTest
//
public NQueensBoard getBoardForIndividual(Individual<Integer> individual) {
int boardSize = individual.length();
NQueensBoard board = new NQueensBoard(boardSize);
for (int i = 0; i < boardSize; i++) {
int pos = individual.getRepresentation().get(i);
board.addQueenAt(new XYLocation(i, pos));
}
return board;
}
public Individual<Integer> generateRandomIndividual(int boardSize) {
List<Integer> individualRepresentation = new ArrayList<Integer>();
for (int i = 0; i < boardSize; i++) {
individualRepresentation.add(new Random().nextInt(boardSize));
}
Individual<Integer> individual = new Individual<Integer>(individualRepresentation);
return individual;
}
public Set<Integer> getFiniteAlphabetForBoardOfSize(int size) {
Set<Integer> fab = new HashSet<Integer>();
for (int i = 0; i < size; i++) {
fab.add(i);
}
return fab;
}
}