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AIMA-Java Core Algorithms from the book Artificial Intelligence a Modern Approach 3rd Ed.

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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, GoalTest {

	private final NQueensGoalTest goalTest = new NQueensGoalTest();

	public NQueensFitnessFunction() {

	}

	//
	// START - Interface FitnessFunction
	public double getValue(Individual 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 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) state));
	}

	// END - Interface GoalTest
	//

	public NQueensBoard getBoardForIndividual(Individual 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 generateRandomIndividual(int boardSize) {

		List individualRepresentation = new ArrayList();
		for (int i = 0; i < boardSize; i++) {
			individualRepresentation.add(new Random().nextInt(boardSize));
		}
		
		Individual individual = new Individual(individualRepresentation);

		return individual;
	}

	public Set getFiniteAlphabetForBoardOfSize(int size) {
		Set fab = new HashSet();

		for (int i = 0; i < size; i++) {
			fab.add(i);
		}

		return fab;
	}
}