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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.Collection;
import java.util.List;
import java.util.Random;

import aima.core.search.framework.problem.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 provide static utility methods for solving the
 * n-queens problem with genetic algorithms. This includes fitness function,
 * goal test, random creation of individuals and convenience methods for
 * translating between between an NQueensBoard representation and the Integer list
 * representation used by the GeneticAlgorithm.
 * 
 * @author Ciaran O'Reilly
 * @author Ruediger Lunde
 * 
 */
public class NQueensGenAlgoUtil {

	public static FitnessFunction getFitnessFunction() {
		return new NQueensFitnessFunction();
	}
	
	public static GoalTest getGoalTest() {
		return new NQueensGenAlgoGoalTest();
	}
	

	public static 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 static Collection getFiniteAlphabetForBoardOfSize(int size) {
		Collection fab = new ArrayList();

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

		return fab;
	}
	
	public static class NQueensFitnessFunction implements FitnessFunction {

		public double apply(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;
		}
	}

	public static class NQueensGenAlgoGoalTest implements GoalTest {
		private final NQueensGoalTest goalTest = new NQueensGoalTest();

		@SuppressWarnings("unchecked")
		public boolean isGoalState(Object state) {
			return goalTest.isGoalState(getBoardForIndividual((Individual) state));
		}
	}

	public static 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;
	}
}