001 package aima.search.nqueens;
002
003 import java.util.HashSet;
004 import java.util.List;
005 import java.util.Random;
006 import java.util.Set;
007
008 import aima.basic.XYLocation;
009 import aima.search.framework.GoalTest;
010 import aima.search.informed.ga.FitnessFunction;
011
012 /**
013 * A class whose purpose is to evaluate the fitness of NQueen individuals
014 * and to provide utility methods for translating between an NQueensBoard
015 * representation and the String representation used by the GeneticAlgorithm.
016 */
017
018 /**
019 * @author Ciaran O'Reilly
020 *
021 */
022 public class NQueensFitnessFunction implements FitnessFunction, GoalTest {
023
024 private final NQueensGoalTest goalTest = new NQueensGoalTest();
025
026 public NQueensFitnessFunction() {
027
028 }
029
030 //
031 // START - Interface FitnessFunction
032 public Double getValue(String individual) {
033 double fitness = 0;
034
035 NQueensBoard board = getBoardForIndividual(individual);
036 int boardSize = board.getSize();
037
038 // Calculate the number of non-attacking pairs of queens (refer to AIMA
039 // page 117).
040 List<XYLocation> qPositions = board.getQueenPositions();
041 for (int fromX = 0; fromX < (boardSize - 1); fromX++) {
042 for (int toX = fromX + 1; toX < boardSize; toX++) {
043 int fromY = qPositions.get(fromX).getYCoOrdinate();
044 boolean nonAttackingPair = true;
045 // Check right beside
046 int toY = fromY;
047 if (board.queenExistsAt(new XYLocation(toX, toY))) {
048 nonAttackingPair = false;
049 }
050 // Check right and above
051 toY = fromY - (toX - fromX);
052 if (toY >= 0) {
053 if (board.queenExistsAt(new XYLocation(toX, toY))) {
054 nonAttackingPair = false;
055 }
056 }
057 // Check right and below
058 toY = fromY + (toX - fromX);
059 if (toY < boardSize) {
060 if (board.queenExistsAt(new XYLocation(toX, toY))) {
061 nonAttackingPair = false;
062 }
063 }
064
065 if (nonAttackingPair) {
066 fitness += 1.0;
067 }
068 }
069 }
070
071 return fitness;
072 }
073
074 // END - Interface FitnessFunction
075 //
076
077 //
078 // START - Interface GoalTest
079 public boolean isGoalState(Object state) {
080 return goalTest.isGoalState(getBoardForIndividual((String) state));
081 }
082
083 // END - Interface GoalTest
084 //
085
086 public NQueensBoard getBoardForIndividual(String individual) {
087 int boardSize = individual.length();
088 NQueensBoard board = new NQueensBoard(boardSize);
089 for (int i = 0; i < boardSize; i++) {
090 int pos = Character
091 .digit(individual.charAt(i), individual.length());
092 board.addQueenAt(new XYLocation(i, pos));
093 }
094
095 return board;
096 }
097
098 public String generateRandomIndividual(int boardSize) {
099 StringBuffer ind = new StringBuffer();
100
101 assert (boardSize >= Character.MIN_RADIX && boardSize <= Character.MAX_RADIX);
102
103 for (int i = 0; i < boardSize; i++) {
104 ind.append(Character.forDigit(new Random().nextInt(boardSize),
105 boardSize));
106 }
107
108 return ind.toString();
109 }
110
111 public Set<Character> getFiniteAlphabetForBoardOfSize(int size) {
112 Set<Character> fab = new HashSet<Character>();
113
114 assert (size >= Character.MIN_RADIX && size <= Character.MAX_RADIX);
115
116 for (int i = 0; i < size; i++) {
117 fab.add(Character.forDigit(i, size));
118 }
119
120 return fab;
121 }
122 }