Tic Tac Toe z Minimax: Komputer czasem przegrywa, gdy gracz idzie pierwszy; działa inaczej

Pracuję nad algorytmem Minimax dla bezkonkurencyjnego Tic Tac Toe. Potrzebuję go, aby działał zarówno w sytuacji, gdy komputer idzie pierwszy, jak i wtedy, gdy gracz idzie pierwszy. W obecnej wersji komputer nigdy nie przegra podczas pierwszego. Wydaje się jednak, że Minimax nigdy nie znajduje najlepszego ruchu (zawsze zwraca -1 jako wynik), jeśli Gracz pójdzie pierwszy.

Co powoduje, że wynik Minimax jest równy -1 dla komputera, jeśli gracz wykona pierwszy ruch?

Przykład:

board.addMark( 1, Mark2.PLAYER ); // add a 'PLAYER' mark to an arbitrary location
Minimax.minimax( board, Mark2.COMPUTER ); // will always return -1

Oto klasa „Minimax”:

public class Minimax {
  public static int minimax( Board board, Mark2 currentMark ) {
    int score = (currentMark == Mark2.COMPUTER) ? -1 : 1;
    int[] availableSpaces = board.getAvailableSpaces();

    if ( board.hasWinningSolution() )
      score = (board.getWinningMark() == Mark2.COMPUTER) ? 1 : -1;
    else if ( availableSpaces.length != 0 ) {
      Mark2 nextMark = (currentMark == Mark2.COMPUTER) ? Mark2.PLAYER : Mark2.COMPUTER;

      for ( int availableIndex = 0; availableIndex < availableSpaces.length; availableIndex++ ) {
        board.addMark( availableSpaces[availableIndex], currentMark );
        int nextScore = minimax( board, nextMark );
        board.eraseMark( availableSpaces[availableIndex] );

        if ( currentMark == Mark2.COMPUTER && nextScore > score
            || currentMark == Mark2.PLAYER && nextScore < score )
          score = nextScore;
      }
    }

    return score;
  }
}

Oto klasa „Board”:

public class Board {
  private Mark2 gameBoard[];
  private int blankSpaces;

  private boolean solutionFound;
  private Mark2 winningMark;

  public final static int winSets[][] = {
      { 0, 1, 2 }, { 3, 4, 5 },
      { 6, 7, 8 }, { 0, 3, 6 },
      { 1, 4, 7 }, { 2, 5, 8 },
      { 0, 4, 8 }, { 2, 4, 6 }
  };

  public Board() {
    gameBoard = new Mark2[9];
    blankSpaces = 9;

    for ( int boardIndex = 0; boardIndex < gameBoard.length; boardIndex++ ) {
      gameBoard[boardIndex] = Mark2.BLANK;
    }
  }

  public void addMark( int spaceIndex, Mark2 mark ) {
    if ( gameBoard[spaceIndex] != mark ) {
      gameBoard[spaceIndex] = mark;

      if ( mark == Mark2.BLANK ) {
        blankSpaces++;
      } else {
        blankSpaces--;
      }
    }
  }

  public void eraseMark( int spaceIndex ) {
    if ( gameBoard[spaceIndex] != Mark2.BLANK ) {
      gameBoard[spaceIndex] = Mark2.BLANK;
      blankSpaces++;
    }
  }

  public int[] getAvailableSpaces() {
    int spaces[] = new int[blankSpaces];
    int spacesIndex = 0;

    for ( int boardIndex = 0; boardIndex < gameBoard.length; boardIndex++ )
      if ( gameBoard[boardIndex] == Mark2.BLANK )
        spaces[spacesIndex++] = boardIndex;

    return spaces;
  }

  public Mark2 getMarkAtIndex( int spaceIndex ) {
    return gameBoard[spaceIndex];
  }

  public boolean hasWinningSolution() {
    this.solutionFound = false;
    this.winningMark = Mark2.BLANK;

    for ( int setIndex = 0; setIndex < winSets.length && !solutionFound; setIndex++ )
      checkSpacesForWinningSolution( winSets[setIndex][0], winSets[setIndex][1], winSets[setIndex][2] );

    return solutionFound;
  }

  public Mark2 getWinningMark() {
    return this.winningMark;
  }

  private void checkSpacesForWinningSolution( int first, int second, int third ) {
    if ( gameBoard[first] == gameBoard[second] && gameBoard[third] == gameBoard[first]
        && gameBoard[first] != Mark2.BLANK ) {
      solutionFound = true;
      winningMark = gameBoard[first];
    }
  }

  public void printBoard() {
    System.out.printf( " %c | %c | %c\n", getMarkCharacter( gameBoard[0] ), getMarkCharacter( gameBoard[1] ),
        getMarkCharacter( gameBoard[2] ) );
    System.out.println( "------------" );
    System.out.printf( " %c | %c | %c\n", getMarkCharacter( gameBoard[3] ), getMarkCharacter( gameBoard[4] ),
        getMarkCharacter( gameBoard[5] ) );
    System.out.println( "------------" );
    System.out.printf( " %c | %c | %c\n", getMarkCharacter( gameBoard[6] ), getMarkCharacter( gameBoard[7] ),
        getMarkCharacter( gameBoard[8] ) );
  }

  public char getMarkCharacter( Mark2 mark ) {
    char result = (mark == Mark2.PLAYER) ? 'O' : ' ';
    result = (mark == Mark2.COMPUTER) ? 'X' : result;
    return result;
  }
}

A oto klasa „Mark2”, jeśli było jakieś zamieszanie:

public enum Mark2 {
  BLANK,
  PLAYER,
  COMPUTER
}

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