checkersgame.java

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      // you give the options of where the piece can move to:
      for(byte i = 0; i < myPossibleMoves.size(); i++) {
        byte[] coordinates = (byte[])myPossibleMoves.elementAt(i);
        if((coordinates[0] == myDestinationX) && 
           (coordinates[1] == myDestinationY)) {
          i++;
          i = (new Integer(i % myPossibleMoves.size())).byteValue();
          coordinates = (byte[])myPossibleMoves.elementAt(i);
          myDestinationX = coordinates[0];
          myDestinationY = coordinates[1];
          break;
        }
      }
    }
  }

  /**
   * If no piece is selected, you select one.  If a piece 
   * is selected, you move it.
   */
  void upPressed() {
    // in the first case the user has not yet selected a 
    // piece to move:
    if(myDestinationX == -1) {
      fixSelection();
    } else {
      // if the source square and destination square 
      // have been chosen, you move the piece:
      move();
    }
  }

  /**
   * If the user decided not to move the selected piece 
   * (and instead wants to select again), this undoes 
   * the selection. This corresponds to pressing the 
   * DOWN key.
   */
  void deselect() {
    // if the player has just completed a jump and 
    // could possibly jump again but decides not to 
    // (i.e. deselects), then the turn ends:
    if(myIsJumping) {
      mySelectedX = -1;
      mySelectedY = -1;
      myDestinationX = -1;
      myDestinationY = -1;
      myIsJumping = false;
      myMoveManager.endTurn();
    } else {
      // setting the destination coordinates to -1 
      // is the signal that the the choice of which 
      // piece to move can be modified:
      myDestinationX = -1;
      myDestinationY = -1;
    }
  }

  //-------------------------------------------------------
  //   internal square selection methods

  /**
   * When the player has decided that the currently selected
   * square contains the piece he really wants to move, this 
   * is called. This method switches to the mode where 
   * the player selects the destination square of the move.
   */
  private void fixSelection() {
    byte[] destination = (byte[])myPossibleMoves.elementAt(0);
    // setting the destination coordinates to valid 
    // coordinates is the signal that the user is done 
    // selecting the piece to move and now is choosing 
    // the destination square:
    myDestinationX = destination[0];
    myDestinationY = destination[1];
  }

  /**
   * This method starts from the currently selected square 
   * and finds the next square that contains a piece that 
   * the player can move.
   */
  private void selectNext() {
    // Test the squares one by one (starting from the 
    // currently selected square) until you find a square 
    // that contains one of the local player's pieces 
    // that can move:
    byte testX = mySelectedX;
    byte testY = mySelectedY;
    while(true) {
      testX++;
      if(testX >= X_LENGTH) {
        testX = 0;
        testY++;
        testY = (new Integer(testY % Y_LENGTH)).byteValue();
      }
      getMoves(testX, testY, myPossibleMoves, false);
      if((myPossibleMoves.size() != 0) || 
           ((testX == mySelectedX) && (testY == mySelectedY))) {
        mySelectedX = testX;
        mySelectedY = testY;
        break;
      }
    }
  }

  /**
   * This method starts from the currently selected square 
   * and finds the next square (to the left) that contains 
   * a piece that the player can move.
   */
  private void selectPrevious() {
    // Test the squares one by one (starting from the 
    // currently selected square) until you find a square 
    // that contains one of the local player's pieces 
    // that can move:
    byte testX = mySelectedX;
    byte testY = mySelectedY;
    while(true) {
      testX--;
      if(testX < 0) {
        testX += X_LENGTH;
        testY--;
        if(testY < 0) {
          testY += Y_LENGTH;
        }
      }
      getMoves(testX, testY, myPossibleMoves, false);
      if((myPossibleMoves.size() != 0) || 
         ((testX == mySelectedX) && (testY == mySelectedY))) {
        mySelectedX = testX;
        mySelectedY = testY;
        break;
      }
    }
  }

  //-------------------------------------------------------
  //   internal utilities

  /**
   * Once the user has selected the move to make, this 
   * updates the data accordingly.
   */
  private void move() {
    // the piece that was on the source square is 
    // now on the destination square:
    myGrid[myDestinationX][myDestinationY] 
      = myGrid[mySelectedX][mySelectedY];
    // the source square is emptied:
    myGrid[mySelectedX][mySelectedY] = 0;
    if(myDestinationY == 0) {
      myGrid[myDestinationX][myDestinationY] = 2;
    }
    // tell the communicator to inform the other player 
    // of this move:
    myMoveManager.move(mySelectedX, mySelectedY, 
                        myDestinationX, myDestinationY);
    // deal with the special rules for jumps::
    if((mySelectedY - myDestinationY > 1) || 
       (myDestinationY - mySelectedY > 1)) {
      int jumpedY = (mySelectedY + myDestinationY)/2;
      int jumpedX = mySelectedX;
      int parity = mySelectedY % 2;
      // the coordinates of the jumped square depend on 
      // what row you're in:
      if((parity > 0) && (myDestinationX > mySelectedX)) {
        jumpedX++;
      } else if((parity == 0) && (mySelectedX > myDestinationX)) {
        jumpedX--;
      }
      // remove the piece that was jumped over:
      myGrid[jumpedX][jumpedY] = 0;
      // now get ready to jump again if possible:
      mySelectedX = myDestinationX;
      mySelectedY = myDestinationY;
      myDestinationX = -1;
      myDestinationY = -1;
      // see if another jump is possible.
      // The "true" argument tells the program to return 
      // only jumps because the player can go again ONLY 
      // if there's a jump:
      getMoves(mySelectedX, mySelectedY, myPossibleMoves, true);
      // if there's another jump possible with the same piece, 
      // allow the player to continue jumping:
      if(myPossibleMoves.size() != 0) {
        myIsJumping = true;
        byte[] landing = (byte[])myPossibleMoves.elementAt(0);
        myDestinationX = landing[0];
        myDestinationY = landing[1];
      } else {
        myIsJumping = false;
        myMoveManager.endTurn();
      }
    } else {
      // since it's not a jump, you just end the turn 
      // by deselecting everything.
      mySelectedX = -1;
      mySelectedY = -1;
      myDestinationX = -1;
      myDestinationY = -1;
      myPossibleMoves.removeAllElements();
      // tell the other player you're done:
      myIsJumping = false;
      myMoveManager.endTurn();
    }
  }
  
  /**
   * Given a square on the grid, get the coordinates 
   * of one of the adjoining (diagonal) squares.
   * 0 = top left
   * 1 = top right
   * 2 = bottom left
   * 3 = bottom right.
   * @return the coordinates or null if the desired corner 
   * is off the board.
   */
  private byte[] getCornerCoordinates(byte x, byte y, byte corner) {
    byte[] retArray = null;
    if(corner < 2) {
      y--;
    } else {
      y++;
    }
    // Where the corner is on the grid depends on 
    // whether this is an odd row or an even row:
    if((corner % 2 == 0) && (y % 2 != 0)) {
      x--;
    } else if((corner % 2 != 0) && (y % 2 == 0)) {
      x++;
    }
    try {
      if(myGrid[x][y] > -15) {
        // you don't really care about the value, this
        // if statement is just there to get it to 
        // throw if the coordinates aren't on the board.
        retArray = new byte[2];
        retArray[0] = x;
        retArray[1] = y;
      }
    } catch(ArrayIndexOutOfBoundsException e) {
      // this throws if the coordinates do not correspond 
      // to a square on the board. It's not a problem, 
      // so you do nothing--you just return null instead 
      // of returning coordinates since no valid 
      // coordinates correspond to the desired corner.
    }
    return(retArray);
  }
  
  /**
   * Determines where the piece in the given 
   * grid location can move.  Clears the Vector
   * and fills it with the locations that 
   * the piece can move to.
   * @param jumpsOnly if you should return only moves that 
   *        are jumps.
   */
  private void getMoves(byte x, byte y, Vector toFill, boolean jumpsOnly) {
    toFill.removeAllElements();
    // if the square does not contain one of the local player's 
    // pieces, then there are no corresponding moves and you just
    // return an empty vector.
    if(myGrid[x][y] <= 0) {
      return;
    }
    // check each of the four corners to see if the 
    // piece can move there:
    for(byte i = 0; i < 4; i++) {
      byte[] coordinates = getCornerCoordinates(x, y, i);
      // if the coordinate array is null, then the corresponding 
      // corner is off the board and you don't deal with it.
      // The later two conditions in the following if statement
      // ensure that either the move is a forward move or the 
      // current piece is a king:
      if((coordinates != null) &&
         ((myGrid[x][y] > 1) || (i < 2))) {
        // if the corner is empty (and you're not looking 
        // for just jumps), then this is a possible move
        // so you add it to the vector of moves:
        if((myGrid[coordinates[0]][coordinates[1]] == 0) && (! jumpsOnly)) {
          toFill.addElement(coordinates);
          // if the space is occupied by an opponent, 
          // see if you can jump it:
        } else if(myGrid[coordinates[0]][coordinates[1]] < 0) {
          byte[] jumpLanding = getCornerCoordinates(coordinates[0], 
                                                 coordinates[1], i);
          // if the space on the far side of the opponent's piece
          // is on the board and is unoccupied, then a jump 
          // is possible, so you add it to the vector of moves:
          if((jumpLanding != null) && 
             (myGrid[jumpLanding[0]][jumpLanding[1]] == 0)) {
            toFill.addElement(jumpLanding);
          }
        }
      }
    } // end for loop
  }
  
}