Making Four-In-A-Row - Part 3: Making Moves

Intro

In this previous blog post, we set up our FourInARowGame class's fields.

Now we're ready to start implementing player moves and updating the state of the game accordingly.

Creating the method

We'll start off by creating a new method in the FourInARowGame class called playMove():

export default class FourInARowGame {  // ..  static createBoard() {    // ..  }  playMove(columnIndex) {    return {      board: this.currentBoard,      winner: Constants.PlayerColor.NONE,      status: {        value: Constants.MoveStatus.SUCCESS,      },      winLine: [],    };  }}

Note: We're just just returning mock data for now. We'll properly implement this method later on.

The playMove method actually takes in a columnIndex argument. From this input, a move will be attempted and the result of the attempted move will be evaluated and returned. The code example above shows the a MoveResult object being returned. It can contain:

• The current state of the board after a move has been played
• The winner
• The status of the move
• The win line - an array of positions of 4 consecutive tokens were located, if found.

Interacting with our state machine

Exposing our state machine object

Now, to try out our code, we'll try interacting with our Four-In-A-Row state machine with the browser.

First, we'll need to expose the object so that the browser's console can access it. One way we can do this is by adding an instance of the FourInARowGame class to the window object. Let's do that right now. Replace the contents of src/index.js with the following:

import FourInARowGame from "./FourInARowGame.js";window.fourInARowGame = new FourInARowGame();

Now, ensure that the a http server is running in the root of project and navigate to the server's address in your web browser.

Open up your the console window in your web browser then enter in: window.fourInARowGame. You should see an output of the FourInARowGame instance object. In Google Chrome it looks like this:

FourInARowGame{startingColor: 'yellow', currentTurn: 'yellow', status: 'start', board: Array(6)}

There is also the options to expand the object so you can view more details about the object.

If you don't see some sort of representation of an FourInARow instance object then please go over your code and check that you've followed the instructions correctly!

Play a (fake) move

Now let's call the playMove method from the FourInARow instance, in the browser console window.

First let's store the state machine in an easier to reference variable called game:

let game = window.fourInARowGame;

Now call the playMove method with the columnIndex parameter set to 0:

game.playMove(0);

Your browser console window should output something resembling the MoveResult object we discussed earlier:

{board: Array(6), winner: 'none', status: {}, winLine: Array(0)}

When expanded, we get more details:

{board: Array(6), winner: 'none', status: {}, winLine: Array(0)}    board: (6) [Uint8Array(7), Uint8Array(7), Uint8Array(7), Uint8Array(7), Uint8Array(7), Uint8Array(7)]    status: {value: 'success'}    winLine: []    winner: "none"    [[Prototype]]: Object

Now that we know the type of result that we're expecting after calling playMove(), let's properly implement the method so that it returns real data based on the state of the board.

Implementing the method (for real this time!)

Firstly, we'll update the playMove() method in our FourInARowGame class so that it checks the current status of the game before allowing the player to make a move.

It wouldn't make sense for a player to be able to perform a move when the game has already ended in a win or a draw.

It also is a good opportunity to update the current status of the game from it's "start" to being "in progress".

playMove(columnIndex) {    switch (this.status) {        case Constants.GameStatus.START:            this.status = GameStatus.IN_PROGRESS;            break;        case Constants.GameStatus.DRAW:        case Constants.GameStatus.WIN:            // The game is over at this point so            // re-evaluate the latest board, returning the same game status            // and board details.            // TODO: Implement this properly            console.log("Game already ended in win or draw. re-evaluating latest game state);        default:            break;    }}

We'll re-evaluate the latest game state when a win or draw happens later.

For now let's continue focusing on allowing a player to make a move that changes the game's state.

We will first create a method called performMove():

performMove(columnIndex) {  // ...}

Now we need to create a copy of the current board that we can modify without changing the value of this.currentBoard.

To do this, we'll create a static method called deepBoardCopy():

static deepBoardCopy(oldBoard) {  let newBoard = new Array(Constants.BoardDimensions.ROWS);  for (let rowIndex = 0; rowIndex < Constants.BoardDimensions.ROWS; rowIndex++) {      newBoard[rowIndex] = new Uint8Array(Constants.BoardDimensions.COLUMNS);      for (let columnIndex = 0; columnIndex < Constants.BoardDimensions.COLUMNS; columnIndex++) {          newBoard[rowIndex][columnIndex] = oldBoard[rowIndex][columnIndex];      }  }  return newBoard;}

Now store the board copy in a variable called nextBoard:

performMove(columnIndex) {  let nextBoard = FourInARowGame.deepBoardCopy(this.currentBoard);}

The next thing we need to do is to actually perform the move on the board, to do this we'll create a method called tryPerformMove():

    tryPerformMove(columnIndex, nextBoard) {        let isMoveValid = false;        for (let i = nextBoard.length - 1; i > -1; i--) {            let boardRow = nextBoard[i];            let boardPosition = boardRow[columnIndex];            if (boardPosition !== Constants.BoardToken.NONE) {                continue;            }            boardRow[columnIndex] = FourInARowGame.playerColorToBoardToken(this.currentTurn);            isMoveValid = true;            break;        }        if (!isMoveValid) {            return {                status: Constants.MoveStatus.INVALID,            };        }        return {            status: Constants.MoveStatus.SUCCESS,            board: nextBoard        };    }

The method above checks the column in the board the player specified from the bottom to the top for an empty position then attempts to add the the current player's token to the board position.

It then returns the result.

There is a static method here called playerColorToBoardToken that was used, this sets the set's the value in the board position to a numeric value that is associated with the current player's colour.

Add it to the FourInARowGame class too:

  static createBoard() {    // ...  }  static playerColorToBoardToken(playerColor) {      switch (playerColor) {          case Constants.PlayerColor.YELLOW:              return Constants.BoardToken.YELLOW;          case Constants.PlayerColor.RED:              return Constants.BoardToken.RED;          default:              return Constants.BoardToken.NONE;      }  }

Now we'll go back to the performMove() method and set the current game board to the board in the object returned from the tryPerformMove() method.

performMove(columnIndex) {  let nextBoard = FourInARowGame.deepBoardCopy(this.currentBoard);  let moveAttemptResult = this.tryPerformMove(columnIndex, nextBoard);  if (moveAttemptResult.status === Constants.MoveStatus.INVALID) {    return {        board: nextBoard,        winner: Constants.PlayerColor.NONE,        status: {            message: "Returned column is filled",            value: Constants.MoveStatus.INVALID        },        winLine: []    }  }  // From this point, the board move was successful.  this.currentBoard = moveAttemptResult.board;}

If the value of the moveAttemptResult.status is MoveStatus.Invalid then we return a MoveResult which has the same structure as the move data we returned in our fake playMove() method implementation.

Lastly, we need create a method called evaluateGame which will be used to check if the status of the game has changed then returns a MoveResult.

For now, we'll just return a MoveResult of a successful move that also indicates that the game is still in progress.

evaluateGame(board) {    // From this point, we can assume that a successful move was made and the game will    // continue on.    return {        board: board,        winner: Constants.PlayerColor.NONE,        status: {            value: Constants.MoveStatus.SUCCESS        },        winLine: []    };}

Now the MoveResult show an updated board value based on the column the player placed their token in.

To finish with our initial proper playMove() method implementation, we'll add a few more lines to our playMove().

playMove(columnIndex) {    switch (this.status) {        case Constants.GameStatus.START:            this.status = Constants.GameStatus.IN_PROGRESS;            break;        case Constants.GameStatus.DRAW:        case Constants.GameStatus.WIN:            // The game is over at this point so            // re-evaluate the latest board, returning the same game status            // and board details.            // TODO: Implement this properly            console.log("Game already ended in win or draw. re-evaluating latest game state);        default:            break;    }    let moveResults = this.performMove(columnIndex);    this.currentTurn = this.currentTurn === Constants.PlayerColor.YELLOW        ? Constants.PlayerColor.RED        : Constants.PlayerColor.YELLOW;    return moveResults;}

We perform a move, update the current turn to the opposing player's turn then return the results of the move.

Testing out our changes

We added quite a few changes, let's test them out.

Assuming your have a local http server running at the root of the project, navigate to the server's address in your web browser and open the browser console window again.

Now enter the following line in the browser console:

let game = window.fourInARowGame;

Now call the playMove method with the columnIndex parameter set to 1 instead of 0 to add the token to the second column on the game board:

game.playMove(1);

Now you'll should see a MoveResult object returned. If you expand the object then expand the board field, you'll see that the value 1 has been added to the bottom of the second column on the board.

If you do, congratulations!

If not, please go over this post carefully to check for mistakes you may have made.