System Design of Tic Tac Toe Game

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Overview

Tic Tac Toe is a board game. Let’s first understand the rules of the game. In this tutorial, we will

  • First, understand the tic tac toe game with an example
  • See the UML diagram of the low-level design
  • Then we will see the low-level design expressed in the Go language
  • Finally, we will see a proper working code of the same with proper design

Let’s first understand what is Tic Tac Toe with an example

  • There is an n*n board and each block in the board can be marked with either a cross or a circle only if the block is empty
  • Max two players play the game at a time with each taking turn.
  • The first player marks a cross at any block in the board in its turn.  While the second player marks a circle at any block in the board in its turn.
  • The objective is to have either an entire row or an entire column or an entire diagonal with any of the one symbol either cross or circle.
  • Both players will try to stop the other player from achieving this objective. Whosoever achieves it first will win.
  • If all blocks in the board are full and none of the players have not been able to mark the entire row, column, or diagonal with its symbol then the game results in a draw.
  • No more moves are allowed after one player wins the game.

Let’s understand this game with an example. Assume a 3*3 grid. Dot (‘.’) represents an empty block

Player 1 Move with Symbol * at Position X:1 Y:1
...
.*.
...

Player 2 Move with Symbol o at Position X:1 Y:2
...
.*o
...

Player 1 Move with Symbol * at Position X:2 Y:0
...
.*o
*..

Player 2 Move with Symbol o at Position X:0 Y:2
..o
.*o
*..

Player 1 Move with Symbol * at Position X:2 Y:2
..o
.*o
*.*

Player 2 Move with Symbol o at Position X:0 Y:0
o.o
.*o
*.*

Player 1 Move with Symbol * at Position X:2 Y:1
o.o
.*o
***

First Player Win
o.o
.*o
***

In the above game First Player Win because the third row is all occupied with symbol cross – ‘*’

UML Diagram

Let’s first see the UML diagram for the same.

Here are some ideas from the UML diagram

  • There is a Symbol Enum which represents different symbols used on the board. The symbol could be a Cross, Circle, or Dot. The Dot represents an empty block in the board
  • There is an iPlayer interface that represents the player in the game.
  • A player can be a human player or a computer player. As such, there is a humanPlayer class and a computerPlayer class that implements the iPlayer interface
  • There is a board class that only captures the details of the board. Depending upon the state of the board it tells whether any of its row,  column, or diagonal has been completed and by which symbol. It knows nothing about a player or the game itself. It knows how to print the board as well. This board class will be used by the Game class as we will later see.
  • There is a Game Status Enum class that defines the different starts of the game. A game can be either Inprogresss, Draw,  FirstPlayerWin, or SecondPlayerWin state.
  • There is a game class that controls the execution of the game. It has the board object,  as well as both players’ objects as its field. Other than that it has other fields as well such as gameStatus, moveIndex, etc.

Low-Level Design

Let’s look at the low-level design of the problem expressed in the Go programming language.

iPlayerInterface

type iPlayer interface {
	getSymbol() Symbol
	getNextMove() (int, int, error)
	getID() int
}

Human Player Class

type humanPlayer struct {
	symbol Symbol
}

func (h *humanPlayer) getSymbol() Symbol

func (h *humanPlayer) getNextMove() (int, int, error)

func (h *humanPlayer) getID() int

Computer Player Class

type computerPlayer struct {
	symbol Symbol
	id     int
}

func (c *computerPlayer) getSymbol() Symbol 

func (c *computerPlayer) getNextMove() (int, int, error) 

func (c *computerPlayer) getID() int

Symbol Enum

type Symbol uint8

const (
	Cross Symbol = iota
	Circle
	Dot
)

GameStatus Enum

type GameStatus uint8

const (
    GameInProgress GameStatus = iota
    GameDraw
    FirstPlayerWin
    SecondPlayerWin
)

Board Class

type board struct {
    square    [][]Symbol
    dimension int
}

func (b *board) markSymbol(i, j int, symbol Symbol) (bool, Symbol, error) 

func (b *board) checkWin(i, j int, symbol Symbol) bool 

func (b *board) printBoard()

Game Class

type game struct {
    board           *board
    firstPlayer     iPlayer
    secondPlayer    iPlayer
    firstPlayerTurn bool
    moveIndex       int
    gameStatus      GameStatus
}

func initGame(b *board, p1, p2 iPlayer) *game 

func (g *game) play() error 

func (g *game) setGameStatus(win bool, symbol Symbol) 

func (g *game) printMove(player iPlayer, x, y int)

func (g *game) printResult()

Program

Here is the full working code

symbol.go

package main

type Symbol uint8

const (
    Cross Symbol = iota
    Circle
    Dot
)

iPlayer.go

package main

type iPlayer interface {
    getSymbol() Symbol
    getNextMove() (int, int, error)
    getID() int
}

humanPlayer.go

package main

import "fmt"

var (
	MovesPlayer1 = [4][2]int{{1, 1}, {2, 0}, {2, 2}, {2, 1}}
	MovesPlayer2 = [4][2]int{{1, 2}, {0, 2}, {0, 0}, {0, 0}}
)

type humanPlayer struct {
	symbol Symbol
	index  int
	id     int
}

func (h *humanPlayer) getSymbol() Symbol {
	return h.symbol
}

func (h *humanPlayer) getNextMove() (int, int, error) {
	if h.symbol == Cross {
		h.index = h.index + 1
		return MovesPlayer1[h.index-1][0], MovesPlayer1[h.index-1][1], nil
	} else if h.symbol == Circle {
		h.index = h.index + 1
		return MovesPlayer2[h.index-1][0], MovesPlayer2[h.index-1][1], nil
	}
	return 0, 0, fmt.Errorf("Invalid Symbol")
}

func (h *humanPlayer) getID() int {
	return h.id
}

computerPlayer.go

package main

type computerPlayer struct {
	symbol Symbol
	id     int
}

func (c *computerPlayer) getSymbol() Symbol {
	return c.symbol
}

func (c *computerPlayer) getNextMove() (int, int, error) {
	//To be implemented
	return 0, 0, nil
}

func (c *computerPlayer) getID() int {
	return c.id
}

gameStatus.go

package main

type GameStatus uint8

const (
	GameInProgress GameStatus = iota
	GameDraw
	FirstPlayerWin
	SecondPlayerWin
)

board.go

package main

import "fmt"

type board struct {
	square    [][]Symbol
	dimension int
}

func (b *board) printBoard() {
	for i := 0; i < b.dimension; i++ {
		for j := 0; j < b.dimension; j++ {
			if b.square[i][j] == Dot {
				fmt.Print(".")
			} else if b.square[i][j] == Cross {
				fmt.Print("*")
			} else {
				fmt.Print("o")
			}

		}
		fmt.Println("")
	}
}

func (b *board) markSymbol(i, j int, symbol Symbol) (bool, Symbol, error) {
	if i > b.dimension || j > b.dimension {
		return false, Dot, fmt.Errorf("index input is greater than dimension")
	}
	if b.square[i][j] != Dot {
		return false, Dot, fmt.Errorf("input square already marked")
	}
	if symbol != Cross && symbol != Circle {
		return false, Dot, fmt.Errorf("incorrect Symbol")
	}
	b.square[i][j] = symbol
	win := b.checkWin(i, j, symbol)

	return win, symbol, nil
}

func (b *board) checkWin(i, j int, symbol Symbol) bool {
	//Check Row
	rowMatch := true
	for k := 0; k < b.dimension; k++ {
		if b.square[i][k] != symbol {
			rowMatch = false
		}
	}

	if rowMatch {
		return rowMatch
	}

	//Check Row
	columnMatch := true
	for k := 0; k < b.dimension; k++ {
		if b.square[k][j] != symbol {
			columnMatch = false
		}
	}

	if columnMatch {
		return columnMatch
	}

	//Check diagonal
	diagonalMatch := false
	if i == j {
		diagonalMatch = true
		for k := 0; k < b.dimension; k++ {
			if b.square[k][k] != symbol {
				diagonalMatch = false
			}
		}
	}

	return diagonalMatch

}

game.go

package main

import "fmt"

type game struct {
    board           *board
    firstPlayer     iPlayer
    secondPlayer    iPlayer
    firstPlayerTurn bool
    moveIndex       int
    gameStatus      GameStatus
}

func initGame(b *board, p1, p2 iPlayer) *game {
    game := &game{
        board:           b,
        firstPlayer:     p1,
        secondPlayer:    p2,
        firstPlayerTurn: true,
        gameStatus:      GameInProgress,
    }
    return game
}

func (g *game) play() error {
    var win bool
    var symbol Symbol
    for {
        if g.firstPlayerTurn {
            x, y, err := g.firstPlayer.getNextMove()
            if err != nil {
                return err
            }
            win, symbol, err = g.board.markSymbol(x, y, g.firstPlayer.getSymbol())
            if err != nil {
                return err
            }
            g.firstPlayerTurn = false
            g.printMove(g.firstPlayer, x, y)
        } else {
            x, y, err := g.secondPlayer.getNextMove()
            if err != nil {
                return err
            }
            win, symbol, err = g.board.markSymbol(x, y, g.secondPlayer.getSymbol())
            if err != nil {
                return err
            }
            g.firstPlayerTurn = true
            g.printMove(g.secondPlayer, x, y)
        }
        g.moveIndex = g.moveIndex + 1
        g.setGameStatus(win, symbol)
        if g.gameStatus != GameInProgress {
            break
        }
    }
    return nil
}

func (g *game) setGameStatus(win bool, symbol Symbol) {
    if win {
        if g.firstPlayer.getSymbol() == symbol {
            g.gameStatus = FirstPlayerWin
            return
        } else if g.secondPlayer.getSymbol() == symbol {
            g.gameStatus = SecondPlayerWin
            return
        }
    }
    if g.moveIndex == g.board.dimension*g.board.dimension {
        g.gameStatus = GameDraw
        return
    }
    g.gameStatus = GameInProgress
}

func (g *game) printMove(player iPlayer, x, y int) {
	symbolString := ""
	symbol := player.getSymbol()
	if symbol == Cross {
		symbolString = "*"
	} else if symbol == Circle {
		symbolString = "o"
	}
	fmt.Printf("Player %d Move with Symbol %s at Position X:%d Y:%d\n", player.getID(), symbolString, x, y)
	g.board.printBoard()
	fmt.Println("")
}

func (g *game) printResult() {
    switch g.gameStatus {
    case GameInProgress:
        fmt.Println("Game in Between")
    case GameDraw:
        fmt.Println("Game Drawn")
    case FirstPlayerWin:
        fmt.Println("First Player Win")
    case SecondPlayerWin:
        fmt.Println("Second Player Win")
    default:
        fmt.Println("Invalid Game Status")
    }
    g.board.printBoard()
}

Output

In the above program, we have fixed the moves for both the player in the humanPlayer.go file. Here is the output based upon those moves.

Player 1 Move with Symbol * at Position X:1 Y:1
...
.*.
...

Player 2 Move with Symbol o at Position X:1 Y:2
...
.*o
...

Player 1 Move with Symbol * at Position X:2 Y:0
...
.*o
*..

Player 2 Move with Symbol o at Position X:0 Y:2
..o
.*o
*..

Player 1 Move with Symbol * at Position X:2 Y:2
..o
.*o
*.*

Player 2 Move with Symbol o at Position X:0 Y:0
o.o
.*o
*.*

Player 1 Move with Symbol * at Position X:2 Y:1
o.o
.*o
***

First Player Win
o.o
.*o
***

Full Working Code:

Here is the full working code in one file

main.go

package main

import "fmt"

type Symbol uint8

const (
	Cross Symbol = iota
	Circle
	Dot
)

type GameStatus uint8

const (
	GameInProgress GameStatus = iota
	GameDraw
	FirstPlayerWin
	SecondPlayerWin
)

type iPlayer interface {
	getSymbol() Symbol
	getNextMove() (int, int, error)
	getID() int
}

var (
	MovesPlayer1 = [4][2]int{{1, 1}, {2, 0}, {2, 2}, {2, 1}}
	MovesPlayer2 = [4][2]int{{1, 2}, {0, 2}, {0, 0}, {0, 0}}
)

type humanPlayer struct {
	symbol Symbol
	index  int
	id     int
}

func (h *humanPlayer) getSymbol() Symbol {
	return h.symbol
}

func (h *humanPlayer) getNextMove() (int, int, error) {
	if h.symbol == Cross {
		h.index = h.index + 1
		return MovesPlayer1[h.index-1][0], MovesPlayer1[h.index-1][1], nil
	} else if h.symbol == Circle {
		h.index = h.index + 1
		return MovesPlayer2[h.index-1][0], MovesPlayer2[h.index-1][1], nil
	}
	return 0, 0, fmt.Errorf("Invalid Symbol")
}

func (h *humanPlayer) getID() int {
	return h.id
}

type computerPlayer struct {
	symbol Symbol
	id     int
}

func (c *computerPlayer) getSymbol() Symbol {
	return c.symbol
}

func (c *computerPlayer) getNextMove() (int, int, error) {
	//To be implemented
	return 0, 0, nil
}

func (c *computerPlayer) getID() int {
	return c.id
}

type board struct {
	square    [][]Symbol
	dimension int
}

func (b *board) printBoard() {
	for i := 0; i < b.dimension; i++ {
		for j := 0; j < b.dimension; j++ {
			if b.square[i][j] == Dot {
				fmt.Print(".")
			} else if b.square[i][j] == Cross {
				fmt.Print("*")
			} else {
				fmt.Print("o")
			}

		}
		fmt.Println("")
	}
}

func (b *board) markSymbol(i, j int, symbol Symbol) (bool, Symbol, error) {
	if i > b.dimension || j > b.dimension {
		return false, Dot, fmt.Errorf("index input is greater than dimension")
	}
	if b.square[i][j] != Dot {
		return false, Dot, fmt.Errorf("input square already marked")
	}
	if symbol != Cross && symbol != Circle {
		return false, Dot, fmt.Errorf("incorrect Symbol")
	}
	b.square[i][j] = symbol
	win := b.checkWin(i, j, symbol)

	return win, symbol, nil
}

func (b *board) checkWin(i, j int, symbol Symbol) bool {
	//Check Row
	rowMatch := true
	for k := 0; k < b.dimension; k++ {
		if b.square[i][k] != symbol {
			rowMatch = false
		}
	}

	if rowMatch {
		return rowMatch
	}

	//Check Row
	columnMatch := true
	for k := 0; k < b.dimension; k++ {
		if b.square[k][j] != symbol {
			columnMatch = false
		}
	}

	if columnMatch {
		return columnMatch
	}

	//Check diagonal
	diagonalMatch := false
	if i == j {
		diagonalMatch = true
		for k := 0; k < b.dimension; k++ {
			if b.square[k][k] != symbol {
				diagonalMatch = false
			}
		}
	}

	return diagonalMatch

}

type game struct {
	board           *board
	firstPlayer     iPlayer
	secondPlayer    iPlayer
	firstPlayerTurn bool
	moveIndex       int
	gameStatus      GameStatus
}

func initGame(b *board, p1, p2 iPlayer) *game {
	game := &game{
		board:           b,
		firstPlayer:     p1,
		secondPlayer:    p2,
		firstPlayerTurn: true,
		gameStatus:      GameInProgress,
	}
	return game
}

func (g *game) play() error {
	var win bool
	var symbol Symbol
	for {
		if g.firstPlayerTurn {
			x, y, err := g.firstPlayer.getNextMove()
			if err != nil {
				return err
			}
			win, symbol, err = g.board.markSymbol(x, y, g.firstPlayer.getSymbol())
			if err != nil {
				return err
			}
			g.firstPlayerTurn = false
			g.printMove(g.firstPlayer, x, y)
		} else {
			x, y, err := g.secondPlayer.getNextMove()
			if err != nil {
				return err
			}
			win, symbol, err = g.board.markSymbol(x, y, g.secondPlayer.getSymbol())
			if err != nil {
				return err
			}
			g.firstPlayerTurn = true
			g.printMove(g.secondPlayer, x, y)
		}
		g.moveIndex = g.moveIndex + 1

		g.setGameStatus(win, symbol)
		if g.gameStatus != GameInProgress {
			break
		}
	}
	return nil
}

func (g *game) setGameStatus(win bool, symbol Symbol) {
	if win {
		if g.firstPlayer.getSymbol() == symbol {
			g.gameStatus = FirstPlayerWin
			return
		} else if g.secondPlayer.getSymbol() == symbol {
			g.gameStatus = SecondPlayerWin
			return
		}
	}
	if g.moveIndex == g.board.dimension*g.board.dimension {
		g.gameStatus = GameDraw
		return
	}
	g.gameStatus = GameInProgress
}

func (g *game) printMove(player iPlayer, x, y int) {
	symbolString := ""
	symbol := player.getSymbol()
	if symbol == Cross {
		symbolString = "*"
	} else if symbol == Circle {
		symbolString = "o"
	}
	fmt.Printf("Player %d Move with Symbol %s at Position X:%d Y:%d\n", player.getID(), symbolString, x, y)
	g.board.printBoard()
	fmt.Println("")
}

func (g *game) printResult() {
	switch g.gameStatus {
	case GameInProgress:
		fmt.Println("Game in Between")
	case GameDraw:
		fmt.Println("Game Drawn")
	case FirstPlayerWin:
		fmt.Println("First Player Win")
	case SecondPlayerWin:
		fmt.Println("Second Player Win")
	default:
		fmt.Println("Invalid Game Status")
	}
	g.board.printBoard()
}

func main() {

	board := &board{
		square:    [][]Symbol{{Dot, Dot, Dot}, {Dot, Dot, Dot}, {Dot, Dot, Dot}},
		dimension: 3,
	}

	player1 := &humanPlayer{
		symbol: Cross,
		id:     1,
	}

	player2 := &humanPlayer{
		symbol: Circle,
		id:     2,
	}

	game := initGame(board, player1, player2)
	game.play()
	game.printResult()

}

Output

In the above program as well, we have fixed the moves for both the player in the humanPlayer class. Here is the output based upon those moves.

Player 1 Move with Symbol * at Position X:1 Y:1
...
.*.
...

Player 2 Move with Symbol o at Position X:1 Y:2
...
.*o
...

Player 1 Move with Symbol * at Position X:2 Y:0
...
.*o
*..

Player 2 Move with Symbol o at Position X:0 Y:2
..o
.*o
*..

Player 1 Move with Symbol * at Position X:2 Y:2
..o
.*o
*.*

Player 2 Move with Symbol o at Position X:0 Y:0
o.o
.*o
*.*

Player 1 Move with Symbol * at Position X:2 Y:1
o.o
.*o
***

First Player Win
o.o
.*o
***

Conclusion

This is all about designing the Tic Tac Toe game. Hope you have liked this tutorial. Please provide your feedback in the comments.

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