TDD-ed TicTacToe

This is a special TicTacToe game implementation, built following the TDD approach, that adheres to the OOP best practices and SOLID principles.

Class diagram

You can also check the exported PNG.

---
title: TicTacToeGame
---
classDiagram
    Position "1" -- "1" Cell
    Piece <.. Cell

    GameResult --> GameOutcome
    GameResult ..> Player

    TicTacToeGame --> Board
    TicTacToeGame --> GameOutcomeResolver
    TicTacToeGame "*" -- "2..*" Player
    XInARowWins --|> GameOutcomeResolver
    Board *-- Cell
    Player "1" -- "1" Piece

    class Board{
        -Cell[][] board
        -Set~String~ unoccupiedPositions
        + fillCell(piece: Piece, position: Position)
        + getRow(rowIndex: number): Cell[]
        + getColumn(colIndex: number): Cell[]
        + getDiagonals(position: Position): Cell[][]
        + getUnoccupiedPositions(): Position[]
        + getColsCount(): number
        + getRowsCount(): number

        - positionIsOutOfBoard(position: Position): boolean
        - rowIndexIsOutOfBounds(rowIndex: number): boolean
        - colIndexIsOutOfBounds(columnIndex: number): boolean
    }
    class Player{
        +Piece piece
    }

    class TicTacToeGame{
        -Board board
        -Queue~Player~ players
        -Set~Piece~ allowedPieces
        -boolean isInProgress
        - getNextTurnPlayer(): Player
        + create(rowsCount: number, colsCount: number, gameOutcomeResolver: GameOutcomeResolver, allowedPieces: Set~Piece~)$
        + addPlayer(player: Player): void
        + startGame(): void
        + play(position: Position): GameResult
    }

    class Cell {
        +Position position
        + setPiece(piece)
        + getPiece(): Piece
        + isOccupied(): boolean
    }

    class Position {
        + row
        + col
    }

    class Piece {
        <<enumeration>>
        X
        O
        Z...
    }

    class GameOutcomeResolver {
        <<interface>>
        getResult(board, lastPlayedPiece, lastPlayedPosition): GameResult
    }

    class XInARowWins {
        +number  count,
        + getResult(board, lastPlayedPiece, lastPlayedPosition): GameResult
        - hasXInARow(): boolean
    }

    class GameResult {
        +Player? winner
        +GameOutcome outcome
    }

    class GameOutcome {
        <<enumeration>>
        END
        DRAW
        NONE
    }

How to use?

It is very easy to build any kind of frontend (CLI/GUI/TUI) on top of this implementation. The game flow tests file simulates a few games with different outcomes.

Why?

The main selling points of this approach are:

• Being able to implement new rules for defining the winner / game outcome without changing any of the already existing code. You only need to implement the GameOutcomeResolver class (Open Closed Principle).
• Each class has very strict logic boundaries and doesn't know that it is used as part of a TicTacToe game. For example, the Board class is very generic and can even be reused for other board based games (Single Responsibility Principle).
• The game settings can be easily changed. For example, it is possible to create a TicTacToe game with a 10x10 board and as many players as needed.
• Easily testable by design, since it was built following the TDD approach.

Assumptions

• The game can't be played with less than 2 players, but has no limit on the maximum number of players.
• The minimum TicTacToe board size is 3x3.