Tic Tac Toe

Level: Beginner 15–30 min

Concepts: StateDesign

Solutions: C# | TypeScript | Python

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Create a program to implement the classic game of Tic Tac Toe.

Requirements

1. Implement game rules:
   • 3x3 grid game board
   • Two players (X and O)
   • Players take turns placing their mark
   • First player to get 3 in a row (horizontally, vertically, or diagonally) wins
   • Game ends in a draw if all spaces are filled with no winner
2. Handle game state:
   • Track current player’s turn
   • Validate moves (prevent playing in occupied spaces)
   • Detect win conditions
   • Detect draw conditions
3. Return appropriate results:
   • Current game state
   • Winner (if any)
   • Error messages for invalid moves

Test Cases

Scenario	Moves	Expected Result	Notes
Empty Board	None	Game in progress	Initial state
Horizontal Win	X(0,0), O(1,0), X(0,1), O(1,1), X(0,2)	X wins	Top row win
Vertical Win	X(0,0), O(0,1), X(1,0), O(1,1), X(2,0)	X wins	Left column win
Diagonal Win	X(0,0), O(0,1), X(1,1), O(0,2), X(2,2)	X wins	Top-left to bottom-right
Draw	X(0,0), O(0,1), X(0,2), O(1,0), X(1,1), O(2,0), X(1,2), O(2,2), X(2,1)	Draw	No winner
Invalid Move	X(0,0), X(0,1)	Error	Same player twice

Edge Cases to Consider

• Playing in an occupied space
• Playing out of turn
• Playing outside the board boundaries
• Continuing play after game end
• Multiple win conditions

Tips

• Start with an empty board representation
• Add move validation next
• Then implement win detection
• Finally add game state management
• Consider using enums for player marks and game states

Reference Walkthrough

Reference implementations in C#, TypeScript, and Python live at tddbuddy-reference-katas/tic-tac-toe. This is an F2 (light builder) kata: one primary entity (Board), a pair of value enums (Cell, Outcome), three domain exceptions for invalid moves, and one small test-folder BoardBuilder (10–30 lines) whose chained .withXAt(r,c) and .withOAt(r,c) calls read as a direct literal of the board state under test.

Scope note — pure domain only. The reference covers placing marks, turn tracking, win/draw detection, and invalid-move rejection. Rendering (ASCII board, HTML view), a CLI loop, a computer opponent, move history/undo, and n-in-a-row generalisation are all out of scope and listed as stretch goals in the repo README. Those responsibilities introduce collaborators (renderers, input parsers, strategy interfaces) — which is F3 territory, not F2.

• C# (.NET 8, xUnit, FluentAssertions 6.12.0) — walkthrough
• TypeScript (Node 20, Vitest 1.6, TS 5 strict) — walkthrough
• Python (3.11, pytest) — walkthrough

This kata ships in Agent Full-Bake mode at middle gear, the F2 (light builder) tier. See the repo’s Gears section for why middle gear is the deliberate choice.