Chess Game - Learning Module

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Requirements Analysis for Chess Game

The Chess Game LLD Challenge

The Chess Game is one of the most fascinating and comprehensive Low-Level Design challenges you will encounter. Unlike simpler systems where the core logic is straightforward, chess presents a unique combination of complex rule enforcement, state management across multiple dimensions, and strategic interaction patterns that tests your ability to create clean, maintainable, and extensible object-oriented designs.

Chess has been played for over a millennium, and its rules have been refined to mathematical precision. Every piece has distinct movement capabilities. Special moves like castling and en passant add conditional complexity. The concept of 'check' and 'checkmate' requires understanding future game states. All this must be modeled in a system that remains understandable, testable, and open for extension (perhaps adding new game variants, AI opponents, or online multiplayer capabilities).

What You Will Master

By the end of this comprehensive case study, you will have designed a complete chess game system from the ground up. You will master entity identification, relationship modeling, move validation strategies, game state management with undo/redo support, and the application of Strategy, Command, and Memento patterns in a cohesive, production-ready design.

Understanding the Problem Domain

Before writing any code or drawing any diagrams, a skilled designer must develop deep domain understanding. Chess is a domain with centuries of established terminology, rules, and conventions. Mastering this vocabulary is essential for creating a design that speaks the language of the domain—a core principle of Domain-Driven Design.

The Game of Chess: A Primer

Chess is a two-player strategy board game played on an 8×8 grid of alternating light and dark squares. Each player controls 16 pieces at the start: one King, one Queen, two Rooks, two Bishops, two Knights, and eight Pawns. The objective is to checkmate the opponent's King—placing it under inescapable threat of capture.

Key Domain Concepts:

Essential Chess Terminology

•Board — An 8×8 grid where each square is identified by file (a-h) and rank (1-8), forming coordinates like 'e4' or 'a1'.
•Piece — One of six types (King, Queen, Rook, Bishop, Knight, Pawn), each with unique movement rules and belonging to either White or Black.
•Move — The action of relocating a piece from one square to another, potentially capturing an opponent's piece.
•Capture — When a piece moves to a square occupied by an opponent's piece, removing it from the game.
•Check — A King is under direct attack and must be defended on the next move.
•Checkmate — A King is in check with no legal moves to escape—the game ends in victory for the attacking player.
•Stalemate — The player to move has no legal moves but is not in check—the game is a draw.
•Castling — A special move involving the King and a Rook, executed under specific conditions for defensive positioning.
•En Passant — A special pawn capture available only immediately after an opponent's pawn advances two squares past a capturable position.
•Promotion — When a pawn reaches the opposite end of the board, it must be promoted to Queen, Rook, Bishop, or Knight.

Domain Immersion Accelerates Design

Spending time truly understanding chess—perhaps playing a few games or watching expert analysis—will make your design work significantly faster and more accurate. Domain experts (chess masters) have precise mental models; your system should reflect their vocabulary naturally.

Functional Requirements

Functional requirements describe what the system must do. For a chess game, these requirements span game setup, move execution, rule enforcement, game termination, and optional features like move history. Let us enumerate requirements with precision.

Core Game Mechanics:

FR-1: Game Initialization

•FR-1.1: The system shall initialize a standard 8×8 chessboard with all 32 pieces in their starting positions.
•FR-1.2: White pieces shall occupy ranks 1 and 2; Black pieces shall occupy ranks 7 and 8.
•FR-1.3: The system shall designate White to move first.
•FR-1.4: The system shall create two players and assign them to White and Black sides respectively.
•FR-1.5: Game clocks (optional) shall be initialized if timed play is enabled.

FR-2: Piece Movement

•FR-2.1: Each piece type shall move according to standard chess rules: King (one square any direction), Queen (any number of squares in any direction), Rook (any number of squares horizontally or vertically), Bishop (any number of squares diagonally), Knight (L-shape: 2+1 squares), Pawn (forward one square, or two from starting position, captures diagonally).
•FR-2.2: A piece shall not move to a square occupied by a friendly piece.
•FR-2.3: A piece (except Knight) shall not jump over other pieces.
•FR-2.4: Capturing an opponent's piece shall remove it from the board permanently.
•FR-2.5: After each move, control shall pass to the opponent (alternating turns).

FR-3: Special Moves

•FR-3.1: Castling: The system shall allow kingside castling (O-O) when: neither the King nor the kingside Rook has moved, no pieces stand between them, the King is not in check, and the King does not pass through or land on an attacked square.
•FR-3.2: Castling: The system shall allow queenside castling (O-O-O) under analogous conditions for the queenside Rook.
•FR-3.3: En Passant: The system shall allow a pawn to capture an opponent's pawn that has just advanced two squares, as if that pawn had only moved one square. This capture is only valid on the immediately following move.
•FR-3.4: Pawn Promotion: When a pawn reaches the opposite rank (rank 8 for White, rank 1 for Black), the player must promote it to a Queen, Rook, Bishop, or Knight.

FR-4: Check and Checkmate

•FR-4.1: The system shall detect when a King is in check (under attack by an opponent's piece).
•FR-4.2: When in check, the system shall only allow moves that remove the King from check (move King, block attack, or capture attacking piece).
•FR-4.3: The system shall detect checkmate: the King is in check and no legal move exists to escape.
•FR-4.4: Upon checkmate, the game shall end immediately with victory for the attacking player.

FR-5: Game Termination Conditions

•FR-5.1: Checkmate — The game ends with a decisive winner.
•FR-5.2: Stalemate — The current player has no legal moves and is not in check; the game is drawn.
•FR-5.3: Resignation — A player may resign, conceding victory to the opponent.
•FR-5.4: Draw Agreement — Both players may agree to a draw at any time.
•FR-5.5: Timeout — In timed games, running out of time loses the game (provided the opponent has sufficient material to checkmate).
•FR-5.6: Insufficient Material — The game is drawn if neither side has sufficient material to deliver checkmate (e.g., King vs. King).
•FR-5.7: Threefold Repetition — The game may be claimed as a draw if the same position occurs three times with the same player to move.
•FR-5.8: Fifty-Move Rule — The game may be claimed as a draw if fifty consecutive moves have been made without a pawn move or capture.

FR-6: Move History and Game Record

•FR-6.1: The system shall maintain a complete history of all moves played in the game.
•FR-6.2: The system shall support undo functionality, allowing moves to be taken back (in casual play).
•FR-6.3: The system shall support redo functionality to replay undone moves.
•FR-6.4: The system shall be capable of exporting the game in standard notation (PGN - Portable Game Notation).

Non-Functional Requirements

Non-functional requirements describe how the system should perform or qualities it must exhibit. These often matter more for long-term success than functional requirements, as they govern maintainability, extensibility, and user experience.

Non-Functional Requirements for Chess Game System
Category	Requirement ID	Description
Performance	NFR-1.1	Move validation shall complete within 50ms even for complex positions.
Performance	NFR-1.2	Check and checkmate detection shall complete within 100ms.
Extensibility	NFR-2.1	Adding new piece types (e.g., for chess variants like Capablanca Chess) shall require creating a new class, not modifying existing code.
Extensibility	NFR-2.2	Adding new game variants (Chess960, King of the Hill) shall be supported through configuration or inheritance, not core logic changes.
Maintainability	NFR-3.1	Move validation logic shall be unit-testable in isolation.
Maintainability	NFR-3.2	The system shall have clear separation between game rules, UI, and persistence.
Usability	NFR-4.1	Invalid moves shall produce meaningful error messages (e.g., 'King is in check; move your King or block the threat').
Correctness	NFR-5.1	All rules shall be FIDE-compliant (Fédération Internationale des Échecs).
Testability	NFR-6.1	The system shall support loading arbitrary board positions for testing edge cases.
Portability	NFR-7.1	Game logic shall be independent of any specific UI framework.

Extensibility is Critical

Chess variants (Chess960, Bughouse, Atomic Chess) are highly popular. A rigid design that hardcodes standard chess rules will require painful rewrites to support variants. Design for extensibility from day one—this will profoundly influence your pattern choices.

Constraints and Assumptions

Every design operates within constraints. Identifying these early prevents wasted effort and guides decision-making throughout the design process.

Technical Constraints

•The system shall be designed as a standalone library, decoupled from any UI framework.
•Persistence is out of scope for the core design (games are in-memory only).
•Multiplayer networking is out of scope; this is a two-player local game.
•AI opponent logic is out of scope (though the design should not preclude adding AI later).
•The system shall be thread-safe for potential concurrent UI updates.

Assumptions

•Standard chess rules (FIDE) are the default; variants may be added later.
•Both players are human users (no AI by default).
•Move input will be provided programmatically (e.g., source and destination squares).
•The UI layer will handle rendering and user input separately.
•Time controls, if implemented, will be handled by a separate component.

Clarifying Questions for Requirements

In a real design session or interview, asking clarifying questions demonstrates thoroughness and prevents scope creep later. Here are questions a thoughtful designer would ask—and the answers we'll assume for this case study:

Clarifying Questions and Answers
Question	Answer for This Design
Do we need to support chess variants?	Yes, the design should be extensible for variants, but we implement standard chess first.
Is timed play (chess clocks) required?	Optional; structure should allow adding time controls later.
Do we need to support saving/loading games?	Out of scope for core design, but undo/redo and move history are in scope.
How will users input moves?	Programmatically, via source and destination coordinates (e.g., e2 to e4).
Is online multiplayer in scope?	No, this is a local two-player game.
Do we need to record games in any standard format?	PGN export capability is desirable.
Should the system support 'takebacks' (undoing moves)?	Yes, for casual play; the Memento pattern will support this.
Is AI opponent in scope?	No, but the design should allow integrating AI players later.

Scope Management in Practice

Notice how we explicitly defer features like AI, networking, and persistence while still designing to accommodate them. This is a hallmark of senior engineering: making scope decisions explicit rather than implicit, and designing for known future directions.

Use Case Analysis

Use cases describe actor interactions with the system. Identifying the primary actors and their goals helps ensure our design addresses all necessary workflows.

Primary Actors

•Player (White) — The human player controlling the white pieces. Initiates the game and moves first.
•Player (Black) — The human player controlling the black pieces. Responds to White's moves.
•System (Game Engine) — Enforces rules, validates moves, detects check/checkmate, maintains game state.

Core Use Cases
Use Case	Actor	Description	Postcondition
Start New Game	Player (either)	Initializes a fresh game with standard setup	Board is set up; White to move; game status is ACTIVE
Make a Move	Current Player	Selects a piece and destination square	If valid: piece moves, turn switches. If invalid: error returned, state unchanged.
Castle	Current Player	Executes kingside or queenside castling	King and Rook swap positions per castling rules
Capture a Piece	Current Player	Moves to square occupied by opponent's piece	Opponent's piece is removed from board
Promote a Pawn	Current Player	Pawn reaches final rank; selects promotion piece	Pawn replaced with selected piece type
Undo Last Move	Either Player	Reverts the game to the previous state	Board and game state restored to before the last move
Redo Move	Either Player	Re-applies an undone move	Board and state move forward to the next recorded state
Resign	Current Player	Concedes the game	Game ends; opponent wins
Offer Draw	Current Player	Proposes a draw to the opponent	If accepted: game drawn. If declined: play continues.
Query Legal Moves	Any Player	Asks which moves are valid for a piece	System returns list of legal destination squares

Critical Use Case: Make a Move (Detailed)

The most frequent and complex use case is making a move. Its flow involves multiple validations:

Actor: Current player (the player whose turn it is)
Precondition: Game is in ACTIVE status; it is this player's turn
Main Flow:
- Player selects a source square containing their own piece
- Player specifies a destination square
- System validates: Is the piece on the source square? Does it belong to the current player? Is the move legal per piece movement rules? Does the move leave or place the King in check? Is a special move (castling, en passant, promotion) being attempted?
- If valid: Execute the move, update board, switch turns, check for check/checkmate/stalemate
- If invalid: Return descriptive error, do not change state
Postcondition: Board state updated, turn passed (or game ended)
Exception Flows: Invalid piece selection, illegal move, player in check must address threat

Requirement Prioritization

Not all requirements are equally critical. Prioritizing helps focus design effort on the most impactful features first—a practice that mirrors agile development principles.

Requirements by Priority (MoSCoW Method)
Priority	Requirements	Rationale
Must Have	Game initialization, piece movement, check/checkmate detection, turn management, stalemate detection	Core gameplay is non-negotiable
Must Have	All special moves (castling, en passant, promotion)	Standard chess is incomplete without these
Should Have	Move history recording, undo/redo functionality	Expected in modern chess apps; enables learning and experimentation
Should Have	Threefold repetition and fifty-move rule detection	Required for tournament-compliant chess
Could Have	PGN export, insufficient material detection	Nice-to-have features for enthusiasts
Won't Have (this phase)	AI opponent, timed play, online multiplayer, persistence	Out of scope; designed for future extensibility

Design for Must-Haves, Architect for All

Your class design should focus on Must-Have features. But your architecture (interfaces, abstractions, patterns) should accommodate all tiers. For example, even though AI is 'Won't Have' now, the Player abstraction should allow an AI implementation seamlessly.

Summary: Requirements Complete

We have completed a thorough requirements analysis for our Chess Game system. This foundational work will guide every design decision that follows.

Key Takeaways

•Domain understanding precedes design — Mastering chess terminology (check, checkmate, castling, en passant) ensures our classes speak the domain language.
•Functional requirements define what — Six requirement groups cover initialization, movement, special moves, check/checkmate, game termination, and move history.
•Non-functional requirements define how — Performance, extensibility, maintainability, and testability guide our architectural choices.
•Constraints bound the solution space — Understanding what's in and out of scope prevents over-engineering.
•Use cases reveal interactions — The 'Make a Move' use case is the central workflow our design must excel at.
•Prioritization focuses effort — Must-have features form the core; should-have and could-have features guide extensibility.

What's Next:

With requirements firmly established, we move to identifying core entities. We will extract the essential classes from our requirements: Board, Piece, Player, Move, Game, and more. We'll distinguish between entities and value objects, apply the noun extraction technique, and create a candidate class list ready for relationship modeling.

Requirements Phase Complete

You now have a comprehensive requirements specification for a chess game system. Every design decision in subsequent pages will trace back to these requirements. This traceability is what separates professional system design from ad-hoc development.