Special moves (castling, en passant) in LLD - Architecture Diagram

System Overview - Special moves (castling, en passant)

This system manages the special chess moves: castling and en passant. It ensures these moves follow chess rules and updates the game state accordingly.

Key requirements include validating move conditions, updating board state, and notifying players of move results.

Architecture Diagram

User
  |
  v
Move Validator Service
  |
  v
Game State Manager
  |      |
  v      v
Castling Handler  En Passant Handler
  |                |
  v                v
Board State Database
  |
  v
Notification Service

Components

User

user

Player who initiates moves

Move Validator Service

service

Checks if moves are legal including special moves

Game State Manager

service

Maintains current game state and coordinates special move handlers

Castling Handler

service

Validates and executes castling moves

En Passant Handler

service

Validates and executes en passant moves

Board State Database

database

Stores current positions of pieces and move history

Notification Service

service

Sends move results and updates to players

Request Flow - 9 Hops

User → Move Validator Service

Move Validator Service → Game State Manager

Game State Manager → Castling Handler

Game State Manager → En Passant Handler

Castling Handler → Game State Manager

En Passant Handler → Game State Manager

Game State Manager → Board State Database

Game State Manager → Notification Service

Notification Service → User

Failure Scenario

Component Fails:Board State Database

Impact:Game state cannot be updated or retrieved, causing move validation and game progress to halt

Mitigation:Use database replication and caching to allow reads from replicas; queue writes for retry; notify users of temporary issues

Architecture Quiz - 3 Questions

Which component first checks if a move is a special move like castling or en passant?

A Game State Manager

B Move Validator Service

C Castling Handler

D Board State Database

Design Principle

This design separates concerns by using dedicated handlers for special moves, ensuring clear validation and execution logic. It uses a central Game State Manager to coordinate and maintain consistency, while the database stores authoritative game state. Notifications keep players informed, supporting a responsive user experience.

Practice

1. Which of the following conditions must be true for castling to be allowed in a chess game?

easy

A. The king is currently in check and moves two squares towards the rook.

B. Neither the king nor the rook involved has moved before, and no pieces are between them.

C. The rook has moved once, but the king has not moved.

D. The king moves diagonally two squares towards the rook.

Solution

1. Step 1: Understand castling rules

   Castling requires that neither the king nor the rook involved has moved before, and the squares between them are empty.

2. Step 2: Check king safety conditions

   The king cannot be in check, nor can it pass through or land on a square under attack during castling.

3. Final Answer:

   Neither the king nor the rook involved has moved before, and no pieces are between them. -> Option B

4. Quick Check:

   Castling conditions = Neither the king nor the rook involved has moved before, and no pieces are between them. [OK]

Hint: Castling needs unmoved king and rook with clear path [OK]

Common Mistakes:

• Allowing castling when king is in check
• Ignoring if rook has moved
• Allowing king to move diagonally during castling

2. Which code snippet correctly checks if an en passant move is possible in a chess game for an opponent pawn landing to the right?

easy

A. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (1, 0): allow_en_passant()

B. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (0, -1): allow_en_passant()

C. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (0, 1): allow_en_passant()

D. if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (-1, 0): allow_en_passant()

Solution

1. Step 1: Understand en passant position logic

   En passant capture happens when an opponent's pawn moves two squares forward and lands beside your pawn horizontally.

2. Step 2: Identify correct position offset

   The opponent pawn must be exactly one square horizontally adjacent (x+1 or x-1), so position + (1, 0) is correct for right side.

3. Final Answer:

   if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (1, 0): allow_en_passant() -> Option A

4. Quick Check:

   En passant horizontal check = if pawn.just_moved_two_squares and opponent_pawn.position == pawn.position + (1, 0): allow_en_passant() [OK]

Hint: En passant checks horizontal adjacency after two-step pawn move [OK]

Common Mistakes:

• Checking vertical instead of horizontal adjacency
• Using wrong coordinate offsets
• Ignoring the two-square move condition

3. Given this simplified code snippet for castling validation, what will be the output if the king has moved before?

def can_castle(king_moved, rook_moved, path_clear):
    if king_moved or rook_moved:
        return False
    if not path_clear:
        return False
    return True

print(can_castle(True, False, True))

medium

A. None

B. True

C. False

D. Error

Solution

1. Step 1: Analyze input parameters

   king_moved is True, rook_moved is False, path_clear is True.

2. Step 2: Follow function logic

   Since king_moved is True, the first if condition triggers and returns False immediately.

3. Final Answer:

   False -> Option C

4. Quick Check:

   King moved disables castling = False [OK]

Hint: If king moved, castling returns False immediately [OK]

Common Mistakes:

• Ignoring king_moved condition
• Assuming path_clear overrides king_moved
• Expecting True despite king having moved

4. Identify the bug in this en passant validation code snippet:

def can_en_passant(pawn_pos, opponent_pawn_pos, last_move):
    if last_move == 'two_squares_forward' and abs(pawn_pos[0] - opponent_pawn_pos[0]) == 1:
        return True
    return False

# Example call
print(can_en_passant((4,4), (5,4), 'two_squares_forward'))

medium

A. The function should return False when pawns are adjacent.

B. The function incorrectly compares x-coordinates instead of y-coordinates.

C. The last_move parameter should be a boolean, not a string.

D. The function does not check if pawns are on the same rank (y-coordinate).

Solution

1. Step 1: Understand en passant position requirements

   En passant requires pawns to be on the same rank (same y-coordinate) and adjacent files (x-coordinates differ by 1).

2. Step 2: Analyze code logic

   The code checks x-coordinate difference but does not verify if y-coordinates are equal, missing a key condition.

3. Final Answer:

   The function does not check if pawns are on the same rank (y-coordinate). -> Option D

4. Quick Check:

   Missing same rank check = The function does not check if pawns are on the same rank (y-coordinate). [OK]

Hint: En passant needs same rank check besides adjacency [OK]

Common Mistakes:

• Ignoring y-coordinate equality
• Assuming x difference alone suffices
• Misusing last_move parameter type

5. You are designing a chess game system that supports castling and en passant. Which design approach best ensures correct validation of these special moves while keeping the system scalable?

hard

A. Track each piece's move history and board state; validate special moves by checking move history and current board conditions.

B. Only check the current board state without tracking move history, assuming special moves are rare.

C. Allow special moves without validation to simplify the system and fix errors later.

D. Hardcode special move rules without tracking piece movement or board state.

Solution

1. Step 1: Understand requirements for special moves

   Castling and en passant depend on move history (e.g., whether king or rook moved, or if pawn moved two squares last turn) and current board state.

2. Step 2: Evaluate design options

   Tracking move history and board state allows accurate validation and supports scalability as game complexity grows.

3. Final Answer:

   Track each piece's move history and board state; validate special moves by checking move history and current board conditions. -> Option A

4. Quick Check:

   Move history + board state = correct scalable validation [OK]

Hint: Track moves and board state for reliable special move validation [OK]

Common Mistakes:

• Ignoring move history for special moves
• Hardcoding rules without flexibility
• Skipping validation to simplify design