Special moves (castling, en passant) in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Castling Conditions

Which of the following is NOT a valid condition for castling in chess?

A The king is currently in check.

B There are no pieces between the king and the rook.

C Neither the king nor the rook involved has moved before.

D The squares the king passes over are not under attack.

💡 Hint

Think about whether the king can castle while in check.

Castling is not allowed if the king is currently in check. The other conditions must be met for castling to be valid.

❓ Architecture

intermediate

Designing En Passant Detection

In designing a chess game system, which component is most responsible for detecting the possibility of an en passant move?

A The piece movement module that only moves pieces without validation.

B The user interface module that highlights possible moves.

C The game timer module that tracks player time.

D The move validation module that checks the last move and current board state.

💡 Hint

Consider which part knows about the last move and rules.

The move validation module must check if the last move was a two-square pawn advance and if an en passant capture is possible.

❓ scaling

advanced

Scaling a Chess Engine for Multiple Games

You are designing a server to handle thousands of simultaneous chess games including special moves like castling and en passant. Which approach best ensures consistent game state and move validation?

A Store game states in a distributed cache without locking mechanisms.

B Allow clients to validate moves locally and send updates without server checks.

C Use a centralized game state service with atomic transactions for each move validation and update.

D Process moves asynchronously without immediate validation to improve throughput.

💡 Hint

Think about consistency and correctness under load.

A centralized service with atomic transactions ensures moves are validated correctly and game state remains consistent, especially for special moves requiring strict rules.

❓ tradeoff

advanced

Tradeoffs in Representing Castling Rights

Which data structure choice best balances memory efficiency and clarity when tracking castling rights in a chess game system?

A Use four boolean flags indicating castling availability for each rook and king side.

B Use a single integer bitmask where each bit represents a castling right.

C Store castling rights as a string describing allowed moves (e.g., "KQkq").

D Maintain a list of moves that have occurred to infer castling rights dynamically.

💡 Hint

Consider compactness and ease of checking rights.

A bitmask uses minimal memory and allows fast bitwise operations to check castling rights, balancing efficiency and clarity.

❓ estimation

expert

Estimating Storage for Game History with Special Moves

Estimate the approximate storage size needed to store the full move history of 1 million chess games including special moves like castling and en passant, assuming each move is stored as a 4-byte record. Average moves per game is 80.

A Approximately 320 MB

B Approximately 80 MB

C Approximately 3.2 GB

D Approximately 800 MB

💡 Hint

Calculate total moves and multiply by bytes per move.

1,000,000 games * 80 moves/game = 80,000,000 moves. 80,000,000 moves * 4 bytes = 320,000,000 bytes = 320 MB.

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