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Openai·Software Engineer·Onsite - Coding / Algorithms·Senior

Senior
Jun 2026

Summary

OpenAI Research Engineer coding round. One meaty OOP design problem that started reasonable and then kept growing with follow-ups until I was basically writing a small game engine.

Questions Asked (3)

Q1

Design classes for a monster battle simulation. You have two teams of monsters that fight in order. Return the winning team and a log of the battle.

System DesignData ModelingAlgorithms & Data Structures
Author's notes

The core design part was fine, Team, Monster, Outcome classes aren't that hard to sketch out.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining core classes like Monster, Team, and Battle. Then design the battle logic to simulate turn-based fights in order, tracking health and logging events. Finally, discuss extensibility and edge cases.

Pro tip: Emphasize separation of concerns: keep battle logic separate from monster data, and use interfaces for abilities to allow easy extension. This shows you think about maintainability and scalability.

1. Clarify Requirements

Ask about monster attributes, turn order, win conditions, and log format. Confirm if monsters can have special abilities or if it's a simple attack exchange.

2. Define Core Classes

Design Monster with health, attack, and possibly defense. Team holds a list of monsters. Battle orchestrates the fight and produces a log.

3. Design Battle Logic

Simulate turns: each monster from team A attacks corresponding monster in team B in order, then vice versa. Track health and remove defeated monsters. Continue until one team has no monsters left.

4. Implement Logging

Record each attack, damage dealt, remaining health, and monster defeats. Ensure log is readable and includes turn order.

5. Discuss Extensibility and Edge Cases

Consider adding abilities, different attack strategies, or multiple monster types. Handle ties, simultaneous defeats, and empty teams.

Key Points to Mention

  • Object-oriented design principles: encapsulation, single responsibility, and open/closed principle.
  • Turn-based simulation with ordered attacks and health tracking.
  • Use of data structures like queues or lists to manage turn order.
  • Logging mechanism to capture battle events for debugging or replay.
  • Edge cases: empty teams, ties, monsters with zero health, and simultaneous attacks.
  • Extensibility: interfaces for abilities, strategy pattern for attack selection, and factory for monster creation.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q2

Extend the simulation to support type weaknesses and resistances between monsters.

System DesignTechnical Trade-offs
Author's notes

Felt like a pretty natural extension once the base was working.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints of the simulation, then design a flexible type effectiveness system that can be easily extended. Discuss trade-offs between different data representations and algorithms, and outline how to integrate the system with minimal disruption to existing code.

Pro tip: Emphasize the importance of separating data from logic: store type matchups in a configuration file or database so that game designers can update them without code changes. This demonstrates foresight and maintainability.

1. Clarify Requirements

Ask questions to understand the scope: How many types? Are matchups symmetric? Should effectiveness multipliers be configurable? What are performance constraints?

2. Design Data Model

Choose a representation for type matchups, such as a 2D matrix or a map of type pairs to multipliers. Consider using an enum for types and a lookup table for effectiveness.

3. Integrate with Combat Logic

Modify the damage calculation to incorporate type effectiveness. Ensure the system is decoupled from specific monsters and types for extensibility.

4. Handle Edge Cases and Extensibility

Address cases like dual-type monsters, immunity (0x damage), and future additions of new types. Discuss how to update the system without breaking existing code.

5. Discuss Trade-offs and Testing

Compare performance of different data structures (e.g., matrix vs. hash map). Outline a testing strategy to verify correctness of matchups and damage calculations.

Key Points to Mention

  • Use of a type effectiveness matrix or map for O(1) lookup
  • Separation of data (matchup table) from logic (damage calculation)
  • Support for dual-type monsters and multiplicative effectiveness
  • Extensibility to add new types or change multipliers without code changes
  • Performance considerations for large numbers of types or monsters
  • Testing strategies including unit tests for matchups and integration tests for combat

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.

Q3

Now give each monster multiple attacks, and each monster should automatically pick the attack that deals the most damage to its current opponent.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

This one tripped me up a bit.

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AI HintsAI Generated

Suggested Approach

First, clarify the requirements: how attacks are defined, how damage is calculated, and what 'most damage' means (e.g., considering resistances). Then, design a data structure to store multiple attacks per monster and implement a selection algorithm that evaluates each attack against the current opponent, picking the one with maximum damage. Finally, discuss trade-offs like time complexity and potential optimizations.

Pro tip: Mention that you would encapsulate the damage calculation and attack selection logic to keep it maintainable and testable, and consider edge cases like ties or no valid attacks.

1. Clarify requirements

Ask questions to understand the attack model: are attacks fixed damage or variable? Do they have types, cooldowns, or costs? How is 'most damage' determined (e.g., after applying resistances)?

2. Design data structures

Represent each monster with a list of attacks, where each attack has properties like damage, type, etc. Ensure the opponent's defensive attributes are accessible for damage calculation.

3. Implement damage calculation

Create a function that computes the effective damage of an attack against a given opponent, accounting for any modifiers such as resistances or vulnerabilities.

4. Select best attack

Iterate through the monster's attacks, compute effective damage for each, and select the attack with the highest damage. Handle ties (e.g., pick first or random) and cases with no attacks.

5. Optimize and discuss trade-offs

Analyze time complexity (O(n) per selection) and suggest optimizations if needed, such as caching or pre-sorting attacks if opponent defenses are static. Discuss maintainability and extensibility.

Key Points to Mention

  • Encapsulation of attack and damage logic for modularity
  • Time complexity of selecting the best attack (linear scan)
  • Handling edge cases: no attacks, ties, zero damage
  • Potential optimizations: caching, pre-computation, or using a priority queue if attacks change dynamically
  • Extensibility: adding new attack types or damage modifiers without major refactoring
  • Testing strategy: unit tests for damage calculation and selection logic

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.