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Hudson River Trading·Software Engineer·Onsite - System Design / Architecture·Intermediate

Intermediate
Apr 2026

Summary

Did a system design round at Hudson River Trading for a software engineering role. The problem was deceptively low-level: wrap a fixed-chunk stream API so callers can request arbitrary byte counts. Felt like a good signal into how they think about infrastructure-level code.

Questions Asked (1)

Q1

You have a low-level stream API that reads exactly 4096 bytes at a time. Design a wrapper `read(n)` that returns any number of bytes the caller requests, buffering leftover bytes between calls.

System DesignAPI & IntegrationsTechnical Trade-offs
Author's notes

This is the kind of question where the core idea clicks fast but the edge cases eat you alive.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements: the wrapper must handle arbitrary n, including n > 4096, and maintain an internal buffer of leftover bytes. Then describe a design with a buffer and a read method that first drains the buffer, then reads from the underlying stream in 4096-byte chunks, copying only what's needed and storing the rest. Finally, discuss edge cases like n=0, EOF, and thread safety.

Pro tip: Mention that you would use a circular buffer or a simple byte array with read/write indices to avoid unnecessary copying, and that you'd handle partial reads from the underlying stream by looping until you get 4096 bytes or EOF.

1. Clarify requirements and constraints

Ask about expected usage: is n always positive? Can n exceed 4096? Should the wrapper be thread-safe? What should happen at EOF? This shows you think about the contract.

2. Design the internal buffer

Propose a buffer (e.g., byte array of size 4096) with read and write pointers to track leftover bytes. Explain that you'll only read from the underlying stream when the buffer is empty.

3. Implement the read(n) logic

Describe the algorithm: first copy min(n, available) from buffer; if more needed, loop reading 4096-byte chunks from the underlying stream, copying directly to the caller's buffer until n bytes are satisfied or EOF. Store any excess in the internal buffer.

4. Handle edge cases and error conditions

Discuss n=0 (return 0), n<0 (throw exception), EOF (return -1 or 0 depending on API), and partial reads from the underlying stream (loop until full chunk or EOF).

5. Discuss performance and trade-offs

Mention that this design minimizes system calls by reading in large chunks, and that using a circular buffer can avoid array shifting. Also note thread-safety considerations if needed.

Key Points to Mention

  • Buffering leftover bytes to avoid data loss between calls
  • Handling n larger than the underlying read size (4096) by looping
  • Minimizing copies by reading directly into the caller's buffer when possible
  • Edge cases: n=0, n<0, EOF, and partial reads from underlying stream
  • Thread-safety and synchronization if the wrapper is used concurrently
  • Performance: reducing system calls by reading in large chunks

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