Sesame AI·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

Sesame AI asked me to implement a ring buffer from scratch, which sounds like a straightforward systems question until you're actually in the middle of it trying to remember which pointer tracks what. Solid technical screen, nothing behavioral, just code and follow-ups.

Questions Asked (1)

Implement a circular buffer with a fixed byte capacity. It should support writing bytes (returning how many were written), reading up to N bytes from the front, and reporting how many bytes are currently available. Use head, tail, and size pointers with modulo arithmetic for wraparound, handle edge cases like a full buffer and reading past what's available, and briefly discuss thread safety.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

I got the basic structure down pretty quickly, head/tail/size, modulo on writes and reads.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints (fixed byte capacity, single-threaded vs multi-threaded). Then design the circular buffer using head, tail, and size with modulo arithmetic, and walk through the implementation of write, read, and available methods. Finally, discuss edge cases and thread safety considerations.

Pro tip: Emphasize that using a separate size variable simplifies distinguishing between full and empty states, avoiding the need to sacrifice a slot. Also, mention that for thread safety, a lock-free approach using atomics can be used if only one producer and one consumer, otherwise a mutex is simpler.

1. Clarify requirements and constraints

Ask about expected usage: single-threaded or multi-threaded? What are the performance requirements? Should reads/writes be blocking or non-blocking? This sets the context for design decisions.

2. Design the data structure

Define a class with a fixed-size byte array, head index (read position), tail index (write position), and size (number of bytes currently stored). Use modulo arithmetic for wraparound.

3. Implement core operations

Write: copy bytes into buffer starting at tail, update tail and size, return number written (may be less if buffer full). Read: copy up to N bytes from head, update head and size, return bytes read. Available: return size.

4. Handle edge cases

Address full buffer (write returns 0 or partial), empty buffer (read returns 0), reading more than available (return only available), and wraparound when head/tail reach end of array.

5. Discuss thread safety

If multi-threaded, mention that without synchronization, race conditions occur. Options: mutex for simplicity, or lock-free with atomics for single-producer single-consumer (SPSC) using acquire/release semantics.

Key Points to Mention

Use of head, tail, and size variables to track buffer state and avoid ambiguity between full and empty.
Modulo arithmetic for index wraparound: (index + 1) % capacity.
Write operation should handle partial writes when buffer is full, returning the number of bytes actually written.
Read operation should return only the number of bytes available, up to N.
Thread safety: mutex vs lock-free (SPSC) with atomics and memory ordering.
Edge cases: full buffer, empty buffer, reading/writing zero bytes, and wraparound.

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