Microsoft·Machine Learning Engineer·Technical Phone Screen·Senior

Senior

Jun 2026

Summary

Microsoft ML engineer interview that went deep on streaming systems. One question, very technical, felt more like a research discussion than a standard coding round.

Questions Asked (1)

You're receiving tokens one at a time from a streaming source (like an LLM decoder) along with one or more delimiter patterns. Build a stateful component that decides, for each new token, whether to emit it, hold it as a potential delimiter prefix, or stop emitting entirely once the delimiter is fully matched. It must never leak a partial delimiter that turns out to be real. Then discuss how you'd extend this to handle multiple delimiters.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

This one took me a minute to even parse what they were asking.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the streaming constraints and the need for a stateful buffer that tracks the longest suffix matching a delimiter prefix. Then present a solution using a trie or Aho-Corasick automaton for multiple delimiters, and discuss trade-offs like memory, latency, and handling overlapping patterns.

Pro tip: Emphasize that you must never emit a token until you're certain it's not part of a delimiter; this often means holding back a suffix of the buffer. Mention that real-world systems (e.g., LLM streaming APIs) often use a simple delimiter like '\n\n' but the same logic generalizes.

1. Clarify requirements and constraints

Ask about token granularity, delimiter set, whether delimiters can overlap, and if partial matches should be held indefinitely. Confirm that the component must be stateful and process tokens one at a time.

2. Design single-delimiter state machine

Maintain a buffer of recent tokens and a pointer to how many characters of the delimiter have matched. For each new token, append to buffer, check for full match (stop), partial match (hold), or no match (emit safe prefix).

3. Extend to multiple delimiters

Use a trie of delimiter patterns to track all possible partial matches simultaneously. Alternatively, use Aho-Corasick for efficient multi-pattern matching, updating state per character.

4. Handle edge cases and correctness

Address overlapping delimiters (e.g., 'ab' and 'abc'), delimiters that are prefixes of others, and the case where a partial match fails and buffered characters must be emitted. Prove no partial delimiter leaks.

5. Discuss trade-offs and optimizations

Compare trie vs. Aho-Corasick in terms of time/space complexity, and discuss buffering strategies (e.g., fixed-size buffer, streaming algorithms). Mention latency implications of holding tokens.

Key Points to Mention

Stateful buffer to hold potential delimiter prefixes
Trie or Aho-Corasick automaton for multiple delimiters
Correctness: never emit a partial delimiter that later completes
Handling overlapping delimiters and prefix relationships
Time and space complexity trade-offs
Real-world examples like LLM stop sequences or streaming parsers

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