Openai·Software Engineer·Technical Phone Screen·Senior

Senior

Apr 2026

Summary

Got a coding screen for a Software Engineer role at OpenAI and the question was about IP address iteration. Niche enough that I had to think carefully about the 32-bit integer representation before writing a single line.

Questions Asked (1)

Design a class that iterates over IPv4 addresses in order. Given a starting IP and either a count or an end IP, implement a next() method that returns the next address as a string and handles wrap-around at the 32-bit boundary without storing all addresses in memory.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

The memory constraint is what makes this interesting.

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AI HintsAI Generated

Suggested Approach

Represent the IP address as a 32-bit unsigned integer internally, and implement next() by incrementing this integer and converting back to dotted-decimal string. Handle wrap-around by using modulo 2^32 arithmetic, and track the number of addresses returned or compare against the end IP to know when to stop.

Pro tip: Mention that using a 32-bit integer avoids string parsing overhead and makes wrap-around trivial; also note that the class should be memory-efficient by not storing addresses, and discuss thread-safety if the iterator might be shared.

1. Clarify requirements and edge cases

Ask about input format (string vs integer), whether the range is inclusive, and how wrap-around should behave (e.g., from 255.255.255.255 to 0.0.0.0). Confirm that next() should return a string and that memory usage must be constant.

2. Choose internal representation

Decide to store the current IP as a 32-bit unsigned integer (e.g., uint32_t) to simplify arithmetic and avoid repeated string conversions. Explain how to convert between dotted-decimal string and integer.

3. Design the iterator logic

Implement next() by incrementing the integer with modulo 2^32 to handle wrap-around. Maintain a counter or compare against the end IP to determine when to stop, returning null or throwing an exception when exhausted.

4. Handle initialization and termination

In the constructor, parse the start IP and either the count or end IP. Compute the total number of addresses if needed, and set up the stopping condition. Ensure that if count is zero or start > end (without wrap), the iterator is immediately exhausted.

5. Discuss trade-offs and optimizations

Mention that integer arithmetic is O(1) per next() call and uses O(1) memory. Discuss potential overflow issues, thread-safety, and whether to precompute the end integer for faster comparison.

Key Points to Mention

Use a 32-bit unsigned integer for internal representation to simplify arithmetic and wrap-around.
Convert between dotted-decimal string and integer using bitwise operations (e.g., shifting and masking).
Implement wrap-around by incrementing with modulo 2^32 (or using unsigned integer overflow).
Track the number of addresses returned or compare against the end IP to know when to stop.
Ensure O(1) memory usage by not storing all addresses; only store current state and stopping condition.
Consider edge cases: start IP equal to end IP, count zero, and wrap-around across the 32-bit boundary.

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