Openai·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026Remote

Summary

Got a coding screen at OpenAI that was pretty much just one problem: build a reverse IPv4 iterator in Python. Straightforward on the surface but the borrow logic across octets tripped me up more than I expected.

Questions Asked (1)

Implement a reverse IPv4 iterator in Python. Given a starting IP string like '192.168.0.255', the iterator should yield every IPv4 address in decreasing order down to 0.0.0.0, using the standard init, iter, and next protocol with correct StopIteration semantics.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The parsing part was fine, split on dots, done.

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AI HintsAI Generated

Suggested Approach

Convert the starting IP to a 32-bit integer, then implement a class with __init__, __iter__, and __next__ that decrements the integer on each call to __next__, yielding the dotted-quad string representation. Stop when the integer goes below zero by raising StopIteration.

Pro tip: Mention that you can use the ipaddress module for robust conversion, but also show you understand the bitwise math behind it—this demonstrates both practical knowledge and depth.

1. Parse and convert the IP

Split the input string by '.' and convert each octet to an integer, then combine them into a single 32-bit integer using bit shifts. Validate the input format and range.

2. Design the iterator class

Define a class with __init__ that stores the current integer value, __iter__ that returns self, and __next__ that handles the decrement and termination logic.

3. Implement next with StopIteration

In __next__, if the current value is less than 0, raise StopIteration. Otherwise, convert the current integer to dotted-quad string, decrement the integer, and return the string.

4. Handle edge cases and validation

Consider invalid input strings, ensure the starting IP is within 0.0.0.0 to 255.255.255.255, and decide whether to include the starting IP in the iteration (typically yes).

5. Test and discuss trade-offs

Walk through an example, test boundary conditions like starting at 0.0.0.0, and mention alternative approaches (e.g., using ipaddress module) and their trade-offs.

Key Points to Mention

Conversion between dotted-quad string and 32-bit integer using bitwise operations
Iterator protocol: __iter__ returns self, __next__ raises StopIteration when exhausted
Efficiency: O(1) space and O(1) time per iteration
Edge cases: starting at 0.0.0.0, invalid input, and ensuring all addresses down to 0.0.0.0 are yielded
Alternative using Python's ipaddress module for clarity vs. manual bit manipulation for performance
Thread safety and reusability considerations (e.g., iterator is single-use)

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