Headway·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

May 2026Remote

Summary

Headway software engineer interview with a debugging scenario built around a log parsing class. Three connected parts, all feeding into each other, which was a bit stressful once I realized fixing part one was the key to everything downstream.

Questions Asked (1)

You're given a LogEntry class where the constructor stores a timestamp token as a string. The downstream speed calculation is broken because of this. Find the bug, fix it, and write a small test to confirm the time-delta math works.

Root Cause AnalysisAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

I actually spotted the type issue pretty fast, tokens[0] staying as a string means any arithmetic on timestamps just blows up.

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AI HintsAI Generated

Suggested Approach

Start by tracing the data flow from LogEntry construction to the speed calculation, focusing on how the timestamp string is parsed and used in time-delta math. Identify the bug (likely a string-to-time conversion issue or incorrect delta computation), fix it by ensuring proper time representation, and write a unit test that verifies the delta calculation with known timestamps.

Pro tip: When fixing time-related bugs, always consider timezone and precision issues; using epoch milliseconds or a dedicated time library can prevent subtle errors. Also, write the test first to reproduce the bug, then fix it—this demonstrates a disciplined debugging approach.

1. Reproduce and Locate the Bug

Trace the code path from LogEntry constructor to the speed calculation. Identify where the timestamp string is used in arithmetic and confirm the incorrect behavior with a sample input.

2. Analyze the Root Cause

Determine why the timestamp string causes incorrect time-delta math. Common issues include parsing the string as a number, ignoring timezone, or using string subtraction instead of converting to a numeric time representation.

3. Implement the Fix

Modify the code to parse the timestamp string into a proper time type (e.g., epoch milliseconds or a DateTime object) before computing deltas. Ensure the fix is minimal and doesn't introduce new dependencies unnecessarily.

4. Write a Unit Test

Create a test that constructs LogEntry objects with known timestamps, computes the speed, and asserts the expected delta. Include edge cases like identical timestamps or large gaps.

5. Verify and Discuss Trade-offs

Run the test to confirm the fix. Discuss alternative approaches (e.g., using a time library) and their trade-offs in terms of precision, performance, and maintainability.

Key Points to Mention

Importance of converting string timestamps to a numeric time representation (e.g., epoch milliseconds) before arithmetic.
Time zone and daylight saving time considerations when parsing timestamps.
Precision loss when using floating-point numbers for time deltas; prefer integer milliseconds.
Writing a focused unit test that isolates the time-delta calculation and covers edge cases.
The value of reproducing the bug first to ensure the fix addresses the root cause.
Trade-offs between using built-in date/time libraries versus manual parsing.

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