Tradedesk·Software Engineer·Onsite - Coding / Algorithms·Intermediate

Intermediate

Apr 2026

Summary

Tradedesk SWE interview had a file system coding problem that looked manageable on the surface but had enough edge cases to keep you honest.

Questions Asked (1)

Implement two functions: one that compresses a file (appends a '.COMPRESSED' suffix and halves its size, but only if the caller owns the file), and one that decompresses it (strips the suffix and doubles the size, but must check for filename collisions and whether there's enough capacity first).

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

The ownership check on compress was fine, that part clicked fast.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, especially around file ownership, atomicity, and error handling. Then outline a design that separates the compression/decompression logic from the file system operations, ensuring that all preconditions are checked before any modifications. Finally, discuss potential edge cases and trade-offs, such as partial failures and concurrency.

Pro tip: Emphasize the importance of atomic operations and proper error handling to prevent data corruption, and mention that you would use temporary files and atomic renames to ensure consistency.

1. Clarify Requirements and Constraints

Ask questions to understand the expected behavior: What does 'halves its size' mean? Is it exactly half? What about file ownership checks? Should operations be atomic? Are there concurrency concerns?

2. Design the Compression Function

Outline the steps: check if the caller owns the file, verify the file exists and is not already compressed, create a new file with '.COMPRESSED' suffix, write half the data (or apply a compression algorithm), and atomically replace the original if needed.

3. Design the Decompression Function

Outline the steps: check if the file has the '.COMPRESSED' suffix, verify the caller owns it, check for filename collision (the target filename without suffix), ensure there is enough disk capacity, then write the decompressed data and remove the compressed file.

4. Address Error Handling and Edge Cases

Discuss how to handle errors such as permission denied, file not found, insufficient disk space, and partial writes. Mention the use of temporary files and atomic operations to avoid corruption.

5. Discuss Trade-offs and Alternatives

Consider trade-offs between simplicity and robustness, e.g., using a real compression algorithm vs. simply halving the size, and the impact on performance and disk usage.

Key Points to Mention

File ownership check: use stat to get the owner and compare with the effective user ID.
Atomicity: use temporary files and rename to ensure operations are atomic and avoid partial writes.
Error handling: check for errors at each step and clean up temporary files on failure.
Concurrency: consider file locking or other mechanisms to prevent race conditions.
Disk space check: use statvfs or similar to ensure enough free space before decompression.
Filename collision: check if the target filename already exists and handle appropriately (e.g., return error or overwrite based on requirements).

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