Extend and optimize an existing CLI tool

Q: Extend and optimize an existing CLI tool

This question evaluates a candidate's ability to analyze an existing CLI codebase, perform gap analysis versus requirements, implement missing features, add tests, and optimize performance while preserving maintainability.

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Question

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You are given:

A short requirements doc for a command-line (CLI) tool that performs a set of concrete tasks (subcommands, flags, inputs/outputs).
An existing codebase that partially implements the tool.

Tasks

Understand the current implementation
- Read the codebase and explain the current architecture (modules, data flow, error handling, parsing of arguments, I/O boundaries).
Gap analysis vs. requirements
- Identify which required features are missing or incorrectly implemented.
- Prioritize what to fix/implement first.
Implement missing features
- Add the missing functionality while keeping the code maintainable and consistent with existing style.
- Add/adjust tests (unit and/or integration) to validate the behavior.
Optimization and quality improvements
- Point out performance bottlenecks or unnecessary work.
- Propose and implement at least one improvement (e.g., avoid repeated parsing, reduce I/O, simplify hot paths), without breaking correctness.

What to discuss during the interview

Trade-offs you make while changing an existing codebase.
How you prevent regressions (tests, logging, incremental refactors).
Edge cases (invalid arguments, empty input, large input, error propagation).

Extend and optimize an existing CLI tool

Quick Overview

Tasks

What to discuss during the interview

Solution

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