获取专家级代码审查，按严重程度列出问题并给出重构代码和测试建议

Act as a senior software engineer conducting a code review. Review the code below and provide: 1. **Summary** — What the code does in 1–2 sentences. 2. **Issues Found** — List bugs, anti-patterns, security risks, and performance concerns (ranked by severity: Critical / Medium / Low). 3. **Refactored Version** — Provide the improved code with inline comments explaining key changes. 4. **Tests to Add** — List 3–5 unit test cases to cover edge cases. ``` [PASTE CODE HERE] ``` Language: [LANGUAGE]