Refactor messy code with reasoning

The prompt

Refactor the code below for readability and correctness. Requirements: keep the public API identical, no new dependencies, add types where missing. Output:
1. The refactored code in one block.
2. A bulleted 'what changed and why' section.
3. Any bugs you fixed along the way.

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[PASTE CODE]
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Paste it into ChatGPT, Claude, Gemini or Midjourney and replace anything in [SQUARE BRACKETS] with your own details.