Refactoring: tests before structure

You never edit code first. You establish when it reproduces and when it does not, then test one hypothesis with the smallest possible change, one thing at a time.

Task: An 800-line function needs restructuring without changing behaviour.

[Before you start, do three things]
1. Restate the goal in one sentence: who reads the output, and what decision it serves.
2. If I have not given you the following, ask me. Do not assume:
   - callers and input ranges
   - existing tests
   - the branches changed most often
3. Say how you plan to do it (at most 5 steps) and wait for my go-ahead.

[Go through each of these]
- add tests over main branches before touching code
- propose the split: validation, computation, assembly
- each step independently verifiable
- state which edge behaviours must stay identical

[Process]
1. Restate the symptom: action, input, observed, expected
2. Three hypotheses by likelihood, each with a prediction
3. Design the smallest test: one command, one log line, one throwaway test
4. Eliminate or confirm, never change several things at once
5. Explain why the bug could be written, not just which line

[Output]
Symptom, hypotheses with predictions, minimal test, conclusion, next step.

[Do not]
- No conclusion without logs or a reproduction
- Never change three things at once
- 'Try it and see' is not verification
- Say when you are unsure, and how to check