Refactoring in small, verified steps

Goal

Improve the structure of existing code while guaranteeing, step by step, that its observable behaviour is unchanged.

Prerequisites

Tests that cover the behaviour of the code to be refactored. If there are none, write characterisation tests first: call the code with representative inputs and assert the current outputs.

Steps

1. Decide the target structure and the smell you are removing; write it in the commit message draft.
2. Apply one named refactoring at a time (extract function, rename, move, inline, introduce parameter object), using editor automation where available.
3. Run the tests after each step. If they fail, undo the step rather than debugging forward.
4. Commit after each meaningful group of steps with a message that says "refactor: …" and no behaviour claims.
5. Never mix a behaviour change into a refactoring commit; reviewers must be able to trust that a refactoring diff is behaviour-neutral.
6. Stop when the smell is gone; do not continue into speculative generality.

Expected result

A series of small commits, each green, each reviewable in minutes, that together produce the intended structure.

Limits and test basis

Tests only protect what they cover; performance and concurrency behaviour may change unnoticed. Large-scale restructuring across modules needs the same discipline plus a plan (see branch by abstraction or the strangler fig pattern). The step catalogue follows the cited reference.