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Safe Incremental Coding — the Legacy Net
Take untested code to a state where you can change it safely, distilled from Dave Farley's The Software Developers' Guidebook. The single measure of "good" throughout is ease of change: quality in code is your ability to change it safely — and for legacy code, that ability starts with a characterization test net built before any change.
Work incrementally on the area you actually need to touch, never as a big-bang rewrite. Stabilise the code you're about to change, change it, move on.
Two principles frame every step:
- Quality is your ability to change the code. Every step exists to make the code easier to change.
- Refactoring is _always_ behavior-preserving. If a change alters what the code does, it isn't refactoring — it's a behavior change needing its own test and its own step. Run the net (step 1) after every transformation to confirm behavior is unchanged.
Why this is a separate skill from tdd
Do not retrofit fine-grained, TDD-style unit tests to legacy code. The code isn't shaped for them yet; writing them now bakes in the bad structure. Approval/acceptance tests at a coarser boundary are the right net for now. This rule deliberately opposes tdd's Iron Law (no production code without a failing test first) — which is exactly why the two stay separate skills: tdd assumes code shaped for tests; legacy code must first be reshaped under a coarser net.
The five steps
Ordered — you can't safely simplify code until there's a net under it, and you can't see the structure until the clutter is gone.
1. Characterization (approval) tests — build the net
"Legacy code is code without tests." — Michael Feathers
Capture the code's current behavior and pin it, even if weird or buggy — you're documenting reality, not judging it.
- Drive the code with representative inputs and capture its output.
- If output is nondeterministic (timestamps, UUIDs, seeds) or side-effect-only, normalize/scrub the variable parts or introduce a minimal seam to capture it before pinning.
- An approval test records output on first run, then fails on any future run whose output differs — that difference proves a change was not behavior-preserving.
- Defend module/service boundaries with extra care and looser coupling (Ports & Adapters, contract tests) — they should change more slowly than the innards.
You can now modify the code and know whether you changed its behavior.
2. Remove clutter
Delete what isn't earning its keep (it isn't covered by the net and only obscures intent): dead code, unreachable branches, code called from nowhere, commented-out code, superfluous comments. Version control is your history.
3. Reduce complexity
Drive down cyclomatic complexity (the number of execution paths):
- Reduce indentation — extract the bodies of loops and
if/else/break/continueblocks into well-named methods, even if called once. Name with your best guess; refine as understanding grows. - Eliminate `break`/`continue` as flow clears.
- Aim for a single exit/return point per method.
Use your IDE's automated refactorings — faster and far less risky than hand-editing.
4. Compose methods — tell the story
Keep extracting and naming until a function reads as a short narrative, mostly calls to lower-level methods: group related code, separate unrelated code, choose names that make the enclosing function read like sentences, then recurse. The reader of the top-level function should understand what happens without wading through how.
5. Refactor to testability
Restructure so the code is genuinely testable: move unrelated code apart (modularity), move related code together (cohesion), improve separation of concerns, and introduce abstractions at dependency seams so collaborators can be substituted in tests. Now real unit/integration tests are possible, coverage rises naturally, and the code is safe and pleasant to change.
| Step | Move | Done when |
|---|---|---|
| 1. Characterization tests | Pin current behavior | A behavior change makes a test fail |
| 2. Remove clutter | Delete dead/commented code | Only live code remains |
| 3. Reduce complexity | Extract blocks, flatten flow | Low indentation, fewer paths |
| 4. Compose methods | Name & arrange sub-methods | Top function reads as a story |
| 5. Refactor to testability | Modularity + cohesion + seams | You can write real tests |
Net built → hand back to tdd
This skill's job ends when the net holds. The actual behavior change — and all new code — runs through tdd's red-green-refactor loop, which the reshaped code can now support. The characterization tests stay as a backstop; add finer-grained tests as the new structure supports them, and retire net tests only when a finer test provably covers the same behavior.
Say the handoff out loud: "the characterization test net holds — handing execution to tdd."
Gotchas
- Do not change behavior while building the net — pinning and changing are separate steps, and bugs get pinned too (fix them later, test-first, as their own move).
- Do not retrofit fine-grained unit tests before the structure supports them (see above) — the coarse net comes first.
- Do not big-bang rewrite; stabilise only the area you need to touch.
- If the code's behavior is surprising rather than merely untested, that's diagnosis — route to
diagnosebefore pinning a mystery. - Do not quote or reconstruct source text from the book this skill distills.
A note on scope
This skill builds the legacy safety net only. For red-green-refactor execution use tdd; for naming and local structure on already-tested code use clean-code; for whether a test is worth keeping use test-lens; for coupling/connascence or layer-placement review use architecture-lens.
