TDD — Test-Driven Development

You write code using strict test-driven development. Follow these rules exactly.

The Two Rules

1. Write new code only when an automated test has failed.
2. Eliminate duplication.

These imply: write tests before code, then refactor to remove duplication. Design emerges from this process.

The Cycle: Red → Green → Refactor

1. Red: Write a small test that fails (or doesn't compile). Run it. Confirm it fails.
2. Green: Make it pass using the simplest possible change. Shortcuts within the current scope are fine — hardcoded values, copy-paste, temporary variables. The only goal is green.
3. Refactor: Remove duplication introduced by getting to green. This includes duplication between test and production code (e.g., a constant in both).

Run the relevant test(s) after every Red and Green step — not the full suite. Commit at green. Refactor at green. Never refactor while red.

Choosing Tests

First test: Start with the simplest degenerate case — empty input, zero, identity, null.

Next test: Pick from the to-do list a test you are confident you can get passing in one step and that will teach you something new about the problem.

Regression test: When a defect is found, write the smallest failing test that reproduces it before writing the fix.

When to delete tests: Only when a test is redundant with another test and removing it does not reduce confidence or clarity.

The to-Do List

Maintain a running list of tests to write (as comments in the test file or inline notes). When a tangential idea arises mid-cycle, add it to the list and stay focused on the current test. Cross off items as done. Add new items as discovered.

Three Strategies to Get to Green

Choose based on confidence:

Step Size

• Smaller steps when uncertain, surprised, or dealing with unfamiliar territory.
• Bigger steps when confident.
• An unexpected red bar means: shift down to smaller steps.
• An unexpected green bar means: review your test — it may be wrong. AI-written tests often mirror what the code does rather than what it should do.

Writing Tests

• Assert first: Start with the assertion (expected outcome), then work backward to the setup and action needed to produce that outcome.
• Evident data: Use literal, readable values so the relationship between input and output is obvious from the test alone (e.g., assertEquals(2.5, five.times(0.5))).
• Test anything that could break: conditionals, loops, operations, polymorphism. Do not test library/framework code or trivial getters/setters.
• Stop writing tests when every branch and boundary condition is exercised and adding more tests teaches you nothing new.

Key Patterns

Child Test

If a test requires too many changes to get to green, write a smaller child test for one piece, get that green, then return to the original test.

Value Objects

Make objects immutable. Operations return new instances. Implement equality and hashing. Eliminates aliasing bugs.

One to Many

Implement for a single element first, then generalize to a collection.

Isolate Change

Before modifying something, isolate it (Extract Method, Extract Object, Method Object) so the change is surgical.

Reconcile Differences

To unify two similar pieces of code: make them identical through tiny changes, then merge.

Mock Objects

For expensive or slow dependencies (DB, network), use fakes that implement the same interface.

Log String

To verify a sequence of calls, append to a log string in each method and assert the final string.

Crash Test Dummy

To test error paths, create a subclass that overrides a method to throw an exception.

Clean Check-In

All tests must pass before committing to shared code. Always.

Test Quality Signals

These smells indicate design problems, not test problems:

• Long setup: Objects are too big. Break them apart.
• Duplicate setup: Too many tightly-coupled objects.
• Long-running tests: Design problem (usually coupling to slow resources).
• Fragile tests: Action at a distance — one change breaks distant tests.

Design Emerges

Do not design ahead. Solve "works" first via red/green, then solve "clean" via refactoring. Design patterns emerge naturally from removing duplication. You don't plan them; you discover them.

The Rhythm

The cycle should be fast — minutes (for a human) per cycle, not hours. Run tests constantly.

If stuck: backtrack. Revert to the last green state and take a smaller step. The tests are your safety net — use them.

Pure Wiring Steps

If a step is pure wiring with no testable behaviour (e.g., bumping a library version, pure refactorings, etc.): implement directly. No test needed.