Test-Driven Development

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.

See tests.md for examples and mocking.md for mocking guidelines.

Anti-Pattern: Horizontal Slices

DO NOT write all tests first, then all implementation. This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code."

This produces crap tests:

• Tests written in bulk test imagined behavior, not actual behavior
• You end up testing the shape of things (data structures, function signatures) rather than user-facing behavior
• Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
• You outrun your headlights, committing to test structure before understanding the implementation

Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.

WRONG (horizontal):
  RED:   test1, test2, test3, test4, test5
  GREEN: impl1, impl2, impl3, impl4, impl5

RIGHT (vertical):
  RED→GREEN: test1→impl1
  RED→GREEN: test2→impl2
  RED→GREEN: test3→impl3
  ...

Workflow

1. Planning

探索代码库时，使用项目领域词汇表，使 test name 与 interface 词汇与项目语言一致，并 respect 你即将改动区域的 ADR。

写任何代码前：

• [ ] 与用户确认需要哪些 interface 变更
• [ ] 与用户确认要测哪些 behavior（prioritize）
• [ ] 识别 deep modules 机会（小 interface、深 implementation）
• [ ] 为 testability 设计 interface
• [ ] 列出要测的 behavior（非 implementation step）
• [ ] 获得用户对 plan 的 approval

Ask: "What should the public interface look like? Which behaviors are most important to test?"

You can't test everything. 与用户确认哪些 behavior 最重要。测试 effort 聚焦 critical path 与 complex logic，非每个 possible edge case。

2. Tracer Bullet

写 一个 test 确认系统 一件事：

RED:   Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes

这是 tracer bullet——证明路径 end-to-end 可行。

3. Incremental Loop

对每个剩余 behavior：

RED:   Write next test → fails
GREEN: Minimal code to pass → passes

Rules:

• One test at a time
• Only enough code to pass current test
• Don't anticipate future tests
• Keep tests focused on observable behavior

4. Refactor

所有 test pass 后，找 refactor candidates：

• [ ] Extract duplication
• [ ] Deepen modules (move complexity behind simple interfaces)
• [ ] Apply SOLID principles where natural
• [ ] Consider what new code reveals about existing code
• [ ] Run tests after each refactor step

Never refactor while RED. Get to GREEN first.

Checklist Per Cycle

[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added