kimchi:tdd

# Test-Driven Development

## Overview

Write the test first. Watch it fail. Write minimal code to pass.

**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.

**Violating the letter of the rules is violating the spirit of the rules.**

## When This Applies

**Always** when writing code that has behavior:
- New features
- Bug fixes
- Refactoring
- Behavior changes

**Exceptions (ask your human partner):**
- Pure configuration files
- Database migrations (tested via schema verification)
- Static content
- Throwaway prototypes

Thinking "skip TDD just this once"? Stop. That's rationalization.

## The Iron Law

```
NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
```

Write code before the test? Delete it. Start over.

**No exceptions:**
- Don't keep it as "reference"
- Don't "adapt" it while writing tests
- Don't look at it
- Delete means delete

Implement fresh from tests. Period.

## Red-Green-Refactor

### RED — Write Failing Test

Write one minimal test showing what should happen.

<Good>
```ruby
it "rejects file over 5MB with clear error" do
  large_file = fixture_file("6mb.jpg")

  result = AvatarUploadService.call(large_file, user_id: user.id)

  expect(result).to be_failure
  expect(result.error).to eq("File too large (max 5MB)")
end
```
Clear name, tests real behavior, one thing
</Good>

<Bad>
```ruby
it "works" do
  mock = double("s3", upload: true)
  allow(S3Client).to receive(:new).and_return(mock)
  result = AvatarUploadService.call(file, user_id: 1)
  expect(mock).to have_received(:upload)
end
```
Vague name, tests mock not code
</Bad>

**Requirements:**
- One behavior per test
- Clear name describing the behavior
- Real code (no mocks unless unavoidable)

### Verify RED — Watch It Fail

**MANDATORY. Never skip.**

```bash
# Run the test, expect failure
bundle exec rspec spec/services/avatars/upload_service_spec.rb
```

Confirm:
- Test fails (not errors)
- Failure message is expected
- Fails because feature missing (not typos)

**Test passes?** You're testing existing behavior. Fix test.

**Test errors?** Fix error, re-run until it fails correctly.

### GREEN — Minimal Code

Write simplest code to pass the test.

<Good>
```ruby
def call(file, user_id:)
  return failure("File too large (max 5MB)") if file.size > 5.megabytes

  url = s3_client.upload(file, key: "avatars/#{user_id}/#{SecureRandom.uuid}")
  success(url: url)
end
```
Just enough to pass
</Good>

<Bad>
```ruby
def call(file, user_id:, options: {})
  strategy = options.fetch(:resize_strategy, :vips)
  validator = ValidatorFactory.for(file.content_type)
  pipeline = ProcessingPipeline.new(strategy: strategy)
  # YAGNI
end
```
Over-engineered
</Bad>

Don't add features, refactor other code, or "improve" beyond the test.

### Verify GREEN — Watch It Pass

**MANDATORY.**

```bash
bundle exec rspec spec/services/avatars/upload_service_spec.rb
```

Confirm:
- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)

**Test fails?** Fix code, not test.

**Other tests fail?** Fix now.

### REFACTOR — Clean Up

After green only:
- Remove duplication
- Improve names
- Extract helpers

Keep tests green. Don't add behavior.

### REPEAT

Next failing test for next feature.

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests passing immediately prove nothing. |
| "Tests after achieve same goals" | Tests-after = "what does this do?" Tests-first = "what should this do?" |
| "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. |
| "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is technical debt. |
| "Keep as reference, write tests first" | You'll adapt it. That's testing after. Delete means delete. |
| "Need to explore first" | Fine. Throw away exploration, start with TDD. |
| "Test hard = design unclear" | Listen to test. Hard to test = hard to use. |
| "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. |
| "Existing code has no tests" | You're improving it. Add tests for the code you touch. |

## Red Flags — STOP and Start Over

- Code before test
- Test after implementation
- Test passes immediately
- Can't explain why test failed
- Tests added "later"
- Rationalizing "just this once"
- "I already manually tested it"
- "Keep as reference" or "adapt existing code"
- "Already spent X hours, deleting is wasteful"
- "This is different because..."

**All of these mean: Delete code. Start over with TDD.**

## Verification Checklist

Before marking work complete:

- [ ] Every new function/method has a test
- [ ] Watched each test fail before implementing
- [ ] Each test failed for expected reason (feature missing, not typo)
- [ ] Wrote minimal code to pass each test
- [ ] All tests pass
- [ ] Output pristine (no errors, warnings)
- [ ] Tests use real code (mocks only if unavoidable)
- [ ] Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

## Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

## When Stuck

| Problem | Solution |
|---------|----------|
| Don't know how to test | Write wished-for API. Write assertion first. Ask your human partner. |
| Test too complicated | Design too complicated. Simplify interface. |
| Must mock everything | Code too coupled. Use dependency injection. |
| Test setup huge | Extract helpers. Still complex? Simplify design. |

## Final Rule

```
Production code → test exists and failed first
Otherwise → not TDD
```

No exceptions without your human partner's permission.