Test-Driven Development

This skill is RIGID. Follow every step exactly. Do not skip RED. Do not write implementation before a failing test exists.

Full test stack documentation: docs/testing.md. Do not duplicate it here -- read it when you need runner details, CI lanes, or fixture locations.

Workflow

Repeat this cycle for every behavior change:

1. RED -- Write exactly one failing test. Run it. Confirm it fails for the expected reason.
2. GREEN -- Write the minimum implementation to make the test pass. Run the test again.
3. REFACTOR -- Clean up test and implementation. Run tests to confirm nothing broke.
4. LINT -- Run ./gradlew staticAnalysis before considering the cycle complete.

Never batch multiple behaviors into one cycle. One test, one behavior, one cycle.

Running Tests

Kotlin/JVM (fast iteration)

# Single test method
./gradlew :core:engine:testDebugUnitTest --tests "ClassName.method name"

# Single test class
./gradlew :core:engine:testDebugUnitTest --tests "ClassName"

# Single module
./gradlew :core:engine:testDebugUnitTest

Replace :core:engine with the target module (:core:service, :core:data, :core:diagnostics).

Rust

# Single test
cargo nextest run --locked -p crate_name test_name

# Single crate
cargo nextest run --locked -p crate_name

# Full workspace
cargo nextest run --locked --workspace

Golden contracts

Golden tests are read-only by default. If your change intentionally alters a contract:

# Bless all golden fixtures
bash scripts/tests/bless-telemetry-goldens.sh

# Manual single-suite bless
RIPDPI_BLESS_GOLDENS=1 ./gradlew :core:engine:testDebugUnitTest
RIPDPI_BLESS_GOLDENS=1 cargo test --locked -p crate_name

Always review blessed diffs before committing. Golden changes require explanation in the commit message.

Test Double Conventions

Fake* classes (no mocking frameworks)

All test doubles are hand-written Fakes in core/engine/src/test/kotlin/com/poyka/ripdpi/core/TestDoubles.kt. The project does not use MockK, Mockito, or any mocking library.

Pattern:

• Name: Fake + interface name (e.g., FakeRipDpiProxyRuntime)
• Track call counts and last arguments as public properties
• Return configurable values set before the test runs

FaultQueue for fault injection

Fault injection uses FaultQueue<T> from core/engine-api/src/main/kotlin/com/poyka/ripdpi/core/testing/FaultModel.kt.

Key types:

• FaultQueue<T> -- ordered queue of faults matched by target enum
• FaultSpec<T> -- target + outcome + scope + optional message/payload
• FaultScope.ONE_SHOT -- fires once then is consumed
• FaultScope.PERSISTENT -- fires on every matching call until cleared
• FaultOutcome -- EXCEPTION, TIMEOUT, DROP, RESET, MALFORMED_PAYLOAD, BLANK_PAYLOAD, PANIC

Usage in tests:

val bindings = FakeRipDpiProxyBindings()
bindings.faults.enqueue(
    FaultSpec(
        target = ProxyBindingFaultTarget.START,
        outcome = FaultOutcome.EXCEPTION,
        message = "simulated native crash",
    )
)

Test Organization by Layer

What you test	Location	Runner
Kotlin business logic	core/*/src/test/	./gradlew :core:*:testDebugUnitTest
Rust native logic	native/rust/crates/*/tests/	cargo nextest run --locked -p crate
JNI integration	app/src/androidTest/.../integration/	connectedGithubFullDebugAndroidTest
Network E2E	app/src/androidTest/.../e2e/	connectedGithubFullDebugAndroidTest
Rust network E2E	native/rust/crates/*/tests/	bash scripts/ci/run-rust-network-e2e.sh

Subagent Strategy

For non-trivial features, use context isolation:

1. Subagent writes the test -- Launch Claude Code's built-in Explore or general-purpose subagent to write the failing test. This keeps test design independent of implementation bias.
2. Main context implements -- Read the test the subagent wrote, then implement the minimum code to pass it.
3. Main context refactors -- Clean up both test and implementation in the same context.

This prevents the "write test and implementation together" anti-pattern.

Rules

1. Never skip RED. Every test must fail before you write implementation.
2. One test at a time. Do not write the next test until the current cycle is complete.
3. Run staticAnalysis before commit. ./gradlew staticAnalysis covers detekt, ktlint, and Android lint. It applies to test code too.
4. Test and implementation in one commit. Never commit a test without its implementation or vice versa.
5. Fake doubles only.* No mocking frameworks. Add new Fakes to TestDoubles.kt.
6. FaultQueue for error paths. Use FaultSpec + FaultQueue, not ad-hoc exception throwing.
7. Golden contracts are read-only by default. Bless intentionally, review the diff, explain in the commit message.
8. Use backtick names in Kotlin tests. @Test fun proxy start propagates native exception().
9. Use snake_case names in Rust tests. fn proxy_start_propagates_native_exception().
10. Prefer unit tests. Only escalate to integration/E2E when the behavior requires real Android or network components.

Common Mistakes

Mistake	Fix
Using MockK or Mockito	Write a Fake* class in TestDoubles.kt
Skipping the RED step	Run the test first. If it passes, your test is wrong.
Multiple assertions per cycle	Split into separate test methods, one behavior each
runBlocking in coroutine tests	Use runTest from kotlinx-coroutines-test
Blessing goldens without review	Run git diff on fixture files before committing
Testing private internals	Test through the public API of the class under test
Putting helpers in test classes	Add shared helpers to TestDoubles.kt
FaultScope.PERSISTENT when ONE_SHOT suffices	Default to ONE_SHOT; use PERSISTENT only for repeated-call scenarios

For deeper coverage of anti-patterns, see references/testing-anti-patterns.md.