bd-test-design

# Test Design

This skill guides the design and implementation of comprehensive tests.

## Core Thinking

Before generating code, establish:
1. **Purpose**: Verify behavior, not implementation.
2. **Scope**: Unit, Integration, or E2E?
3. **Inputs/Outputs**: Define strict expectations.

## Design Criteria

Use these techniques to select inputs:

### Equivalence Partitioning (EP)
Divide inputs into classes where all values are treated the same. Test one value from each valid and invalid class.
*Example*: For `age` (0-12 child, 13-19 teen), test values: `-1`, `5`, `15`.

### Boundary Value Analysis (BVA)
Bugs hide at edges. Test exact boundaries and one step before/after.
*Example*: For range 0-12, test: `-1`, `0`, `12`, `13`.

### Advanced Heuristics
For complex validation logic (CORRECT, Right-BICEP, Zero-One-Many), see [references/heuristics.md](references/heuristics.md).

## Test Structure

### Arrange-Act-Assert (AAA)
Ensure all tests follow this pattern:
1. **Arrange**: Set up independent state/fixtures.
2. **Act**: Execute the behavior.
3. **Assert**: Verify outcome.

See [references/aaa-examples.md](references/aaa-examples.md) for code examples.

### Isolation & Naming
- **Isolation**: No shared mutable state. Mocks/stubs for external deps.
- **Naming**: `should_[outcome]_when_[scenario]` or `test_[scenario]_[expected_outcome]`.

## Domain & Strategy Guides

Select the specific guide based on the user's context:

- **Property-Based Testing**: For invariants, algorithms, and round-trip data. See [references/property-testing.md](references/property-testing.md).
- **Edge Cases**: For checklists of required edge cases (Happy/Sad/Ugly). See [references/edge-cases.md](references/edge-cases.md).
- **Platform Specifics**: Tooling and patterns for **Mobile, Web, Backend, and AI**. See [references/platforms.md](references/platforms.md).

## Anti-Patterns
- **Interdependence**: Tests relying on execution order.
- **Shared Mutable State**: State bleeding between tests.
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