property-based-testing

# Property-Based Testing Guide

Use this skill proactively during development when you encounter patterns where PBT provides stronger coverage than example-based tests.

## When to Invoke (Automatic Detection)

**Invoke this skill when you detect:**

- **Serialization pairs**: `encode`/`decode`, `serialize`/`deserialize`, `toJSON`/`fromJSON`, `pack`/`unpack`
- **Parsers**: URL parsing, config parsing, protocol parsing, string-to-structured-data
- **Normalization**: `normalize`, `sanitize`, `clean`, `canonicalize`, `format`
- **Validators**: `is_valid`, `validate`, `check_*` (especially with normalizers)
- **Data structures**: Custom collections with `add`/`remove`/`get` operations
- **Mathematical/algorithmic**: Pure functions, sorting, ordering, comparators
- **Smart contracts**: Solidity/Vyper contracts, token operations, state invariants, access control

**Priority by pattern:**

| Pattern | Property | Priority |
|---------|----------|----------|
| encode/decode pair | Roundtrip | HIGH |
| Pure function | Multiple | HIGH |
| Validator | Valid after normalize | MEDIUM |
| Sorting/ordering | Idempotence + ordering | MEDIUM |
| Normalization | Idempotence | MEDIUM |
| Builder/factory | Output invariants | LOW |
| Smart contract | State invariants | HIGH |

## When NOT to Use

Do NOT use this skill for:
- Simple CRUD operations without transformation logic
- One-off scripts or throwaway code
- Code with side effects that cannot be isolated (network calls, database writes)
- Tests where specific example cases are sufficient and edge cases are well-understood
- Integration or end-to-end testing (PBT is best for unit/component testing)

## Property Catalog (Quick Reference)

| Property | Formula | When to Use |
|----------|---------|-------------|
| **Roundtrip** | `decode(encode(x)) == x` | Serialization, conversion pairs |
| **Idempotence** | `f(f(x)) == f(x)` | Normalization, formatting, sorting |
| **Invariant** | Property holds before/after | Any transformation |
|