property-based-testing

name	property-based-testing
description	Use when: test invariants over generated inputs instead of a few hand-picked examples.

Goal: find edge cases by checking properties across many inputs.

Use for:

• logic with broad input ranges or many edge cases
• parsers, serializers, and data transformations
• complementing example-based tests

Workflow:

1. Identify an invariant that must always hold.
2. Define generators for the relevant input space.
3. Let the framework run many randomized cases.
4. Let it shrink failures to a minimal counterexample.
5. Add the counterexample as a regression test.
6. Refine generators to cover meaningful edges.

Useful properties:

• round-trip: decode(encode(x)) == x
• invariant: output always satisfies a rule
• equivalence: two implementations agree
• idempotence: f(f(x)) == f(x)

Rules:

• assert properties, not specific outputs
• constrain generators to valid, meaningful inputs
• pin discovered counterexamples as fixed tests
• use seeds so failures reproduce