ADR-0022 — Property-based tests, weekly link check, mutation tool deferred

Status: Accepted

Context

Example-based tests check a few hand-computed values; a formula can pass them and still be wrong elsewhere (sign, quadrant, degenerate input). The library is made of small pure functions composed like formulas — the case where random testing of properties works best (REF-QUICKCHECK). The feature audit (ADR-0020) mutates functions by hand and re-verifies sources by hand; sources can disappear between audits.

Decision

1. Property-based tests with fast-check and @fast-check/vitest (test.prop), in addition to the hand-computed examples, for math and geometry functions with a stateable invariant (symmetry, bounds, periodicity, composition). Examples stay mandatory: they carry the sourced values; properties widen the input space. Runs are seeded by fast-check and a failure prints its seed and shrunk counterexample (REF-FASTCHECK-PBT). New stories use them from EN-005; AUD-001 adds them to the functions of F01 already merged.
2. Weekly link check of all Markdown with lychee (lycheeverse/lychee-action, links.yml, also run on demand): external URLs and local file links, exclusions in .lycheeignore (REF-LYCHEE-ACTION).
3. Mutation testing tool deferred: StrykerJS with Vitest 5 runs no test against mutants (REF-STRYKER-6210; trial: 4 % score, every covered mutant survived). The audit's manual mutation spot-checks remain. Review when the issue is closed and released.

Consequences

• Two dev dependencies (docs/tooling/dependencies.md), no runtime dependency.
• A property failure is a finding: either the function is wrong or the property is; both are recorded.
• A broken source link fails the weekly workflow and notifies the owner; the reference is fixed or marked [unverified].

References

REF-QUICKCHECK, REF-FASTCHECK-PBT, REF-GOOGLE-MUTATION, REF-STRYKER-6210, REF-LYCHEE-ACTION, ADR-0020, research note 0002