# Property-Based Tests (FsCheck)

## Module
Policy

## Status
IMPLEMENTED

## Description
Property-based tests using FsCheck for canonical JSON determinism, SBOM/VEX ordering invariants, floating-point stability, digest computation determinism, smart-diff properties, and VEX lattice merge commutativity.

## Implementation Details
- **DecayPropertyTests**: `src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/PropertyTests/DecayPropertyTests.cs`
  - FsCheck properties for DecayedConfidenceCalculator: monotonicity, bounds, floor enforcement, half-life correctness
  - Verifies: decay factor always in [0, 1], result always >= floor, older observations have lower scores
- **DeterminismPropertyTests**: `src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/PropertyTests/DeterminismPropertyTests.cs`
  - FsCheck properties for deterministic evaluation: same inputs produce same outputs
  - Verifies: ObservationDecay CalculateDecay is deterministic, content-addressed ID generation is stable
- **ObservationDecayTests**: `src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Models/ObservationDecayTests.cs`
  - Unit tests for ObservationDecay model: Create, Fresh, WithSettings, CalculateDecay, CheckIsStale
- **DeterminizationResultTests**: `src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Models/DeterminizationResultTests.cs`
  - Unit tests for DeterminizationResult model
- **DecayedConfidenceCalculatorTests**: `src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/DecayedConfidenceCalculatorTests.cs`
  - Unit tests for DecayedConfidenceCalculator: boundary values, half-life verification, floor enforcement
- **DeterminizationGateTests**: `src/Policy/__Tests/StellaOps.Policy.Engine.Tests/Gates/Determinization/DeterminizationGateTests.cs`
  - Integration tests for DeterminizationGate in gate pipeline
- **DeterminizationPolicyTests**: `src/Policy/__Tests/StellaOps.Policy.Engine.Tests/Policies/DeterminizationPolicyTests.cs`
  - Policy-level tests for determinization enforcement

## E2E Test Plan
- [ ] Run DecayPropertyTests; verify all FsCheck properties pass (monotonicity, bounds, floor, half-life)
- [ ] Run DeterminismPropertyTests; verify deterministic evaluation property holds for 1000+ random inputs
- [ ] Verify DecayedConfidenceCalculator boundary: baseConfidence=0.0 always returns floor
- [ ] Verify DecayedConfidenceCalculator boundary: ageDays=0 always returns baseConfidence
- [ ] Verify ObservationDecay.Fresh(now).CheckIsStale(now) == false
- [ ] Verify decay monotonicity: older age always produces lower or equal score
- [ ] Verify content-addressed ID generation produces same ID for identical inputs (1000 iterations)
- [ ] Verify DeterminizationGate integration with DecayedConfidenceCalculator in gate pipeline
- [ ] Verify all test suites pass in CI with frozen test clock (no time-dependent flakiness)