Property-Based Testing

Property-based testing verifies that invariants hold across thousands of randomly generated inputs. The Sovereign AI Stack uses proptest for numerical correctness and data structure validation.

Core Concept

Instead of testing specific pairs, define properties that must always be true:

#![allow(unused)]
fn main() {
use proptest::prelude::*;

proptest! {
    #[test]
    fn normalize_produces_unit_vector(v in prop::collection::vec(-1000.0f32..1000.0, 3..128)) {
        let normalized = normalize(&v);
        let magnitude: f32 = normalized.iter().map(|x| x * x).sum::<f32>().sqrt();
        prop_assert!((magnitude - 1.0).abs() < 1e-5);
    }
}
}

Common Property Patterns

Property	Description	Example
Round-trip	encode then decode equals original	serialize/deserialize
Idempotent	applying twice equals once	normalize, deduplicate
Invariant	condition always holds	sorted output, non-negative
Oracle	matches known-good implementation	Rust vs Python output

Strategy Composition

Build complex input generators from simple ones:

#![allow(unused)]
fn main() {
fn model_config_strategy() -> impl Strategy<Value = ModelConfig> {
    (1usize..=32, 64usize..=4096, 1usize..=64)
        .prop_map(|(layers, hidden, heads)| ModelConfig {
            num_layers: layers,
            hidden_size: hidden - (hidden % heads),
            num_heads: heads,
        })
}
}

Shrinking

When proptest finds a failure, it shrinks to the minimal reproduction:

Minimal failing input: ModelConfig { num_layers: 1, hidden_size: 64, num_heads: 65 }

Combining with Mutation Testing

Property tests are excellent mutation killers. A mutation changing < to <= will likely violate an invariant across thousands of inputs:

make mutants-fast    # Find surviving mutants
# Write property tests targeting survivors
make mutants         # Verify mutations are killed

CI Integration

Property tests run as standard cargo test. CI can increase case count:

#![allow(unused)]
fn main() {
proptest! {
    #![proptest_config(ProptestConfig::with_cases(10_000))]
    #[test]
    fn exhaustive_check(input in any::<u32>()) { /* ... */ }
}
}

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