Shrinkable randomness

Code that takes a Random\Randomizer — a jittered backoff, a shuffle, a weighted pick, a replica selector — can be property-tested by drawing a seed and building new Randomizer(new Mt19937($seed)). That reproduces a failure; it does not shrink it. The smallest failing seed is another opaque number, and the sequence of random decisions behind the failure stays as long and as noisy as it was.

Gen::randomEngine() yields a Random\Engine whose every generate() is one in-body draw of eight bytes. The code under test sees the native API and needs no seam; the engine records each call on the replay tape, replays it by position on shrink trials, and shrinks it through the bytes' own tree.

use Rasuvaeff\PropertyTesting\Gen;

#[Property]
public function backoffStaysUnderCap(int $attempt, \Random\Randomizer $randomizer): void
{
    $delay = (new JitteredBackoff($randomizer))->delayMs($attempt);

    Assert::true($delay <= 60_000);
}

Under auto, a Random\Engine parameter is derived as Gen::randomEngine() and a Random\Randomizer parameter as Gen::randomizer() — the native Randomizer over the drawn engine; both are also there to pass explicitly.

What "smaller" means

The bytes shrink toward "\0", and the direction is worth knowing before reading a counterexample: Randomizer::getInt($min, $max) maps all-zero bytes to $min, getFloat() to the lower end, shuffleArray() to a near-identity permutation. For most code that is the natural minimum — the shortest jitter, the first replica, the unshuffled list — and the counterexample says which random decisions were needed to fail, not just that some were.

Each engine call is one tape position, reported as draw#N. The descent is bounded like every in-body draw, so a body that consumes thousands of random values per run shrinks only as far as that cap allows. The engine is valid only inside a run — outside one, generate() throws the way Gen::draw() does — and is not seedable: the tape is the seed. A counterexample that carries draws is stored in the corpus as a seed entry, as always.