seed

Deterministic pseudo-random seed.

The implementation is the 32-bit “Marsaglia xorshift” PRNG. It uses only shifts and xors — no multiplication — so BEAM (bignum integers) and JavaScript (53-bit safe doubles) produce bit-identical streams. An LCG-style multiplier would overflow JavaScript’s double precision well before the 32-bit mask and cause subtle distribution drift.

Statistical quality is not the goal here — determinism and portability are. Property-based testing is well-served even by modest PRNGs because the failure search is shrinker-driven, not statistical.

Types

Seed

opaque </>

Internal state of the PRNG. Always non-negative and ≤ 0xFFFFFFFF. The xorshift family has 0 as a fixed point, so a masked-to-zero input is silently replaced with a non-zero default.

pub opaque type Seed

Values

next_int

</>

pub fn next_int(s: Seed) -> #(Int, Seed)

Advance the seed once and return the next non-negative integer alongside the advanced seed.

next_int_in

</>

pub fn next_int_in(s: Seed, lo: Int, hi: Int) -> #(Int, Seed)

Return an integer uniformly in the closed interval [lo, hi].

If lo > hi the bounds are swapped. If lo == hi the bound is returned without consuming randomness (the seed is still advanced for determinism).

random_seed

</>

pub fn random_seed() -> Seed

Construct a seed from the system clock. Useful for ad-hoc local runs; CI should pin a value via metamon.with_seed(metamon.seed(_)).

seed

</>

pub fn seed(value: Int) -> Seed

Construct a seed from an integer. Negative or out-of-range values are normalised by masking to the 32-bit positive window so the stream stays target-portable.

split

</>

pub fn split(s: Seed) -> #(Seed, Seed)

Split a seed into two statistically independent seeds. The implementation derives the second seed by xor-ing the first with a constant before stepping, which on a 32-bit LCG produces a stream that does not align with the original — sufficient for shrinking independent generator components without correlation artefacts.

state

</>

pub fn state(s: Seed) -> Int

The raw integer state. Used by the regression-file format to serialise a seed into a reproduction key.