Engines

import { xoshiro128pp, pcg32, mt19937 } from "ransu/engine";

new Random(42); // xoshiro128++
new Random(42, { engine: pcg32 });
new Random(mt19937(42)); // a constructed engine

Engine	State	Period	Use it when
xoshiro128pp	4×32	2^128−1	The default. 32-bit only, which is what JavaScript is fast at
xoshiro256pp	4×64	2^256−1	A stream has to line up with a Rust or C implementation
pcg32	64+64	2^64	You need advance(n): skip or rewind N draws in O(log N)
sfc32	4×32	~2^127	Minimal code, no jump-ahead needed
mulberry32	1×32	2^32	Code size dominates. Too short for long runs
mt19937	624×32	2^19937−1	Interoperating with existing Mersenne Twister data
chacha20	key+counter	effectively unbounded	Seeded and hard to predict
nativeMath	—	—	Math.random. Fastest, not seedable
cryptoRandom	—	—	The platform CSPRNG. Not seedable

Writing your own

One required method:

const myEngine = {
  algorithm: "my-prng",
  seedable: false,
  nextUint32: () => 0, // 32 uniform bits
};

new Random(myEngine);

Optional members — nextFloat64, fillBytes, getState, clone, split, jump, reseed — are detected once when the engine is adopted and used as a fast path when present.

A bare () => number in [0, 1) also works, so Math.random, seedrandom and similar drop straight in.

MT19937 is for compatibility only

It is slow to seed, carries 2.5 KB of state, and fails statistical tests the others pass. It is here so you can read existing data, not for new work.

ChaCha20 is not a crypto API

It gives a stream that is both seeded and hard to predict — a prize draw you can replay for an auditor. ransu does not expose it as a cryptographic primitive.