simulator/engine/rng.js

// Seeded PRNG for the engine (never Math.random). mulberry32 by Tommy Ettinger
// (public domain), plus a Box–Muller normal deviate. Pure; no DOM.

export function mulberry32(seed) {
  let a = (Number(seed) >>> 0) || 0x9e3779b9;
  const next = () => {
    a = (a + 0x6d2b79f5) >>> 0;
    let t = a;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
  return next;
}

export function createRng(seed = 1) {
  const uniform = mulberry32(seed);
  let spare = null;
  const normal = () => {
    if (spare !== null) { const s = spare; spare = null; return s; }
    let u = 0, v = 0;
    while (u <= Number.EPSILON) u = uniform();
    v = uniform();
    const r = Math.sqrt(-2 * Math.log(u));
    spare = r * Math.sin(2 * Math.PI * v);
    return r * Math.cos(2 * Math.PI * v);
  };
  return { uniform, normal };
}