Initial

src/rng.js

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+/* ============================================================
+   rng.js — deterministic seeding & sampling
+   Every stochastic decision in the generator draws from a
+   salted sub-stream so that changing one parameter does not
+   reshuffle unrelated parts of the scene.
+   ============================================================ */
+
+/* cyrb53 — fast 53-bit string hash. Stable across runs/machines. */
+export function cyrb53(str, seed = 0) {
+  let h1 = 0xdeadbeef ^ seed, h2 = 0x41c6ce57 ^ seed;
+  for (let i = 0; i < str.length; i++) {
+    const ch = str.charCodeAt(i);
+    h1 = Math.imul(h1 ^ ch, 2654435761);
+    h2 = Math.imul(h2 ^ ch, 1597334677);
+  }
+  h1 = Math.imul(h1 ^ (h1 >>> 16), 2246822507) ^ Math.imul(h2 ^ (h2 >>> 13), 3266489909);
+  h2 = Math.imul(h2 ^ (h2 >>> 16), 2246822507) ^ Math.imul(h1 ^ (h1 >>> 13), 3266489909);
+  return (4294967296 * (2097151 & h2) + (h1 >>> 0)).toString(16).padStart(13, '0');
+}
+
+/* mulberry32 — tiny fast PRNG, returns [0,1). */
+export function mulberry32(seed) {
+  let a = seed >>> 0;
+  return function () {
+    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;
+  };
+}
+
+/* makeRng — seed string + salt → independent PRNG stream. */
+export function makeRng(seedStr, salt = '') {
+  const h = cyrb53(seedStr + '::' + salt);
+  return mulberry32(parseInt(h.slice(0, 8), 16));
+}
+
+/* ---------- Distributions ---------- */
+
+/* Standard normal via Box-Muller. */
+export function gauss(rng) {
+  const u = 1 - rng(), v = rng();
+  return Math.sqrt(-2 * Math.log(u)) * Math.cos(2 * Math.PI * v);
+}
+
+/* Log-normal — heavy-tailed positive sample. */
+export function logNormal(rng, mu, sigma) {
+  return Math.exp(mu + sigma * gauss(rng));
+}
+
+/* Uniform in [lo, hi). */
+export function range(rng, lo, hi) {
+  return lo + (hi - lo) * rng();
+}
+
+/* Random int in [lo, hi]. */
+export function randInt(rng, lo, hi) {
+  return lo + Math.floor(rng() * (hi - lo + 1));
+}
+
+/* Pick one element. */
+export function pick(rng, arr) {
+  return arr[Math.floor(rng() * arr.length)];
+}
+
+/* Bernoulli. */
+export function chance(rng, p) {
+  return rng() < p;
+}