/* ============================================================
   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;
}