/** Seeded RNG (mulberry32) so every synthesized dataset is reproducible
 *  from (base seed, run index) alone. */
export type Rng = () => number;

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

/** Fisher-Yates shuffle in place. */
export function shuffle<T>(arr: T[], rng: Rng): T[] {
  for (let i = arr.length - 1; i > 0; i--) {
    const j = Math.floor(rng() * (i + 1));
    const t = arr[i]; arr[i] = arr[j]; arr[j] = t;
  }
  return arr;
}