/** A 32-bit unsigned integer seed. */
export type Seed = number;

export interface Rng {
  /** Float in [0, 1). */
  next(): number;
  /** Integer in [min, max], inclusive. */
  int(min: number, max: number): number;
  /** A uniformly chosen element. Throws on an empty list. */
  pick<T>(list: readonly T[]): T;
}

/** Seeded PRNG (mulberry32). The same seed always yields the same sequence. */
export function createRng(seed: Seed): Rng {
  let state = seed >>> 0;

  const next = () => {
    state = (state + 0x6d2b79f5) >>> 0;
    let t = state;
    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,
    int(min, max) {
      return min + Math.floor(next() * (max - min + 1));
    },
    pick(list) {
      if (list.length === 0) throw new Error("Cannot pick from an empty list");
      return list[Math.floor(next() * list.length)]!;
    },
  };
}

/** Derives an independent child seed from a parent seed and a label (FNV-1a, then a final mix). */
export function deriveSeed(parent: Seed, label: string): Seed {
  let hash = 0x811c9dc5 ^ (parent >>> 0);
  for (let i = 0; i < label.length; i++) {
    hash ^= label.charCodeAt(i);
    hash = Math.imul(hash, 0x01000193);
  }
  hash ^= hash >>> 16;
  hash = Math.imul(hash, 0x85ebca6b);
  hash ^= hash >>> 13;
  hash = Math.imul(hash, 0xc2b2ae35);
  hash ^= hash >>> 16;
  return hash >>> 0;
}