/**
 * Deterministic pseudo-random number generator.
 *
 * Uses mulberry32 — small, fast, 32-bit state, good enough for test
 * action selection. Same seed → same sequence, so chaos runs are
 * reproducible.
 */

export interface Rng {
  /** Returns a float in [0, 1). */
  next(): number;
  /** Returns the seed used to initialise this generator. */
  readonly seed: number;
}

export function createRng(seed: number): Rng {
  let state = seed >>> 0;
  return {
    seed,
    next(): number {
      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;
    },
  };
}

/** Generate a random 32-bit seed from Math.random (for when the user didn't pass one). */
export function randomSeed(): number {
  return Math.floor(Math.random() * 0x100000000) >>> 0;
}

/** Pick one item from `items` using weights (sum > 0 required). */
export function weightedPick<T>(
  items: readonly T[],
  weightOf: (item: T) => number,
  rng: Rng
): T {
  if (items.length === 0) {
    throw new Error("weightedPick: items is empty");
  }
  let total = 0;
  for (const item of items) total += weightOf(item);
  if (total <= 0) return items[items.length - 1]!;

  let roll = rng.next() * total;
  for (const item of items) {
    roll -= weightOf(item);
    if (roll <= 0) return item;
  }
  return items[items.length - 1]!;
}

/** Integer in [0, max). */
export function randomInt(rng: Rng, max: number): number {
  return Math.floor(rng.next() * max);
}