// Seeded, serializable random number generator.
//
// Every draw the game makes comes from here, so a run is fully reproducible
// from its seed plus the player's choice sequence. That makes tests
// deterministic and playtest reports replayable: the seed and the generator's
// position both live in the save file and the feedback log.

/** Hash an arbitrary string seed down to a uint32. */
function hashSeed(seed) {
  let h = 1779033703 ^ String(seed).length;
  for (let i = 0; i < String(seed).length; i++) {
    h = Math.imul(h ^ String(seed).charCodeAt(i), 3432918353);
    h = (h << 13) | (h >>> 19);
  }
  h = Math.imul(h ^ (h >>> 16), 2246822507);
  h = Math.imul(h ^ (h >>> 13), 3266489909);
  return (h ^ (h >>> 16)) >>> 0;
}

/**
 * mulberry32: one uint32 of state, which is the whole point — the generator's
 * position round-trips through JSON with no special handling.
 */
export function createRng(seed, position) {
  let state = position === undefined ? hashSeed(seed) : position >>> 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 {
    seed,
    /** Float in [0, 1). */
    next,
    /** Integer in [0, n). */
    int: (n) => Math.floor(next() * n),
    /** Uniform pick from a non-empty array. */
    pick(items) {
      if (!items.length) throw new RangeError('cannot pick from an empty array');
      return items[Math.floor(next() * items.length)];
    },
    /**
     * Weighted pick. `weightOf` must return a non-negative number; entries
     * weighing zero are never drawn. Returns undefined if nothing has weight.
     */
    weighted(items, weightOf) {
      let total = 0;
      for (const item of items) total += Math.max(0, weightOf(item));
      if (total <= 0) return undefined;
      let roll = next() * total;
      for (const item of items) {
        roll -= Math.max(0, weightOf(item));
        if (roll < 0) return item;
      }
      return items[items.length - 1];
    },
    /** Current position, for saving mid-run. */
    getPosition: () => state,
  };
}