/**
 * llmindex.io — deterministic seeded RNG for item generation/perturbation
 * Author:  Simon-Pierre Boucher
 * Contact: contact@spboucher.ai
 * License: Proprietary — © Simon-Pierre Boucher, all rights reserved
 */

/** FNV-1a 32-bit hash of a string seed. */
export function hashSeed(seed: string): number {
  let h = 0x811c9dc5;
  for (let i = 0; i < seed.length; i++) {
    h ^= seed.charCodeAt(i);
    h = Math.imul(h, 0x01000193);
  }
  return h >>> 0;
}

export interface Rng {
  /** Uniform float in [0,1). */
  next(): number;
  /** Uniform integer in [min,max] inclusive. */
  int(min: number, max: number): number;
  /** Pick one element. */
  pick<T>(arr: readonly T[]): T;
  /** Fisher-Yates shuffle (copy). */
  shuffle<T>(arr: readonly T[]): T[];
}

/** mulberry32 PRNG — fast, deterministic, good enough for item perturbation. */
export function createRng(seed: string): Rng {
  let a = hashSeed(seed);
  const next = (): number => {
    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;
  };
  return {
    next,
    int: (min, max) => min + Math.floor(next() * (max - min + 1)),
    pick: (arr) => {
      const el = arr[Math.floor(next() * arr.length)];
      if (el === undefined) throw new Error('pick from empty array');
      return el;
    },
    shuffle: (arr) => {
      const out = [...arr];
      for (let i = out.length - 1; i > 0; i--) {
        const j = Math.floor(next() * (i + 1));
        [out[i], out[j]] = [out[j]!, out[i]!];
      }
      return out;
    },
  };
}