meteor/packages/random/AleaRandomGenerator.js

import RandomGenerator from './AbstractRandomGenerator';

// Alea PRNG, which is not cryptographically strong
// see http://baagoe.org/en/wiki/Better_random_numbers_for_javascript
// for a full discussion and Alea implementation.
function Alea(seeds) {
  function Mash() {
    let n = 0xefc8249d;

    const mash = (data) => {
      data = data.toString();
      for (let i = 0; i < data.length; i++) {
        n += data.charCodeAt(i);
        let h = 0.02519603282416938 * n;
        n = h >>> 0;
        h -= n;
        h *= n;
        n = h >>> 0;
        h -= n;
        n += h * 0x100000000; // 2^32
      }
      return (n >>> 0) * 2.3283064365386963e-10; // 2^-32
    };

    mash.version = 'Mash 0.9';
    return mash;
  }

  let s0 = 0;
  let s1 = 0;
  let s2 = 0;
  let c = 1;
  if (seeds.length === 0) {
    seeds = [+new Date];
  }
  let mash = Mash();
  s0 = mash(' ');
  s1 = mash(' ');
  s2 = mash(' ');

  for (let i = 0; i < seeds.length; i++) {
    s0 -= mash(seeds[i]);
    if (s0 < 0) {
      s0 += 1;
    }
    s1 -= mash(seeds[i]);
    if (s1 < 0) {
      s1 += 1;
    }
    s2 -= mash(seeds[i]);
    if (s2 < 0) {
      s2 += 1;
    }
  }
  mash = null;

  const random = () => {
    const t = (2091639 * s0) + (c * 2.3283064365386963e-10); // 2^-32
    s0 = s1;
    s1 = s2;
    return s2 = t - (c = t | 0);
  };

  random.uint32 = () => random() * 0x100000000; // 2^32
  random.fract53 = () => random() +
        ((random() * 0x200000 | 0) * 1.1102230246251565e-16); // 2^-53

  random.version = 'Alea 0.9';
  random.args = seeds;
  return random;
}

// options:
// - seeds: an array
//   whose items will be `toString`ed and used as the seed to the Alea
//   algorithm
export default class AleaRandomGenerator extends RandomGenerator {
  constructor ({ seeds = [] } = {}) {
    super();
    if (!seeds) {
      throw new Error('No seeds were provided for Alea PRNG');
    }
    this.alea = Alea(seeds);
  }

  /**
   * @name Random.fraction
   * @summary Return a number between 0 and 1, like `Math.random`.
   * @locus Anywhere
   */
  fraction () {
    return this.alea();
  }
}