Seeding the random number generator in Javascript Ask Question

No, it is not possible to seed Math.random(). The ECMAScript specification is intentionally vague on the subject, providing no means for seeding nor require that browsers even use the same algorithm. So such a function must be externally provided, which thankfully isn't too difficult.

I've implemented a number of good, short and fast Pseudorandom number generator (PRNG) functions in plain JavaScript. All of them can be seeded and provide high quality numbers. These are not intended for security purposes--if you need a seedable CSPRNG, look into ISAAC.

First of all, take care to initialize your PRNGs properly. To keep things simple, the generators below have no built-in seed generating procedure, but accept one or more 32-bit numbers as the initial seed state of the PRNG. Similar or sparse seeds (e.g. a simple seed of 1 and 2) have low entropy, and can cause correlations or other randomness quality issues, sometimes resulting in the output having similar properties (such as randomly generated levels being similar). To avoid this, it is best practice to initialize PRNGs with a well-distributed, high entropy seed and/or advancing past the first 15 or so numbers.

There are many ways to do this, but here are two methods. Firstly, hash functions are very good at generating seeds from short strings. A good hash function will generate very different results even when two strings are similar, so you don't have to put much thought into the string. Here's an example hash function:

function cyrb128(str) {
    let h1 = 1779033703, h2 = 3144134277,
        h3 = 1013904242, h4 = 2773480762;
    for (let i = 0, k; i < str.length; i++) {
        k = str.charCodeAt(i);
        h1 = h2 ^ Math.imul(h1 ^ k, 597399067);
        h2 = h3 ^ Math.imul(h2 ^ k, 2869860233);
        h3 = h4 ^ Math.imul(h3 ^ k, 951274213);
        h4 = h1 ^ Math.imul(h4 ^ k, 2716044179);
    }
    h1 = Math.imul(h3 ^ (h1 >>> 18), 597399067);
    h2 = Math.imul(h4 ^ (h2 >>> 22), 2869860233);
    h3 = Math.imul(h1 ^ (h3 >>> 17), 951274213);
    h4 = Math.imul(h2 ^ (h4 >>> 19), 2716044179);
    h1 ^= (h2 ^ h3 ^ h4), h2 ^= h1, h3 ^= h1, h4 ^= h1;
    return [h1>>>0, h2>>>0, h3>>>0, h4>>>0];
}
// Side note: Only designed & tested for seed generation,
// may be suboptimal as a general 128-bit hash.

// Create cyrb128 state:
var seed = cyrb128("apples");
// Four 32-bit component hashes provide the seed for sfc32.
var rand = sfc32(seed[0], seed[1], seed[2], seed[3]);

// Or... only one 32-bit component hash is needed for splitmix32.
var rand = splitmix32(seed[0]);

// You can now generate repeatable sequences of random numbers:
rand(); // 0.8865117691457272
rand(); // 0.24518639338202775

var seed = 1337 ^ 0xDEADBEEF; // 32-bit seed with optional XOR value
// Pad seed with Phi, Pi and E.
// https://en.wikipedia.org/wiki/Nothing-up-my-sleeve_number
var rand = sfc32(0x9E3779B9, 0x243F6A88, 0xB7E15162, seed);
for (var i = 0; i < 15; i++) rand();

function sfc32(a, b, c, d) {
  return function() {
    a |= 0; b |= 0; c |= 0; d |= 0;
    let t = (a + b | 0) + d | 0;
    d = d + 1 | 0;
    a = b ^ b >>> 9;
    b = c + (c << 3) | 0;
    c = (c << 21 | c >>> 11);
    c = c + t | 0;
    return (t >>> 0) / 4294967296;
  }
}

const seedgen = () => (Math.random()*2**32)>>>0;
const getRand = sfc32(seedgen(), seedgen(), seedgen(), seedgen());
for(let i=0; i<10; i++) console.log(getRand());

function splitmix32(a) {
 return function() {
   a |= 0;
   a = a + 0x9e3779b9 | 0;
   let t = a ^ a >>> 16;
   t = Math.imul(t, 0x21f0aaad);
   t = t ^ t >>> 15;
   t = Math.imul(t, 0x735a2d97);
   return ((t = t ^ t >>> 15) >>> 0) / 4294967296;
  }
}

const prng = splitmix32((Math.random()*2**32)>>>0)
for(let i=0; i<10; i++) console.log(prng());

function mulberry32(a) {
  return function() {
    let t = a += 0x6D2B79F5;
    t = Math.imul(t ^ t >>> 15, t | 1);
    t ^= t + Math.imul(t ^ t >>> 7, t | 61);
    return ((t ^ t >>> 14) >>> 0) / 4294967296;
  }
}

const getRand = mulberry32((Math.random()*2**32)>>>0)
for(let i=0; i<10; i++) console.log(getRand());

function xoshiro128ss(a, b, c, d) {
  return function() {
    let t = b << 9, r = b * 5;
    r = (r << 7 | r >>> 25) * 9;
    c ^= a;
    d ^= b;
    b ^= c;
    a ^= d;
    c ^= t;
    d = d << 11 | d >>> 21;
    return (r >>> 0) / 4294967296;
  }
}

const seedgen = () => (Math.random()*2**32)>>>0;
const getRand = xoshiro128ss(seedgen(), seedgen(), seedgen(), seedgen());
for (let i = 0; i < 10; i++) console.log(getRand());

function jsf32(a, b, c, d) {
  return function() {
    a |= 0; b |= 0; c |= 0; d |= 0;
    let t = a - (b << 27 | b >>> 5) | 0;
    a = b ^ (c << 17 | c >>> 15);
    b = c + d | 0;
    c = d + t | 0;
    d = a + t | 0;
    return (d >>> 0) / 4294967296;
  }
}

const seedgen = () => (Math.random()*2**32)>>>0;
const getRand = jsf32(seedgen(), seedgen(), seedgen(), seedgen());
for (let i = 0; i < 10; i++) console.log(getRand());