random / seeding / shuffle

[inoas]

Caveat: This will rely on default args for good ergonomics

1. list.shuffle should take an optional callback that can be used instead statically relying on float.random(0.0, 1.0).
2. Add an optional arg to float.random and int.random to make it easier to generate slower but better values across the Erlang and Javascript by relying on
   • Erlang: https://www.erlang.org/doc/man/crypto.html#strong_rand_bytes-1
   • JavaScript https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
3. Add random_float and random_int functions to gleam/erlang and maybe gleam/javascript that allow to set a seeder and even constant seed values, for Erlang see: https://www.erlang.org/doc/man/rand.html#description -
   It seems https://www.erlang.org/doc/man/rand.html#seed-1 can use the value of https://www.erlang.org/doc/man/rand.html#export_seed-0 to capture and then return to the previous global state?

[NicklasXYZ]

This is kind of the reason why I ended up implementing my own PRNG as it's then also possible ensure consistency across the targets: https://github.com/nicklasxyz/gleam_stats/blob/v1.1.0/src/gleam_stats/generators.gleam#L166-L166

Alg: https://www.pcg-random.org/

Aligning the functionality across the two platforms becomes a bit of a mess otherwise.

[spicychickensauce]

In case someone finds this looking for a solution to seed int.random and float.random without passing around seed values, I leave my solution here. I need it only for the JS target, but for erlang it would be much easier, e.g. just call rand.seed. For JS I monkey patched Math.random. Of course this is quite hacky, but I think totally acceptable for tests.

pub fn foo_test() {
  seed("whatever")
  // randomness is now deterministic
}

@external(javascript, "./helpers.js", "seed")
pub fn seed(str: String) -> Nil

// Mostly copied from: https://stackoverflow.com/a/47593316
export function seed(str) {
  const [a, b, c, d] = cyrb128(str);
  const rand = sfc32(a, b, c, d);

  // advance state a bit
  for (var i = 0; i < 15; i++) rand();

  Math.random = function () {
    return 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;
  };
}

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];
}