Seedable Xorshift128 PRNG

prng-xorshift128.js

// RNG = Math.random
RNG = xorshift128

function nonsense() {
  // if all you need 
  // is a random seed
  // of type string 
  // then this thing
  // does the deed, hey 
  xorshift128_seed(Date.now() + '');
  const sample = (a) => a[Math.floor(RNG() * a.length)];
  const tok = x => () => sample(x.split(' '));
  const title = s => s.substring(0, 1).toUpperCase() + s.substring(1);
  const vow = tok('a aa e ee i o oo au eu');
  const con1 = tok('b d f k l n p r z');
  const con2 = tok('k l r s t');
  const syl = () => con1() + vow() + sample([con2(), '']);
  const word = () => [...$G.loop(1 + irand(3), syl)].join('');
  return title([...$G.loop(2 + irand(2), word)].join(' '));
}

// TODO: test against ALEA generator
const xs_state = Uint32Array.of(0xC9A5, 0x5996, 0x5696, 0x9A33); // must not be all zero
function xorshift128() {
  let s, t = xs_state[3];
  xs_state[3] = xs_state[2];
  xs_state[2] = xs_state[1];
  xs_state[1] = s = xs_state[0];
  t ^= t << 11;
  t ^= t >>> 8;
  xs_state[0] = t ^ s ^ (s >>> 19);
  return xs_state[0] / 0x100000000;
}

function xorshift128_seed(seed_str) {
  const FNV_prime = 16777619;
  xs_state.fill(2166136261);
  for (let i = 0; i < seed_str.length; i++) {
    const j = i & 3;
    xs_state[j] ^= seed_str.charCodeAt(i);
    xs_state[j] *= FNV_prime; 
  }
}