Towards an JS xor128

script.js

// This is half the maximum `Uint32Array` length, which I need to
// scale the output down to between 0 and 1.
// https://github.com/stdlib-js/constants-int32-max
const INT32_MAX = 2147483647

// https://en.wikipedia.org/wiki/Xorshift
export function xor128(seed = Math.random()) {
  // Using the djb2 hashing function to initialise state.
  // http://www.cse.yorku.ca/~oz/hash.html
  const state = new Uint32Array(4).fill(5381)
  const x = String(seed)

  for (let i = 0; i < x.length; i += 1) {
    const j = i % state.length

    state[j] = Math.imul(state[j], 33) + x.charCodeAt(i)
  }

  return () => {
    const [s,,,a] = state
    const b = a ^ (a << 11)
    const t = b ^ (b >> 8)

    state[3] = state[2]
    state[2] = state[1]
    state[1] = s
    state[0] = t ^ s ^ (s >> 19)

    // Fit result between 0 and 1.
    return state[0] / INT32_MAX
  }
}