prng-bytes.js

const { ceil, random } = Math

const BITS_PER_BYTE = 8

const endianByteNum = (() => {
  const buf32 = new Uint32Array(1)
  const buf8 = new Uint8Array(buf32.buffer)
  buf32[0] = 0xff
  return (buf8[0] === 0xff) ? [2, 3, 4, 5, 6, 7] : [0, 1, 2, 3, 6, 7]
})()

const prngBytes = (count) => {
  const BYTES_USED_PER_RANDOM_CALL = 6
  const randCount = ceil(count / BYTES_USED_PER_RANDOM_CALL)

  const buffer = new ArrayBuffer(count)
  const dataView = new DataView(new ArrayBuffer(BITS_PER_BYTE))
  for (let rNum = 0; rNum < randCount; rNum += 1) {
    dataView.setFloat64(0, random())
    for (let n = 0; n < BYTES_USED_PER_RANDOM_CALL; n += 1) {
      const fByteNum = endianByteNum[n]
      const bByteNum = (rNum * BYTES_USED_PER_RANDOM_CALL) + n
      if (bByteNum < count) {
        buffer[bByteNum] = dataView.getUint8(fByteNum)
      }
    }
  }
  return buffer
}

module.exports = {
  prngBytes
}