const fromHexString = hexString =>
  new Uint8Array(hexString.match(/.{1,2}/g).map(byte => parseInt(byte, 16)));

const toHexString = bytes =>
  bytes.reduce((str, byte) => str + byte.toString(16).padStart(2, '0'), '');

onmessage = async (e) => {
  const seed = fromHexString(e.data.seedHex);
  const target = e.data.target;

  const fullBuf = new ArrayBuffer(seed.byteLength*2);

  const fullBufSeed = new Uint8Array(fullBuf, 0, seed.byteLength);
  seed.forEach((v, i) => fullBufSeed[i] = v);

  const randBuf = new Uint8Array(fullBuf, seed.byteLength);

  while (true) {
    crypto.getRandomValues(randBuf);
    const digest = await crypto.subtle.digest('SHA-512', fullBuf);
    const digestView = new DataView(digest);
    if (digestView.getUint32(0) < target) {
      postMessage(toHexString(randBuf));
      return;
    }
  }

};