// prep-work
const _data = new Uint8Array(16);
const _hex: string[] = [];
for (let i = 0; i < 256; i++) {
  _hex.push(i.toString(16).padStart(2, '0'));
}

// todo@jrieken
// 1. node nodejs use`crypto#randomBytes`, see: https://nodejs.org/docs/latest/api/crypto.html#crypto_crypto_randombytes_size_callback
let _fillRandomValues: (bucket: Uint8Array) => Uint8Array;

declare const crypto:
  | undefined
  | { getRandomValues(data: Uint8Array): Uint8Array };

if (
  typeof crypto === 'object' &&
  typeof crypto.getRandomValues === 'function'
) {
  // browser
  _fillRandomValues = crypto.getRandomValues.bind(crypto);
} else {
  _fillRandomValues = function (bucket: Uint8Array): Uint8Array {
    for (let i = 0; i < bucket.length; i++) {
      bucket[i] = Math.floor(Math.random() * 256);
    }
    return bucket;
  };
}

export function generateUuid(): string {
  // get data
  _fillRandomValues(_data);

  // set version bits
  _data[6] = (_data[6] & 0x0f) | 0x40;
  _data[8] = (_data[8] & 0x3f) | 0x80;

  // print as string
  let i = 0;
  let result = '';
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += '-';
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += '-';
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += '-';
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += '-';
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  result += _hex[_data[i++]];
  return result;
}