firetruss/src/KeyGenerator.js

const ALPHABET = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz';

const getRandomValues = window.crypto && window.crypto.getRandomValues &&
  window.crypto.getRandomValues.bind(window.crypto);

export default class KeyGenerator {
  constructor() {
    this._lastUniqueKeyTime = 0;
    this._lastRandomValues = [];
  }

  generateUniqueKey(now) {
    now = now || Date.now();
    const chars = new Array(20);
    let prefix = now;
    for (let i = 7; i >= 0; i--) {
      chars[i] = ALPHABET.charAt(prefix & 0x3f);  // eslint-disable-line no-bitwise
      prefix = Math.floor(prefix / 64);
    }
    if (now === this._lastUniqueKeyTime) {
      let i = 11;
      while (i >= 0 && this._lastRandomValues[i] === 63) {
        this._lastRandomValues[i] = 0;
        i -= 1;
      }
      if (i === -1) {
        throw new Error('Internal assertion failure: ran out of unique IDs for this millisecond');
      }
      this._lastRandomValues[i] += 1;
    } else {
      this._lastUniqueKeyTime = now;
      if (getRandomValues) {
        /* global Uint8Array */
        const array = new Uint8Array(12);
        getRandomValues(array);
        for (let i = 0; i < 12; i++) {
          // eslint-disable-next-line no-bitwise
          this._lastRandomValues[i] = array[i] & (i ? 0x3f : 0x0f);
        }
      } else {
        for (let i = 0; i < 12; i++) {
          // Make sure to leave some space for incrementing in the top nibble.
          this._lastRandomValues[i] = Math.floor(Math.random() * (i ? 64 : 16));
        }
      }
    }
    for (let i = 0; i < 12; i++) {
      chars[i + 8] = ALPHABET[this._lastRandomValues[i]];
    }
    return chars.join('');
  }
}