code: func String(n int) string {
  	b := make([]byte, n)
  	rng.Lock()
  	defer rng.Unlock()
  
  	randomInt63 := rng.rand.Int63()
  	remaining := maxAlphanumsPerInt
  	for i := 0; i < n; {
  		if remaining == 0 {
  			randomInt63, remaining = rng.rand.Int63(), maxAlphanumsPerInt
  		}
  		if idx := int(randomInt63 & alphanumsIdxMask); idx < len(alphanums) {
  			b[i] = alphanums[idx]
  			i++
  		}
  		randomInt63 >>= alphanumsIdxBits
  		remaining--
  	}
  	return string(b)
  }
docs: // String generates a random alphanumeric string, without vowels, which is n
  // characters long.  This will panic if n is less than zero.
  // How the random string is created:
  // - we generate random int63's
  // - from each int63, we are extracting multiple random letters by bit-shifting and masking
  // - if some index is out of range of alphanums we neglect it (unlikely to happen multiple times in a row)