code: func RandStringBytesMaskImprSrc(n int) string {
  	const letterBytes = "abcdefghijklmnopqrstuvwxyz0123456789"
  	const (
  		letterIdxBits = 6                    // 6 bits to represent a letter index
  		letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
  		letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
  	)
  	src := rand.NewSource(time.Now().UnixNano())
  	b := make([]byte, n)
  	// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
  	for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
  		if remain == 0 {
  			cache, remain = src.Int63(), letterIdxMax
  		}
  		if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
  			b[i] = letterBytes[idx]
  			i--
  		}
  		cache >>= letterIdxBits
  		remain--
  	}
  
  	return string(b)
  }
docs: // from https://stackoverflow.com/questions/22892120/how-to-generate-a-random-string-of-a-fixed-length-in-golang