code: func base64NeededEncodedLength(n int) int {
  	// Returns -1 indicate the result will overflow.
  	if strconv.IntSize == 64 {
  		// len(arg)            -> len(to_base64(arg))
  		// 6827690988321067803 -> 9223372036854775804
  		// 6827690988321067804 -> -9223372036854775808
  		if n > 6827690988321067803 {
  			return -1
  		}
  	} else {
  		// len(arg)   -> len(to_base64(arg))
  		// 1589695686 -> 2147483645
  		// 1589695687 -> -2147483646
  		if n > 1589695686 {
  			return -1
  		}
  	}
  
  	length := (n + 2) / 3 * 4
  	return length + (length-1)/76
  }
docs: // base64NeededEncodedLength return the base64 encoded string length.