code: func StrLenOfInt64Fast(x int64) int {
  	size := 0
  	if x < 0 {
  		size = 1 // add "-" sign on the length count
  	}
  	return size + StrLenOfUint64Fast(uint64(abs(x)))
  }
docs: // StrLenOfInt64Fast efficiently calculate the string character lengths of an int64 as input