The algorithm is simple, which is why it is also suitable for textbooks. However, it is far from representing the state of the art. In case you are interested in the latest development, I have recently published a new distinct counting algorithm where the estimate does not depend on the processing order, which can be merged and requires only 3.5kB of memory to guarantee a standard error of 1.13% for counts up to exa-scale: https://arxiv.org/abs/2402.13726