The HyperLogLog implementation in Redis is extremely well documented and amazingly quick. It drops down to non-probabilistic counting for smaller sample sizes, providing 100% accuracy with comparable speeds. As a fun exercise, which I wrote about [0], I estimated the number of unique words throughout the major works of Charles Dickens. Side note, that man had an amazing hairdo [1].

[0] https://blog.codeship.com/counting-distinct-values-with-hype... [1] https://upload.wikimedia.org/wikipedia/commons/a/aa/Dickens_...