Something important to note is that the authors use normalized compression distance (NCD). NCD is a way to approximate Kolmogorov complexity. This is a pretty

Something important to note is that the authors use normalized compression distance (NCD). NCD is a way to approximate Kolmogorov complexity. This is a pretty old idea, see [1,2]. Old but still very useful, like perceptrons. [1] Li and Vitanyi. An Introduction to Kolmogorov Complexity and Its Applications. [2] Clustering by compression. https://arxiv.org/pdf/cs/0312044 https://arxiv.org/pdf/cs/0312044