Universal Graph Compression: Stochastic Block Models

Alankrita Bhatt, Ziao Wang, Chi Wang, Lele Wang

Abstract: Motivated by the prevalent data science applications of processing large-scale graph data such as social networks, web graphs, and biological networks, as well as the high I/O and communication costs of storing and transmitting such data, this paper investigates universal compression of data appearing in the form of a labeled graph. In particular, we consider a widely used random graph model, stochastic block model (SBM), which captures the clustering effects in social networks. A universal graph compressor is proposed, which achieves the optimal compression rate for a wide family of SBMs with edge probabilities from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$O$</tex> (1) to Ω(1/ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$n$</tex> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-∊</sup> ) for any 0 < ∊ < 1. Existing universal compression techniques are developed mostly for stationary ergodic one-dimensional sequences with entropy linear in the number of variables. However, the adjacency matrix of SBM has complex two-dimensional correlations and sublinear entropy in the sparse regime. These challenges are alleviated through a carefully designed transform that converts two-dimensional correlated data into almost i.i.d. blocks. The blocks are then compressed by a Krichevsky-Trofimov compressor, whose length analysis is generalized to arbitrarily correlated processes with identical marginals.