# A “learned” approach to quicken and compress rank/select dictionaries

Antonio Boffa,
Paolo Ferragina,
Giorgio Vinciguerra

January, 2021

### Abstract

We address the well-known problem of designing, implementing and experimenting compressed data structures for supporting $\textsf{rank}$ and $\textsf{select}$ queries over a dictionary of integers. This problem has been studied far and wide since the end of the ‘80s with tons of important theoretical and practical results.

Following a recent line of research on the so-called learned data structures, we first show that this problem has a surprising connection with the geometry of a set of points in the Cartesian plane suitably derived from the input integers. We then build upon some classical results in computational geometry to introduce the first “learned” scheme for implementing a compressed rank/select dictionary. We prove theoretical bounds on its time and space performance both in the worst case and in the case of input distributions with finite mean and variance.

We corroborate these theoretical results with a large set of experiments over datasets originating from a variety of sources and applications (Web, DNA sequencing, information retrieval and natural language processing), and we show that a carefully engineered version of our approach provides new interesting space-time trade-offs with respect to several well-established implementations of Elias-Fano, RRR-vector, and random-access vectors of Elias $\gamma$/$\delta$-coded gaps.

Publication

Proceedings of the 23rd SIAM Symposium on Algorithm Engineering and Experiments (ALENEX)