# A Framework for Similarity Search with Space-Time Tradeoffs using Locality Sensitive Filtering

Tobias Lybecker Christiani

Publikation: Konference artikel i Proceeding eller bog/rapport kapitelKonferencebidrag i proceedingsForskningpeer review

## Abstrakt

We present a framework for similarity search based on Locality-Sensitive Filtering~(LSF),generalizing the Indyk-Motwani (STOC 1998) Locality-Sensitive Hashing~(LSH) framework to support space-time tradeoffs. Given a family of filters, defined as a distribution over pairs of subsets of space that satisfies certain locality-sensitivity properties, we can construct a dynamic data structure that solves the approximate near neighbor problem in $d$-dimensional space with query time $dn^{\rho_q + o(1)}$, update time $dn^{\rho_u + o(1)}$, and space usage $dn + n^{1 + \rho_u + o(1)}$ where $n$ denotes the number of points in the data structure.The space-time tradeoff is tied to the tradeoff between query time and update time (insertions/deletions), controlled by the exponents $\rho_q, \rho_u$ that are determined by the filter family. \\ Locality-sensitive filtering was introduced by Becker et al. (SODA 2016) together with a framework yielding a single, balanced, tradeoff between query time and space, further relying on the assumption of an efficient oracle for the filter evaluation algorithm.We extend the LSF framework to support space-time tradeoffs and through a combination of existing techniques we remove the oracle assumption. \\Laarhoven (arXiv 2015), building on Becker et al., introduced a family of filters with space-time tradeoffs for the high-dimensional unit sphere under inner product similarity and analyzed it for the important special case of random data.We show that a small modification to the family of filters gives a simpler analysis that we use, together with our framework, to provide guarantees for worst-case data. Through an application of Bochner's~Theorem from harmonic analysis by Rahimi \& Recht (NIPS 2007), we are able to extend our solution on the unit sphere to $\real^d$ under the class of similarity measures corresponding to real-valued characteristic functions.For the characteristic functions of $s$-stable distributions we obtain a solution to the $(r, cr)$-near neighbor problem in $\ell_s^d$-spaces with query and update exponents $\rho_q = \frac{c^s (1+\lambda)^2}{(c^s + \lambda)^2}$ and $\rho_u = \frac{c^s (1-\lambda)^2}{(c^s + \lambda)^2}$where $\lambda \in [-1,1]$ is a tradeoff parameter. This result improves upon the space-time tradeoff of Kapralov (PODS 2015) and is shown to be optimal in the case of a balanced tradeoff, matching the LSH lower bound by O'Donnell et al.~\mbox{(ITCS 2011)} and a similar LSF lower bound proposed in this paper.Finally, we show a lower bound for the space-time tradeoff on the unit sphere that matches Laarhoven's and our own upper bound in the case of random data.
Originalsprog Engelsk Proceedings of the Twenty-Eighth Annual ACM-SIAM Symposium on Discrete Algorithms Society for Industrial and Applied Mathematics 2017 31-46 978-1-61197-478-2 https://doi.org/10.1137/1.9781611974782.3 Udgivet - 2017

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