A General Technique for Searching in Implicit Sets via Function Inversion

Abstract: Given a function $f$ from the set $[N]$ to a $d$-dimensional integer grid, we consider data structures that allow efficient orthogonal range searching queries in the image of $f$, without explicitly storing it. We show that, if $f$ is of the form $[N]\to [2^{w}]^d$ for some $w=\mathrm{polylog} (N)$ and is computable in constant time, then, for any $0<\alpha <1$, we can obtain a data structure using $\tilde {O}(N^{1-\alpha / 3})$ words of space such that, for a given $d$-dimensional axis-aligned box $B$, we can search for some $x\in [N]$ such that $f(x) \in B$ in time $\tilde{O}(N^{\alpha})$. This result is obtained simply by combining integer range searching with the Fiat-Naor function inversion scheme, which was already used in data-structure problems previously. We further obtain - data structures for range counting and reporting, predecessor, selection, ranking queries, and combinations thereof, on the set $f([N])$, - data structures for preimage size and preimage selection queries for a given value of $f$, and - data structures for selection and ranking queries on geometric quantities computed from tuples of points in $d$-space. These results unify and generalize previously known results on 3SUM-indexing and string searching, and are widely applicable as a black box to a variety of problems. In particular, we give a data structure for a generalized version of gapped string indexing, and show how to preprocess a set of points on an integer grid in order to efficiently compute (in sublinear time), for points contained in a given axis-aligned box, their Theil-Sen estimator, the $k$th largest area triangle, or the induced hyperplane that is the $k$th furthest from the origin.