Testing Pattern-Freeness

Understanding the local behaviour of structured multi-dimensional data is a fundamental problem in various areas of computer science. As the amount of data is often huge, it is desirable to obtain sublinear time algorithms, and specifically property testers, to understand local properties of the data. We focus on the natural local problem of testing pattern freeness: given a large d-dimensional array A and a fixed d-dimensional pattern P over a finite alphabet Γ, we say that A is P -free if it does not contain a copy of the forbidden pattern P as a consecutive subarray. The distance of A to P -freeness is the fraction of the entries of A that need to be modified to make it P -free. For any ∈ [0, 1] and any large enough pattern P over any alphabet – other than a very small set of exceptional patterns – we design a tolerant tester that distinguishes between the case that the distance is at least and the case that the distance is at most ad , with query complexity and running time cd −1, where ad < 1 and cd depend only on the dimension d. The testers only need to access uniformly random blocks of samples from the input A. To analyze the testers we establish several combinatorial results, including the following ddimensional modification lemma, which might be of independent interest: for any large enough d-dimensional pattern P over any alphabet (excluding a small set of exceptional patterns for the binary case), and any d-dimensional array A containing a copy of P , one can delete this copy by modifying one of its locations without creating new P -copies in A. Our results address an open question of Fischer and Newman, who asked whether there exist efficient testers for properties related to tight substructures in multi-dimensional structured data. They serve as a first step towards a general understanding of local properties of multidimensional arrays, as any such property can be characterized by a fixed family of forbidden patterns.