Out-of-Order Membership to Regular Languages
Antoine Amarilli, Sebastien Labbe, Charles Paperman
TL;DR
The paper studies out-of-order membership for fixed regular languages and its algebraic analogue, out-of-order evaluation on finite monoids and semigroups. It introduces a robust algebraic framework and deriving tight space bounds: a complete space-trichotomy for out-of-order monoid evaluation (constant space for $M\in\mathbf{Com}$, $\Theta(\log n)$ space for $M\in\mathbf{FL}\lor\mathbf{Com}$, and $\Theta(n)$ space otherwise) and constant-space results for semigroups in $\mathbf{Li}\lor\mathbf{Com}$ (with $\Theta(\log n)$ lower bounds outside this class). The work also provides practical upper and lower bounds via fooling sets and a first-last (FL) subword methodology, plus illustrative examples and partial results for more complex complexity regimes (including $\Theta(n)$ and $O(\sqrt{n})$ gaps). Overall, the results illuminate how the algebraic structure of the target language or semigroup governs streaming space complexity under adversarial reveal orders, with implications for distributed processing, networking, and pattern-detection in out-of-order data streams.
Abstract
We introduce the task of out-of-order membership to a formal language L, where the letters of a word w are revealed one by one in an adversarial order. The length |w| is known in advance, but the content of w is streamed as pairs (i, w[i]), received exactly once for each position i, in arbitrary order. We study efficient algorithms for this task when L is regular, seeking tight complexity bounds as a function of |w| for a fixed target language. Most of our results apply to an algebraically defined variant dubbed out-of-order evaluation: this problem is defined for a fixed finite monoid or semigroup S, and our goal is to compute the ordered product of the streamed elements of w. We show that, for any fixed regular language or finite semigroup, both problems can be solved in constant time per streamed symbol and in linear space. However, the precise space complexity strongly depends on the algebraic structure of the target language or evaluation semigroup. Our main contributions are therefore to show (deterministic) space complexity characterizations, which we do for out-of-order evaluation of monoids and semigroups. For monoids, we establish a trichotomy: the space complexity is either Θ(1), Θ(log n), or Θ(n), where n = |w|. More specifically, the problem admits a constant-space solution for commutative monoids, while all non-commutative monoids require Ω(log n) space. We further identify a class of monoids admitting an O(log n)-space algorithm, and show that all remaining monoids require Ω(n) space. For general semigroups, the situation is more intricate. We characterize a class of semigroups admitting constant-space algorithms for out-of-order evaluation, and show that semigroups outside this class require at least Ω(log n) space.