Out-of-Order Membership in Regular Languages

TL;DR

This paper studies the complexity of recognizing regular languages and evaluating finite algebraic structures when input symbols are revealed in an adversarial, out-of-order manner, providing tight space bounds.

Contribution

It introduces the out-of-order membership problem and characterizes its space complexity for regular languages and algebraic structures like monoids and semigroups.

Findings

Constant space algorithms exist for certain classes of monoids and semigroups.

Non-commutative monoids require logarithmic space, while commutative ones require constant space.

The paper provides a detailed space complexity classification for out-of-order evaluation tasks.

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 {\Theta}(1), {\Theta}(log n), or {\Theta}(n), where n = |w|. More specifically, the problem admits a constant-space solution for commutative monoids, while all non-commutative monoids require {\Omega}(log n) space. We further identify a class of monoids admitting an O(log n)-space algorithm, and show that all remaining monoids require {\Omega}(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 {\Omega}(log n) space.