History-deterministic Parikh Automata

TL;DR

This paper introduces and studies history-deterministic Parikh automata, a class of automata with on-the-fly nondeterminism resolution, balancing expressiveness and desirable computational properties.

Contribution

It defines history-deterministic Parikh automata, analyzes their expressiveness, closure properties, and complexity, filling a gap between deterministic and nondeterministic automata.

Findings

More expressive than deterministic automata

Incomparable with unambiguous automata

Retain most closure properties of deterministic automata

Abstract

Parikh automata extend finite automata by counters that can be tested for membership in a semilinear set, but only at the end of a run. Thereby, they preserve many of the desirable properties of finite automata. Deterministic Parikh automata are strictly weaker than nondeterministic ones, but enjoy better closure and algorithmic properties. This state of affairs motivates the study of intermediate forms of nondeterminism. Here, we investigate history-deterministic Parikh automata, i.e., automata whose nondeterminism can be resolved on the fly. This restricted form of nondeterminism is well-suited for applications which classically call for determinism, e.g., solving games and composition. We show that history-deterministic Parikh automata are strictly more expressive than deterministic ones, incomparable to unambiguous ones, and enjoy almost all of the closure properties of deterministic automata. Finally, we investigate the complexity of resolving nondeterminism in history-deterministic Parikh automata.