
arXiv: 2512.00314
In this work, we study the fundamental problems of counting and sampling traces that a regular language touches. Formally, one fixes the alphabet Σ and an independence relation I ⊆ Σ × Σ. The computational problems we address take as input a regular language L over Σ, presented as a finite automaton with m states, together with a natural number n (presented in unary). For the counting problem, the output is the number of Mazurkiewicz traces (induced by I) that intersect the n th slice L n = L ∩ Σ n of L , i.e., traces that have at least one linearization in L n . For the sampling problem, the output is a trace drawn from a distribution that is approximately uniform over all such traces. These problems are motivated by applications such as bounded model checking based on partial-order reduction, where an a priori estimate of the size of the state space can significantly improve usability, as well as testing approaches for concurrent programs that use partial-order-aware random sampling, where uniform exploration is desirable for effective bug detection. We first show that the counting problem is #P-hard even when the automaton accepting the language L is deterministic, which is in sharp contrast to the corresponding problem for counting the words of a DFA, which is solvable in polynomial time. We then show that the counting problem remains in the class #P for both NFAs and DFAs, independent of whether L is trace-closed. Finally, our main contributions are a fully polynomial-time randomized approximation scheme (FPRAS) that, with high probability, estimates the desired count within a specified accuracy parameter, and a fully polynomial-time almost uniform sampler (FPAUS) that generates traces while ensuring that the distribution induced on them is approximately uniform with high probability.
FOS: Computer and information sciences, Computational Complexity, Logic in Computer Science, Formal Languages and Automata Theory (cs.FL), Programming Languages, Computational Complexity (cs.CC), Formal Languages and Automata Theory, Logic in Computer Science (cs.LO), Programming Languages (cs.PL)
FOS: Computer and information sciences, Computational Complexity, Logic in Computer Science, Formal Languages and Automata Theory (cs.FL), Programming Languages, Computational Complexity (cs.CC), Formal Languages and Automata Theory, Logic in Computer Science (cs.LO), Programming Languages (cs.PL)
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