Enumerating Regular Languages with Bounded Delay
Résumé
We study the task, for a given language $L$, of enumerating the (generally
infinite) sequence of its words, without repetitions, while bounding the
delay between two consecutive words. To allow for delay bounds that do not depend on the current word length,
we assume a model where we produce each word by editing the preceding word with
a small edit script, rather than writing out the word from scratch. In
particular, this witnesses that the language is orderable, i.e., we can
write its words as an infinite sequence such that the Levenshtein edit distance
between any two consecutive words is bounded by a value that depends only on the language. For instance,
$(a+b)^*$ is orderable (with a variant of the Gray code), but $a^* + b^*$ is
not.
We characterize which regular languages are enumerable in this sense, and
show that this can be decided in PTIME in an input deterministic finite
automaton (DFA) for the language. In fact, we show that, given a DFA $A$,
we can compute in PTIME automata $A_1, \ldots, A_t$ such that $L(A)$ is partitioned
as $L(A_1) \sqcup \ldots \sqcup L(A_t)$ and every $L(A_i)$ is orderable in
this sense. Further, we show that the value of $t$ obtained is optimal, i.e., we cannot partition $L(A)$
into less than $t$ orderable languages.
In the case where $L(A)$ is orderable (i.e., $t=1$), we show that the ordering can be
produced by a bounded-delay algorithm: specifically, the algorithm runs in a
suitable pointer machine model, and produces a sequence of bounded-length edit
scripts to visit the words of $\L(A)$ without repetitions, with bounded delay -- exponential in $|A|$ --
between each script. In fact, we show that we can achieve this while only
allowing the edit operations push and pop at the beginning and
end of the word, which implies that the word can in fact be maintained in
a double-ended queue.
By contrast, when fixing the distance bound $d$ between consecutive words
and the number of classes of the partition, it is NP-hard in the input DFA $A$
to decide if $L(A)$ is orderable in this sense, already for finite languages.
Last, we study the model where push-pop edits are only allowed at the end of
the word, corresponding to a case where the word is maintained on a stack.
We show that these operations are strictly weaker and that the slender
languages are precisely those that can be partitioned into finitely many
languages that are orderable in this sense. For the slender languages, we can again
characterize the minimal number of languages in the partition, and achieve
bounded-delay enumeration.