On the Communication Complexity of Shortcut Set Computation
A recent breakthrough of Kogan and Parter (SODA '22) provided the first polynomial improvement in decades on the size/diameter tradeoff available for shortcut sets. However, algorithmic applications of shortcut sets have mostly not taken advantage of these new bounds. A barrier is that many shortcut set applications occur in \emph{decentralized} settings, and we generally lack decentralized algorithms implementing the Kogan-Parter bounds. To this end, we study the \emph{communication complexity} of constructing Kogan-Parter shortcut sets. The edges of an $n$-node directed graph are split between two players, Alice and Bob, who are also given a target diameter $D$. Our main result is a protocol that, for $D \le n^{1/3}$, allows Alice and Bob to agree on a shortcut set matching the Kogan-Parter tradeoff between size and diameter, while exchanging only $\Oish(n^2 / D^2)$ bits of communication. We also provide (non-matching) lower bounds on the communication complexity required to find a correct $D$-diameter shortcut set of any size. The main technical ingredient for the upper bounds is a low-communication protocol for finding a \emph{path cover}, a common subroutine in shortcut set constructions that has attracted some independent interest. Our protocol crucially avoids applications of Menger's theorem or antichain decompositions, as often done in prior work. On the lower bounds side, we relate the communication complexity of the shortcut set problem to the size of bridge-free path systems.