Parallel Approximate Undirected Shortest Paths Via Low Hop Emulators

We present a $(1+\varepsilon)$-approximate parallel algorithm for computing shortest paths in undirected graphs, achieving $\mathrm{poly}(\log n)$ depth and $m\mathrm{poly}(\log n)$ work for $n$-nodes $m$-edges graphs. Although sequential algorithms with (nearly) optimal running time have been known for several decades, near-optimal parallel algorithms have turned out to be a much tougher challenge. For $(1+\varepsilon)$-approximation, all prior algorithms with $\mathrm{poly}(\log n)$ depth perform at least $Ω(mn^{c})$ work for some constant $c>0$. Improving this long-standing upper bound obtained by Cohen (STOC'94) has been open for $25$ years. We develop several new tools of independent interest. One of them is a new notion beyond hopsets --- low hop emulator --- a $\mathrm{poly}(\log n)$-approximate emulator graph in which every shortest path has at most $O(\log\log n)$ hops (edges). Direct applications of the low hop emulators are parallel algorithms for $\mathrm{poly}(\log n)$-approximate single source shortest path (SSSP), Bourgain's embedding, metric tree embedding, and low diameter decomposition, all with $\mathrm{poly}(\log n)$ depth and $m\mathrm{poly}(\log n)$ work. To boost the approximation ratio to $(1+\varepsilon)$, we introduce compressible preconditioners and apply it inside Sherman's framework (SODA'17) to solve the more general problem of uncapacitated minimum cost flow (a.k.a., transshipment problem). Our algorithm computes a $(1+\varepsilon)$-approximate uncapacitated minimum cost flow in $\mathrm{poly}(\log n)$ depth using $m\mathrm{poly}(\log n)$ work. As a consequence, it also improves the state-of-the-art sequential running time from $m\cdot 2^{O(\sqrt{\log n})}$ to $m\mathrm{poly}(\log n)$.