$(1+\epsilon)$-Approximate Shortest Paths in Dynamic Streams

TL;DR

This paper presents a new streaming algorithm for efficiently computing approximate shortest paths in dynamic graphs with significantly fewer passes and space, improving upon previous methods especially for unweighted graphs.

Contribution

It introduces novel dynamic streaming constructions of spanners and hopsets that enable constant-pass approximate shortest path computations for unweighted graphs and polylogarithmic passes for weighted graphs.

Findings

Achieves $(1+psilon)$-approximate shortest paths with O(n^{1+1/ppa}) space.

Uses constant passes for unweighted graphs, polylogarithmic for weighted graphs.

Extends to multi-source shortest paths with up to O(n^{1/ppa}) sources.

Abstract

Computing approximate shortest paths in the dynamic streaming setting is a fundamental challenge that has been intensively studied during the last decade. Currently existing solutions for this problem either build a sparse multiplicative spanner of the input graph and compute shortest paths in the spanner offline, or compute an exact single source BFS tree. Solutions of the first type are doomed to incur a stretch-space tradeoff of $2\kappa-1$ versus $n^{1+1/\kappa}$, for an integer parameter $\kappa$. (In fact, existing solutions also incur an extra factor of $1+\epsilon$ in the stretch for weighted graphs, and an additional factor of $\log^{O(1)}n$ in the space.) The only existing solution of the second type uses $n^{1/2 - O(1/\kappa)}$ passes over the stream (for space $O(n^{1+1/\kappa})$), and applies only to unweighted graphs. In this paper we show that $(1+\epsilon)$-approximate single-source shortest paths can be computed in this setting with $\tilde{O}(n^{1+1/\kappa})$ space using just \emph{constantly} many passes in unweighted graphs, and polylogarithmically many passes in weighted graphs (assuming $\epsilon$ and $\kappa$ are constant). Moreover, in fact, the same result applies for multi-source shortest paths, as long as the number of sources is $O(n^{1/\kappa})$. We achieve these results by devising efficient dynamic streaming constructions of $(1 + \epsilon, \beta)$-spanners and hopsets. We believe that these constructions are of independent interest.