Path-Reporting Distance Oracles with Linear Size

Given an undirected weighted graph, an (approximate) distance oracle is a data structure that can (approximately) answer distance queries. A Path-Reporting Distance Oracle, or PRDO, is a distance oracle that must also return a path between the queried vertices. Given a graph on n vertices and an integer parameter k ≥ 1, Thorup and Zwick [22] showed a PRDO with stretch 2k − 1, size O(k · n^1+1/k) and query time O(k) (for the query time of PRDOs, we omit the time needed to report the path itself). Subsequent works [20, 7, 8] improved the size to O(n^1+1/k) and the query time to O(1). However, these improvements produce distance oracles which are not path-reporting. Several other works [12, 13] focused on small size PRDO for general graphs, but all known results on distance oracles with linear size suffer from polynomial stretch, polynomial query time, or not being path-reporting. In this paper we devise the first linear size PRDO with poly-logarithmic stretch and low query time O(log log n). More generally, for any integer k ≥ 1, we obtain a PRDO with stretch at most O(k^4.82), size O(n^1+1/k), and query time O(log k). In addition, we can make the size of our PRDO as small as n + o(n), at the cost of increasing the query time to poly-logarithmic. For unweighted graphs, we improve the stretch to O(k²). We also consider pairwise PRDO, which is a PRDO that is only required to answer queries from a given set of pairs P. An exact PRDO of size O(n + |P|²) and constant query time was provided in [13]. In this work we dramatically improve the size, at the cost of slightly increasing the stretch. Specifically, given any ϵ > 0, we devise a pairwise PRDO with stretch 1 + ϵ, constant query time, and near optimal size n^o(1) · (n + |P|).