Deterministic decremental reachability, scc, and shortest paths via directed expanders and congestion balancing

Aaron Bernstein, Maximilian Probst Gutenberg, Thatchaphol Saranurak

    Research output: Chapter in Book/Report/Conference proceedingConference contribution

    Abstract

    Let G=(V, E, w) be a weighted, directed graph subject to a sequence of adversarial edge deletions. In the decremental single-source reachability problem (SSR), we are given a fixed source s and the goal is to maintain a data structure that can answer path-queries s rightarrowtail v for any v in V. In the more general single-source shortest paths (SSSP) problem the goal is to return an approximate shortest path to v, and in the SCC problem the goal is to maintain strongly connected components of G and to answer path queries within each component. All of these problems have been very actively studied over the past two decades, but all the fast algorithms are randomized and, more significantly, they can only answer path queries if they assume a weaker model: They assume an oblivious adversary which is not adaptive and must fix the update sequence in advance. This assumption significantly limits the use of these data structures, most notably preventing them from being used as subroutines in static algorithms. All the above problems are notoriously difficult in the adaptive setting. In fact, the state-of-the-art is still the Even and Shiloach tree, which dates back all the way to 1981 [1] and achieves total update time O(mn). We present the first algorithms to break through this barrier. •deterministic decremental SSR/SSC with total update time mn{2/3+o(1)} •deterministic decremental SSSP with total update time n{2+2/3+o(1)} To achieve these results, we develop two general techniques for working with dynamic graphs. The first generalizes expander-based tools to dynamic directed graphs. While these tools have already proven very successful in undirected graphs, the underlying expander decomposition they rely on does not exist in directed graphs. We thus need to develop an efficient framework for using expanders in directed graphs, as well as overcome several technical challenges in processing directed expanders. We establish several powerful primitives that we hope will pave the way for other expander-based algorithms in directed graphs. The second technique, which we call congestion balancing, provides a new method for maintaining flow under adversarial deletions. The results above use this technique to maintain an embedding of an expander.

    Original languageEnglish (US)
    Title of host publicationProceedings - 2020 IEEE 61st Annual Symposium on Foundations of Computer Science, FOCS 2020
    PublisherIEEE Computer Society
    Pages1123-1134
    Number of pages12
    ISBN (Electronic)9781728196213
    DOIs
    StatePublished - Nov 2020
    Event61st IEEE Annual Symposium on Foundations of Computer Science, FOCS 2020 - Virtual, Durham, United States
    Duration: Nov 16 2020Nov 19 2020

    Publication series

    NameProceedings - Annual IEEE Symposium on Foundations of Computer Science, FOCS
    Volume2020-November
    ISSN (Print)0272-5428

    Conference

    Conference61st IEEE Annual Symposium on Foundations of Computer Science, FOCS 2020
    Country/TerritoryUnited States
    CityVirtual, Durham
    Period11/16/2011/19/20

    Keywords

    • dynamic algorithm
    • single-source reachability
    • single-source shortest paths
    • strongly-connected components

    ASJC Scopus subject areas

    • General Computer Science

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