PACE Solver Description: Hydra Prime

Authors Yosuke Mizutani , David Dursteler , Blair D. Sullivan

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Author Details

Yosuke Mizutani
  • University of Utah, Salt Lake City, UT, USA
David Dursteler
  • University of Utah, Salt Lake City, UT, USA
Blair D. Sullivan
  • University of Utah, Salt Lake City, UT, USA

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Yosuke Mizutani, David Dursteler, and Blair D. Sullivan. PACE Solver Description: Hydra Prime. In 18th International Symposium on Parameterized and Exact Computation (IPEC 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 285, pp. 36:1-36:5, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)


This note describes our submission to the 2023 PACE Challenge on the computation of twin-width. Our solver Hydra Prime combines modular decomposition with a collection of upper- and lower-bound algorithms, which are alternatingly applied on the prime graphs resulting from the modular decomposition. We introduce two novel approaches which contributed to the solver’s winning performance in the Exact Track: timeline encoding and hydra decomposition. Timeline encoding is a new data structure for computing the width of a given contraction sequence, enabling faster local search; the hydra decomposition is an iterative refinement strategy featuring a small vertex separator.

Subject Classification

ACM Subject Classification
  • Theory of computation → Graph algorithms analysis
  • Twin-width
  • PACE 2023


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