An efficient algorithm for partial order production

  • Authors:
  • Jean Cardinal;Samuel Fiorini;Gwenaël Joret;Raphaël M. Jungers;J. Ian Munro

  • Affiliations:
  • Université Libre de Bruxelles (ULB), Brussels, Belgium;Université Libre de Bruxelles (ULB), Brussels, Belgium;Université Libre de Bruxelles (ULB), Brussels, Belgium;Université catholique de Louvain (UCL), Louvain-la-Neuve, Belgium;University of Waterloo, Waterloo, Canada

  • Venue:
  • Proceedings of the forty-first annual ACM symposium on Theory of computing
  • Year:
  • 2009

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Abstract

We consider the problem of partial order production: arrange the elements of an unknown totally ordered set T into a target partially ordered set S, by comparing a minimum number of pairs in T. Special cases of this problem include sorting by comparisons, selection, multiple selection, and heap construction. We give an algorithm performing ITLB + o(ITLB) + O(n) comparisons in the worst case. Here, n denotes the size of the ground sets, and ITLB denotes a natural information-theoretic lower bound on the number of comparisons needed to produce the target poset. The overall complexity of our algorithm is polynomial. This answers a question of Yao (SICOMP, 1989). Our strategy is to extend the poset S to a weak order W whose corresponding information-theoretic lower bound is provably not much larger than that for S. Taking W instead of S as a target poset, we then solve the problem by applying a multiple selection algorithm that performs not much more than ITLB comparisons. We base our analysis on the entropy of the target poset S, a quantity that can be efficiently computed and provides a good estimate of ITLB.