Optimal in-place and cache-oblivious algorithms for 3-d convex hulls and 2-d segment intersection

  • Authors:
  • Timothy M. Chan;Eric Y. Chen

  • Affiliations:
  • Cheriton School of Computer Science, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada;Cheriton School of Computer Science, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

  • Venue:
  • Computational Geometry: Theory and Applications
  • Year:
  • 2010

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Abstract

We describe the first optimal randomized in-place algorithm for the basic 3-d convex hull problem (and, in particular, for 2-d Voronoi diagrams). The algorithm runs in O(nlogn) expected time using only O(1) extra space; this improves the previous O(nlog^3n) bound by Bronnimann, Chan, and Chen (2004) [10]. The same approach leads to an optimal randomized in-place algorithm for the 2-d line segment intersection problem, with O(nlogn+K) expected running time for output size K, improving the previous O(nlog^2n+K) bound by Vahrenhold (2007) [42]. As a bonus, we also point out a simplification of a known optimal cache-oblivious (non-in-place) algorithm by Kumar and Ramos (2002) [33] for 3-d convex hulls, and observe its applicability to 2-d segment intersection, extending a recent result for red/blue segment intersection by Arge, Molhave, and Zeh (2008) [3]. Our results are all obtained by standard random sampling techniques, with some interesting twists.