Timothy M. Chan's Publications: Geometric algorithms with limited space


Subquadratic encodings for point configurations

(with
Jean Cardinal, John Iacono, Stefan Langerman, and Aurelien Ooms)

For many algorithms dealing with sets of points in the plane, the only relevant information carried by the input is the combinatorial configuration of the points: the orientation of each triple of points in the set (clockwise, counterclockwise, or collinear). This information is called the order type of the point set. In the dual, realizable order types and abstract order types are combinatorial analogues of line arrangements and pseudoline arrangements. Too often in the literature we analyze algorithms in the real-RAM model for simplicity, putting aside the fact that computers as we know them cannot handle arbitrary real numbers without some sort of encoding. Encoding an order type by the integer coordinates of a realizing point set is known to yield doubly exponential coordinates in some cases. Other known encodings can achieve quadratic space or fast orientation queries, but not both. In this contribution, we give a compact encoding for abstract order types that allows efficient query of the orientation of any triple: the encoding uses O(n^2) bits and an orientation query takes O(log n) time in the word-RAM model with word size w >= log n. This encoding is space-optimal for abstract order types. We show how to shorten the encoding to O(n^2 (loglog n)^2 / log n) bits for realizable order types, giving the first subquadratic encoding for those order types with fast orientation queries. We further refine our encoding to attain O(log n / loglog n) query time at the expense of a negligibly larger space requirement. In the realizable case, we show that all those encodings can be computed efficiently. Finally, we generalize our results to the encoding of point configurations in higher dimension.


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

(with Eric Y. Chen)

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(n log n) expected time using only O(1) extra space; this improves the previous O(n log^3 n) bound by Bronnimann, Chan, and Chen [SoCG'04]. The same approach leads to an optimal randomized in-place algorithm for the 2-d line segment intersection problem, with O(n log n + K) expected running time for output size K, improving the previous O(n log^2 n + K) bound by Vahrenhold [WADS'05]. As a bonus, we also point out a simplification of a known optimal cache-oblivious (non-in-place) algorithm by Kumar and Ramos (2002) 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 [ESA'08]. Our results are all obtained by standard random sampling techniques, with some interesting twists.


In-place 2-d nearest neighbor search

(with Eric Y. Chen)

We revisit a classic problem in computational geometry: preprocessing a planar n-point set to answer nearest neighbor queries. In SoCG 2004, Bronnimann, Chan, and Chen showed that it is possible to design an efficient data structure that takes no extra space at all other than the input array holding a permutation of the points. The best query time known for such "in-place data structures" is O(log^2 n). In this paper, we break the O(log^2 n) barrier by providing a method that answers nearest neighbor queries in time

O((log n)^{log_{3/2} 2} loglog n) = O(log^{1.71} n).
The new method uses divide-and-conquer (based on planar separators) in a way that is quite unlike traditional point location methods, and extends previous 1-d data structuring techniques (specifically the van Emde Boas layout). The method has further applications, for example, in answering extreme point queries for a 3-d point set on the boundary of a convex set of constant complexity.


Space-efficient algorithms for Klee's measure problem

(with Eric Y. Chen)

We give space-efficient geometric algorithms for two related problems. Given a set of n axis-aligned rectangles in the plane, we calculate the area covered by the union of these rectangles (Klee's measure problem) in O(n^{3/2} log n) time with O(sqrt{n}) extra space. If the input can be destroyed and there are no degenerate cases and input coordinates are all integers, we can solve Klee's measure problem in O(n log^2 n) time with O(log^2 n) extra space. Given a set of n points in the plane, we find the axis-aligned unit square that covers the maximum number of points in O(n log^3 n) time with O(log^2 n) extra space.


Multi-pass geometric algorithms

(with Eric Y. Chen)

We initiate the study of exact geometric algorithms that require limited storage and make only a small number of passes over the input. Fundamental problems such as low-dimensional linear programming and convex hulls are considered.


Towards in-place geometric algorithms and data structures

(with
Hervé Brönnimann, and Eric Y. Chen)

For many geometric problems, there are efficient algorithms that surprisingly use very little extra space other than the given array holding the input. For many geometric query problems, there are efficient data structures that need no extra space at all other than an array holding a permutation of the input. In this paper, we obtain the first such space-economical solutions for a number of fundamental problems, including three-dimensional convex hulls, two-dimensional Delaunay triangulations, fixed-dimensional range queries, and fixed-dimensional nearest neighbor queries.


Space-efficient algorithms for computing the convex hull of a simple polygonal line in linear time

(with
Hervé Brönnimann)

We present space-efficient algorithms for computing the convex hull of a simple polygonal line in-place, in linear time. It turns out that the problem is as hard as stable partition, i.e., if there were a truly simple solution then stable partition would also have a truly simple solution, and vice versa. Nevertheless, we present a simple self-contained solution that uses O(log n) space, and indicate how to improve it to O(1) space with the same techniques used for stable partition. If the points inside the convex hull can be discarded, then there is a truly simple solution that uses a single call to stable partition, and even that call can be spared if only extreme points are desired (and not their order). If the polygonal line is closed, then the problem admits a very simple solution which does not call for stable partitioning at all.


A space-efficient algorithm for segment intersection

(with Eric Y. Chen)

We examine the space requirement for the classic line-segment intersection problem. Using so-called implicit data structures, we show how to make the standard sweep-line algorithm run in O((n+k)log^2 n) time with only O(log^2 n) extra space, where n is the number of line segments and k is the number of intersections. If division is allowed and input can be destroyed, the algorithm can run in O((n+k)log n) time with O(1) extra space.


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Timothy Chan (Last updated September 2018)