Stephane Durocher

University of Manitoba, Winnipeg, Manitoba, Canada

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Publications (83)20.77 Total impact

  • D. Mondal · H. Hemmati · S. Durocher
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    ABSTRACT: Test case selection is a classic testing technique to choose a subset of existing test cases for execution, due to the limited budget and tight deadlines. While 'code coverage' is the state of practice among test case selection heuristics, recent literature has shown that 'test case diversity' is also a very promising approach. In this paper, we first compare these two heuristics for test case selection in several real-world case studies (Apache Ant, Derby, JBoss, NanoXML and Math). The results show that neither of the two techniques completely dominates the other, but they can potentially be complementary. Therefore, we next propose a novel approach that maximizes both code coverage and diversity among the selected test cases using NSGA-II multi- objective optimization, and the results show a significant improvement in fault detection rate. Specifically, sometimes this novel approach detects up to 16\%(Ant), 10\%(JBoss), and 14\% (Math) more faults compared to either of coverage or diversity-based approaches, when the testing budget is less than 20\% of the entire test suite execution cost.
    No preview · Article · May 2015
  • Prosenjit Bose · Jean-Lou De Carufel · Stephane Durocher
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    ABSTRACT: We revisit the problem of searching for a target at an unknown location on a line when given upper and lower bounds on the distance D that separates the initial position of the searcher from the target. Prior to this work, only asymptotic bounds were known for the optimal competitive ratio achievable by any search strategy in the worst case. We present the first tight bounds on the exact optimal competitive ratio achievable, parameterized in terms of the given bounds on D, along with an optimal search strategy that achieves this competitive ratio. We prove that this optimal strategy is unique. We characterize the conditions under which an optimal strategy can be computed exactly and, when it cannot, we explain how numerical methods can be used efficiently. In addition, we answer several related open questions, including the maximal reach problem, and we discuss how to generalize these results to m rays, for any .
    No preview · Article · Mar 2015 · Theoretical Computer Science
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    ABSTRACT: We consider several variations of the problems of covering a set of barriers (modeled as line segments) using sensors that can detect any intruder crossing any of the barriers. Sensors are initially located in the plane and they can relocate to the barriers. We assume that each sensor can detect any intruder in a circular area centered at the sensor. Given a set of barriers and a set of sensors located in the plane, we study three problems: the feasibility of barrier coverage, the problem of minimizing the largest relocation distance of a sensor (MinMax), and the problem of minimizing the sum of relocation distances of sensors (MinSum). When sensors are permitted to move to arbitrary positions on the barrier, the problems are shown to be NP-complete. We also study the case when sensors use perpendicular movement to one of the barriers. We show that when the barriers are parallel, both the MinMax and MinSum problems can be solved in polynomial time. In contrast, we show that even the feasibility problem is NP-complete if two perpendicular barriers are to be covered, even if the sensors are located at integer positions, and have only two possible sensing ranges. On the other hand, we give an O(n 3/2) algorithm for a natural special case of this last problem.
    No preview · Article · Feb 2015 · Theoretical Computer Science
  • Stephane Durocher · Debajyoti Mondal · Md.Saidur Rahman
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    ABSTRACT: Let T be an edge weighted tree and let d min ,d max be two nonnegative real numbers. Then the pairwise compatibility graph (PCG) of T is a graph G such that each vertex of G corresponds to a distinct leaf of T and two vertices are adjacent in G if and only if the weighted distance between their corresponding leaves in T is in the interval [d min ,d max ]. Similarly, a given graph G is a PCG if there exist suitable T,d min ,d max , such that G is a PCG of T. Yanhaona, Bayzid and Rahman proved that there exists a graph with 15 vertices that is not a PCG. On the other hand, Calamoneri, Frascaria and Sinaimeri proved that every graph with at most seven vertices is a PCG. In this paper we construct a graph of eight vertices that is not a PCG, which strengthens the result of Yanhaona, Bayzid and Rahman, and implies optimality of the result of Calamoneri, Frascaria and Sinaimeri. We then construct a planar graph with sixteen vertices that is not a PCG. Finally, we prove a variant of the PCG recognition problem to be NP-complete.
    No preview · Article · Jan 2015 · Theoretical Computer Science
  • Stephane Durocher · Debajyoti Mondal

    No preview · Conference Paper · Sep 2014
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    Stephane Durocher · Debajyoti Mondal

    Full-text · Conference Paper · Sep 2014
  • Sharma V. Thankachan · Stephane Durocher · Ian Munro · Hicham El-Zein
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    ABSTRACT: Let $\set$ be a set of $n$ points in an $[n]^d$ grid, such that each point is assigned a color. Given a query range $\Q= [a_1, b_1] \times [a_2, b_2] \times \ldots \times [a_d, b_d]$, the geometric range mode query problem asks to report the most frequent color (i.e., a mode) of the multiset of colors corresponding to points in $\set \cap \Q$. When $d=1$, Chan et al.~(STACS 2012 \cite{chan2012linear}) gave a data structure that requires $O(n+(n/\Delta)^2/w)$ words of space and supports range mode queries in $O(\Delta)$ time for any $\Delta \geq 1$, where $w = \Omega(\log n)$ is the word size. Chan et al.\ also proposed a data structures for higher dimensions (i.e., $d \geq 2$) with $O(s_n+(n/\Delta)^{2d})$ space and $O(\Delta\cdot t_n)$ query time, where $s_n$ and $t_n$ denote the space and query time of a data structure that supports orthogonal range counting queries on the set $\set$. In this paper we show that the space can be improved without any increase to the query time, by presenting an $O(s_n+(n/\Delta)^{2d}/w)$-space data structure that supports orthogonal range mode queries on a set of $n$ points in $d$ dimensions in $O(\Delta \cdot t_n)$ time, for any $\Delta \geq 1$. When $d=1$, these space and query time costs match those achieved by the current best known one-dimensional data structure.
    No preview · Conference Paper · Aug 2014
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    Stephane Durocher · Debajyoti Mondal
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    ABSTRACT: Dujmovi, Eppstein, Suderman, and Wood showed that every 3-connected plane graph G with n vertices ad-mits a straight-line drawing with at most 2.5n − 3 seg-ments, which is also the best known upper bound when restricted to plane triangulations. On the other hand, they showed that there exist triangulations requiring 2n − 6 segments. In this paper we show that every plane triangulation admits a straight-line drawing with at most (7n − 2∆ 0 − 10)/3 ≤ 2.33n segments, where ∆ 0 is the number of cyclic faces in the minimum re-alizer of G. If the input triangulation is 4-connected, then our algorithm computes a drawing with at most (9n − 9)/4 ≤ 2.25n segments. For general plane graphs with n vertices and m edges, our algorithm requires at most (16n − 3m − 28)/3 ≤ 5.33n − m segments, which is smaller than 2.5n − 3 for all m ≥ 2.84n.
    Full-text · Conference Paper · Aug 2014
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    ABSTRACT: The sequence of adjacent nodes (graph walk) visited by a routing algorithm on a graph G between given source and target nodes s and t is a c-competitive route if its length in G is at most c times the length of the shortest path from s to t in G. We present 21.766-, 17.982- and 15.479-competitive online routing algorithms on the Delaunay triangulation of an arbitrary given set of points in the plane. This improves the competitive ratio on Delaunay triangulations from the previous best of 45.749. We present a 7.621-competitive online routing algorithm for Delaunay triangulations of point sets in convex position.
    No preview · Article · Jul 2014
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    ABSTRACT: We consider how to preprocess n colored points in the plane such that later, given a multiset of colors, we can quickly find an axis-aligned rectangle containing a subset of the points with exactly those colors, if one exists. We first give an index that uses o(n 4 ) space and o(n) query time when there are O(1) distinct colors. We then restrict our attention to the case in which there are only two distinct colors. We give an index that uses O(n) bits and O(1) query time to detect whether there exists a matching rectangle. Finally, we give a O(n)-space index that returns a matching rectangle, if one exists, in O(lg 2 n/lglgn) time.
    No preview · Conference Paper · Jun 2014
  • Stephane Durocher · Stefan Felsner · Saeed Mehrabi · Debajyoti Mondal
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    ABSTRACT: A strict orthogonal drawing of a graph G=(V,E) in ℝ 2 is a drawing of G such that each vertex is mapped to a distinct point and each edge is mapped to a horizontal or vertical line segment. A graph G is HV-restricted if each of its edges is assigned a horizontal or vertical orientation. A strict orthogonal drawing of an HV-restricted graph G is good if it is planar and respects the edge orientations of G. In this paper we give a polynomial-time algorithm to check whether a given HV-restricted plane graph (i.e., a planar graph with a fixed combinatorial embedding) admits a good orthogonal drawing preserving the input embedding, which settles an open question posed by Maňuch, Patterson, Poon and Thachuk (GD 2010). We then examine HV-restricted planar graphs (i.e., when the embedding is not fixed). Here we completely characterize the 2-connected maximum-degree-three HV-restricted outerplanar graphs that admit good orthogonal drawings.
    No preview · Conference Paper · Mar 2014
  • Stephane Durocher · Alexandre Leblanc · Jason Morrison · Matthew Skala
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    ABSTRACT: In this paper we present a novel nonparametric method for simplifying piecewise linear curves and we apply this method as a statistical approximation of structure within sequential data in the plane. Specifically, given a sequence P of n points in the plane that determine a simple polygonal chain consisting of n-1 segments, we describe algorithms for selecting a subsequence Q subset of P (including the first and last points of P) that determines a second polygonal chain to approximate P, such that the number of crossings between the two polygonal chains is maximized, and the cardinality of Q is minimized among all such maximizing subsets of P. Our algorithms have respective running times O(n(2) log n) (respectively, O(n(2) root log n))when P is monotonic and O(n(2) log(2) n) (respectively, O(n(2) log(4/3) n)) when P is any simple polygonal chain in the Real RAM model (respectively, in the Word RAM model).
    No preview · Article · Jan 2014 · International Journal of Computational Geometry & Applications

  • No preview · Article · Jan 2014
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    Stephane Durocher · David S. Gunderson · Pak Ching Li · Matthew Skala
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    ABSTRACT: We conjecture that the balanced complete bipartite graph $K_{\lfloor n/2 \rfloor,\lceil n/2 \rceil}$ contains more cycles than any other $n$-vertex triangle-free graph, and we make some progress toward proving this. We give equivalent conditions for cycle-maximal triangle-free graphs; show bounds on the numbers of cycles in graphs depending on numbers of vertices and edges, girth, and homomorphisms to small fixed graphs; and use the bounds to show that among regular graphs, the conjecture holds. We also consider graphs that are close to being regular, with the minimum and maximum degrees differing by at most a positive integer $k$. For $k=1$, we show that any such counterexamples have $n\le 91$ and are not homomorphic to $C_5$; and for any fixed $k$ there exists a finite upper bound on the number of vertices in a counterexample. Finally, we describe an algorithm for efficiently computing the matrix permanent (a $#P$-complete problem in general) in a special case used by our bounds.
    Full-text · Article · Oct 2013 · Discrete Mathematics
  • Stephane Durocher · Rahul Shah · Matthew Skala · Sharma V. Thankachan
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    ABSTRACT: We present a data structure for the following problem: Given a tree $\mathcal{T}$, with each of its nodes assigned a color in a totally ordered set, preprocess $\mathcal{T}$ to efficiently answer queries for the top $k$ distinct colors on the path between two nodes, reporting the colors sorted in descending order. Our data structure requires linear space of $O(n)$ words and answers queries in $O(k)$ time.
    No preview · Conference Paper · Oct 2013
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    Prosenjit Bose · Jean-Lou De Carufel · Stephane Durocher
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    ABSTRACT: We revisit the problem of searching for a target at an unknown location on a line when given upper and lower bounds on the distance D that separates the initial position of the searcher from the target. Prior to this work, only asymptotic bounds were known for the optimal competitive ratio achievable by any search strategy in the worst case. We present the first tight bounds on the exact optimal competitive ratio achievable, parameterized in terms of the given bounds on D, along with an optimal search strategy that achieves this competitive ratio. We prove that this optimal strategy is unique. We characterize the conditions under which an optimal strategy can be computed exactly and, when it cannot, we explain how numerical methods can be used efficiently. In addition, we answer several related open questions, including the maximal reach problem, and we discuss how to generalize these results to m rays, for any m >= 2.
    Preview · Article · Oct 2013
  • Stephane Durocher · Rahul Shah · Matthew Skala · Sharma V. Thankachan
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    ABSTRACT: We present O(n)-space data structures to support various range frequency queries on a given array A[0:n − 1] or tree T with n nodes. Given a query consisting of an arbitrary pair of pre-order rank indices (i,j), our data structures return a least frequent element, mode, or α-minority of the multiset of elements in the unique path with endpoints at indices i and j in A or T. We describe a data structure that supports range least frequent element queries on arrays in \(O(\sqrt{n / w})\) time, improving the \(\Theta(\sqrt{n})\) worst-case time required by the data structure of Chan et al. (SWAT 2012), where w ∈ Ω(logn) is the word size in bits. We describe a data structure that supports range mode queries on trees in \(O(\log\log n \sqrt{n / w})\) time, improving the \(\Theta(\sqrt{n} \log n)\) worst-case time required by the data structure of Krizanc et al. (ISAAC 2003). Finally, we describe a data structure that supports range α-minority queries on trees in O(α − 1 loglogn) time, where α ∈ [0,1] is specified at query time.
    No preview · Conference Paper · Aug 2013
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    Stephane Durocher · Omrit Filtser · Robert Fraser · Ali Mehrabi · Saeed Mehrabi
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    ABSTRACT: Consider a sliding camera that travels back and forth along an orthogonal line segment $s$ inside an orthogonal polygon $P$ with $n$ vertices. The camera can see a point $p$ inside $P$ if and only if there exists a line segment containing $p$ that crosses $s$ at a right angle and is completely contained in $P$. In the minimum sliding cameras (MSC) problem, the objective is to guard $P$ with the minimum number of sliding cameras. In this paper, we give an $O(n^{5/2})$-time $(7/2)$-approximation algorithm to the MSC problem on any simple orthogonal polygon with $n$ vertices, answering a question posed by Katz and Morgenstern (2011). To the best of our knowledge, this is the first constant-factor approximation algorithm for this problem.
    Full-text · Article · Aug 2013
  • Stephane Durocher · Debajyoti Mondal
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    ABSTRACT: We give an O(nlog3n)-time linear-space algorithm that, given a plane 3-tree G with n vertices and a set S of n points in the plane, determines whether G has a point-set embedding on S (i.e., a planar straight-line drawing of G where each vertex is mapped to a distinct point of S), improving the O(n 4/3 + ε )-time O(n 4/3)-space algorithm of Moosa and Rahman. Given an arbitrary plane graph G and a point set S, Di Giacomo and Liotta gave an algorithm to compute 2-bend point-set embeddings of G on S using O(W 3) area, where W is the length of the longest edge of the bounding box of S. Their algorithm uses O(W 3) area even when the input graphs are restricted to plane 3-trees. We introduce new techniques for computing 2-bend point-set embeddings of plane 3-trees that takes only O(W 2) area. We also give approximation algorithms for point-set embeddings of plane 3-trees. Our results on 2-bend point-set embeddings and approximate point-set embeddings hold for partial plane 3-trees (e.g., series-parallel graphs and Halin graphs).
    No preview · Conference Paper · Aug 2013
  • Stephane Durocher · Saeed Mehrabi
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    ABSTRACT: Let P be an orthogonal polygon. Consider a sliding camera that travels back and forth along an orthogonal line segment s ⊆ P as its trajectory. The camera can see a point p ∈ P if there exists a point q ∈ s such that pq is a line segment normal to s that is completely contained in P. In the minimum-cardinality sliding cameras problem, the objective is to find a set S of sliding cameras of minimum cardinality to guard P (i.e., every point in P can be seen by some sliding camera in S) while in the minimum-length sliding cameras problem the goal is to find such a set S so as to minimize the total length of trajectories along which the cameras in S travel. In this paper, we first settle the complexity of the minimum-length sliding cameras problem by showing that it is polynomial tractable even for orthogonal polygons with holes, answering a question posed by Katz and Morgenstern [9]. Next we show that the minimum-cardinality sliding cameras problem is NP-hard when P is allowed to have holes, which partially answers another question posed by Katz and Morgenstern [9].
    No preview · Conference Paper · Aug 2013

Publication Stats

333 Citations
20.77 Total Impact Points

Institutions

  • 2007-2015
    • University of Manitoba
      • Department of Computer Science
      Winnipeg, Manitoba, Canada
    • McGill University
      • School of Computer Science
      Montréal, Quebec, Canada
  • 2012
    • University of Victoria
      • Department of Computer Science
      Victoria, British Columbia, Canada
  • 2007-2011
    • University of Waterloo
      • David R. Cheriton School of Computer Science
      Waterloo, Ontario, Canada
  • 2000-2006
    • University of British Columbia - Vancouver
      • Department of Computer Science
      Vancouver, British Columbia, Canada