Spanners and Reachability Oracles for Directed Transmission Graphs

The vertex connectivity of an m -edge n -vertex undirected graph is the smallest number of vertices whose removal disconnects the graph or leaves only a singleton vertex. In 1974, Aho Hopcroft and Ullman asked if vertex connectivity can be computed in linear time. Despite the substantial effort in the past five decades, the best-known running time is \(\tilde{O}(mn)\) by Henzinger-Rao-Gabow (FOCS 1996). Indeed, no algorithm with an o ( mn ) running time is known even if we assume a linear-time max-flow algorithm. In this article, we give an affirmative answer to this long-standing open problem (up to a sub-polynomial factor). We present a randomized reduction from the vertex connectivity problem to the max-flow problem which incurs only a poly-logarithmic overhead in runtime. Using this reduction, we can solve vertex connectivity in almost linear time by using the celebrated almost-linear-time max-flow algorithms by Chen-Kyng-Liu-Peng-Probst Gutenberg-Sachdeva (FOCS 2022) and Brand-Chen-Kyng-Liu-Peng-Probst Gutenberg-Sachdeva-Sidford (FOCS 2023). Using our new techniques, we also obtain an algorithm for directed vertex connectivity with a running time of n 2 + o (1) time which improves the best-known bound of \(\tilde{O}(mn)\) by Henzinger-Rao-Gabow (FOCS 1996).

We present an improved space/query-time tradeoff for the general simplex range searching problem, matching known lower bounds up to small polylogarithmic factors. In particular, we construct a linear-space simplex range searching data structure withO(n 1−1/d) query time, which is optimal ford=2 and probably also ford>2. Further, we show that multilevel range searching data structures can be built with only a polylogarithmic overhead in space and query time per level (the previous solutions require at least a small fixed power ofn). We show that Hopcroft's problem (detecting an incidence amongn lines andn points) can be solved in time\(n^{4/3} 2^{O(\log ^4 n)}\). In all these algorithms we apply Chazelle's results on computing optimal cuttings.

Consider an ordinal tree T on n nodes, each of which is assigned a category from an alphabet [σ]={1,2,…,σ}. We preprocess the tree T in order to support categorical path counting queries, which ask for the number of distinct categories occurring on the path in T between two query nodes x and y. For this problem, we propose a linear-space data structure with query time O(nlg⁡lg⁡σlg⁡w), where w=Ω(lg⁡n) is the word size in the word-RAM. As shown in our proof, from the assumption that matrix multiplication cannot be solved in time polynomially faster than cubic (with only combinatorial methods), our result is optimal, save for polylogarithmic speed-ups. For a trade-off parameter 1≤t≤n, we propose an O(n+n2t2)-word, O(tlg⁡lg⁡σlg⁡w) query time data structure. We also consider c-approximate categorical path counting queries, which must return an approximation to the number of distinct categories occurring on the query path, by counting each such category at least once and at most c times. We describe a linear-space data structure that supports 2-approximate categorical path counting queries in O(lg⁡n/lg⁡lg⁡n) time.Next, we generalize the categorical path counting queries to weighted trees. Here, a query specifies two nodes x,y and an orthogonal range Q. The answer to thus formed categorical path range counting query is the number of distinct categories occurring on the path from x to y, if only the nodes with weights falling inside Q are considered. We propose an O(nlg⁡lg⁡n+(n/t)4)-word data structure with O(tlg⁡lg⁡n) query time, or an O(n+(n/t)4)-word data structure with O(tlgϵ⁡n) query time. For an appropriate choice of the trade-off parameter t, this implies a linear-space data structure with O(n3/4lgϵ⁡n) query time. We then extend the approach to the trees weighted with vectors from [n]d, where d is a constant integer greater than or equal to 2. We present a data structure with O(nlgd−1+ϵ⁡n+(n/t)2d+2) words of space and O(tlgd−1⁡n(lg⁡lg⁡n)d−2) query time. For an O(n⋅polylogn)-space solution, one thus has O(n2d+12d+2⋅polylogn) query time.The inherent difficulty revealed by the lower bound we proved motivated us to consider data structures based on sketching. In unweighted trees, we propose a sketching data structure to solve the approximate categorical path counting problem which asks for a (1±ϵ)-approximation (i.e. within 1±ϵ of the true answer) of the number of distinct categories on the given path, with probability 1−δ, where 0<ϵ,δ<1 are constants. The data structure occupies O(n+ntlg⁡n) words of space, for the query time of O(tlg⁡n). For trees weighted with d-dimensional weight vectors (d≥1), we propose a data structure with O((n+ntlg⁡n)lgd⁡n) words of space and O(tlgd+1⁡n) query time.All these problems generalize the corresponding categorical range counting problems in Euclidean space Rd+1, for respective d, by replacing one of the dimensions with a tree topology.

We study \emph{dynamic} algorithms for maintaining spectral vertex sparsifiers of graphs with respect to a set of terminals $T$ of our choice. Such objects preserve pairwise resistances, solutions to systems of linear equations, and energy of electrical flows between the terminals in $T$. We give a data structure that supports insertions and deletions of edges, and terminal additions, all in sublinear time. Our result is then applied to the following problems. (1) A data structure for maintaining solutions to Laplacian systems $\mathbf{L} \mathbf{x} = \mathbf{b}$, where $\mathbf{L}$ is the Laplacian matrix and $\mathbf{b}$ is a demand vector. For a bounded degree, unweighted graph, we support modifications to both $\mathbf{L}$ and $\mathbf{b}$ while providing access to $\epsilon$-approximations to the energy of routing an electrical flow with demand $\mathbf{b}$, as well as query access to entries of a vector $\tilde{\mathbf{x}}$ such that $\left\lVert \tilde{\mathbf{x}}-\mathbf{L}^{\dagger} \mathbf{b} \right\rVert_{\mathbf{L}} \leq \epsilon \left\lVert \mathbf{L}^{\dagger} \mathbf{b} \right\rVert_{\mathbf{L}}$ in $\tilde{O}(n^{11/12}\epsilon^{-5})$ expected amortized update and query time. (2) A data structure for maintaining All-Pairs Effective Resistance. For an intermixed sequence of edge insertions, deletions, and resistance queries, our data structure returns $(1 \pm \epsilon)$-approximation to all the resistance queries against an oblivious adversary with high probability. Its expected amortized update and query times are $\tilde{O}(\min(m^{3/4},n^{5/6} \epsilon^{-2}) \epsilon^{-4})$ on an unweighted graph, and $\tilde{O}(n^{5/6}\epsilon^{-6})$ on weighted graphs. These results represent the first data structures for maintaining key primitives from the Laplacian paradigm for graph algorithms in sublinear time without assumptions on the underlying graph topologies.

Many dynamic graph algorithms have an amortized update time, rather than a stronger worst-case guarantee. But amortized data structures are not suitable for real-time systems, where each individual operation has to be executed quickly. For this reason, there exist many recent randomized results that aim to provide a guarantee stronger than amortized expected. The strongest possible guarantee for a randomized algorithm is that it is always correct (Las Vegas) and has high-probability worst-case update time, which gives a bound on the time for each individual operation that holds with high probability. In this article, we present the first polylogarithmic high-probability worst-case time bounds for the dynamic spanner and the dynamic maximal matching problem. (1) For dynamic spanner, the only known o ( n ) worst-case bounds were O ( n 3/4 ) high-probability worst-case update time for maintaining a 3-spanner and O ( n 5/9 ) for maintaining a 5-spanner. We give a O (1) k log 3 ( n ) high-probability worst-case time bound for maintaining a ( 2k-1 )-spanner, which yields the first worst-case polylog update time for all constant k . (All the results above maintain the optimal tradeoff of stretch 2k-1 and Õ( n 1+1/k ) edges.) (2) For dynamic maximal matching, or dynamic 2-approximate maximum matching, no algorithm with o(n) worst-case time bound was known and we present an algorithm with O (log 5 ( n )) high-probability worst-case time; similar worst-case bounds existed only for maintaining a matching that was (2+ϵ)-approximate, and hence not maximal. Our results are achieved using a new approach for converting amortized guarantees to worst-case ones for randomized data structures by going through a third type of guarantee, which is a middle ground between the two above: An algorithm is said to have worst-case expected update time ɑ if for every update σ, the expected time to process σ is at most ɑ. Although stronger than amortized expected, the worst-case expected guarantee does not resolve the fundamental problem of amortization: A worst-case expected update time of O(1) still allows for the possibility that every 1/ f(n) updates requires ϴ ( f(n) ) time to process, for arbitrarily high f(n) . In this article, we present a black-box reduction that converts any data structure with worst-case expected update time into one with a high-probability worst-case update time: The query time remains the same, while the update time increases by a factor of O (log 2(n) ). Thus, we achieve our results in two steps: (1) First, we show how to convert existing dynamic graph algorithms with amortized expected polylogarithmic running times into algorithms with worst-case expected polylogarithmic running times. (2) Then, we use our black-box reduction to achieve the polylogarithmic high-probability worst-case time bound. All our algorithms are Las-Vegas-type algorithms.

Specializing programs with respect to run time values has been shown to drastically improve code performance on realistic programs ranging from operating systems to graphics. Recently, various approaches to specializing code at run time have been proposed. However, these approaches still suffer from shortcomings that limit their applicability: they are manual, too expensive, or require programs to be written in a dedicated language. We solve these problems by introducing new techniques to implement run time specialization. The key to our approach is the use of code templates. Templates are automatically generated from ordinary programs and are optimized before run time, allowing high quality code to be quickly generated at run time. Experimental results obtained on scientific and graphics code indicate that our approach is highly effective. Little run time overhead is introduced, since code generation primarily consists of copying instructions. Run time specialized programs run up to 10 times faster, and are nearly as fast as fully optimized programs (80% on average). The combination of low run time overhead and high code quality enables specialization to be amortized in as few as 3 runs. Although this approach is highly effective, its implementation is relatively simple since it exploits existing partial evaluation and compiler technologies.

We show tight upper and lower bounds for time-space trade-offs for the c-approximate Near Neighbor Search problem. For the d-dimensional Euclidean space and n-point datasets, we develop a data structure with space n1+ρu+o(1) + O(dn) and query time nρq +o(1) + dno(1) for every ρu, ρq ≥ 0 with: In particular, for the approximation c = 2 we get: Space n1.77… and query time no(1), significantly improving upon known data structures that support very fast queries [IM98, KOR00]; Space n1.14… and query time n0.14…, matching the optimal data-dependent Locality-Sensitive Hashing (LSH) from [AR15]; Space n1+o(1) and query time n0‘43 ‘, making significant progress in the regime of near-linear space, which is arguably of the most interest for practice [LJW+07]. This is the first data structure that achieves sublinear query time and near-linear space for every approximation factor c > 1, improving upon [Kap15]. The data structure is a culmination of a long line of work on the problem for all space regimes; it builds on Spherical Locality-Sensitive Filtering [BDGL16] and data- dependent hashing [AINR14, AR15]. Our matching lower bounds are of two types: conditional and unconditional. First, we prove tightness of the whole trade-off (0.1) in a restricted model of computation, which captures all known hashing-based approaches. We then show unconditional cell-probe lower bounds for one and two probes that match (0.1) for ρq = 0, improving upon the best known lower bounds from [PTW10]. In particular, this is the first space lower bound (for any static data structure) for two probes which is not polynomially smaller than the one-probe bound. To show the result for two probes, we establish and exploit a connection to locally-decodable codes.

We present a number of new results about range searching for colored (or categorical) data: 1) For a set of n colored points in three dimensions, we describe randomized data structures with O(n polylog n) space that can report the distinct colors in any query orthogonal range (axis-aligned box) in O(k polyloglog n) expected time, where k is the number of distinct colors in the range, assuming that coordinates are in {1,…,n}. Previous data structures require O((log n)/(log log n) + k) query time. Our result also implies improvements in higher constant dimensions. 2) Our data structures can be adapted to halfspace ranges in three dimensions (or circular ranges in two dimensions), achieving O(k log n) expected query time. Previous data structures require O(k log²n) query time. 3) For a set of n colored points in two dimensions, we describe a data structure with O(n polylog n) space that can answer colored type-2 range counting queries: report the number of occurrences of every distinct color in a query orthogonal range. The query time is O((log n)/(log log n) + k log log n), where k is the number of distinct colors in the range. Naively performing k uncolored range counting queries would require O(k (log n)/(log log n)) time. Our data structures are designed using a variety of techniques, including colored variants of randomized incremental construction (which may be of independent interest), colored variants of shallow cuttings, and bit-packing tricks.

Article Free Access Share on Exact sampling and approximate counting techniques Author: Mark Huber School of Operations Research and Industrial Engineering, Cornell University, Ithaca, NY School of Operations Research and Industrial Engineering, Cornell University, Ithaca, NYView Profile Authors Info & Claims STOC '98: Proceedings of the thirtieth annual ACM symposium on Theory of computingMay 1998 Pages 31–40https://doi.org/10.1145/276698.276709Published:23 May 1998Publication History 25citation666DownloadsMetricsTotal Citations25Total Downloads666Last 12 Months66Last 6 weeks15 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

As large amounts of garbage are generated in concurrent logic programs, it becomes necessary to salvage used memory frequently and efficiently, with a garbage collector. Another approach is to detect when a data structure becomes garbage and reuse it. In concurrent languages it is particularly difficult to determine when a data structure is garbage and suitable for destructive update. Dynamic schemes, such as reference counting, incur space and time overheads that can be unacceptable. In contrast, static-analysis techniques identify data objects whose storage may be reused. Information from static analysis can be used by the compiler in generating appropriate instructions for reuse, incurring little or no runtime overhead. In this paper we present a new static-analysis technique for reuse detection. We present empirical performance measurements comparing the new scheme against binary reference counting (MRB). It is shown that, a simple static analysis can achieve most of the benefits of MRB and improve the execution time.

Given a set S of n (distinct) keys from key space [U], each associated with a value from Σ, the static dictionary problem asks to preprocess these (key, value) pairs into a data structure, supporting value-retrieval queries: for any given x∈ [U], valRet(x) must return the value associated with x if x∈ S, or return ⊥ if x∉ S. The special case where |Σ|=1 is called the membership problem. The “textbook” solution is to use a hash table, which occupies linear space and answers each query in constant time. On the other hand, the minimum possible space to encode all (key, value) pairs is only OPT:= ⌈lg2( U n )+nlg2|Σ|⌉ bits, which could be much less. In this paper, we design a randomized dictionary data structure using OPT+lgn+O(lglglglglgU) bits of space, and it has expected constant query time, assuming the query algorithm can access an external lookup table of size n 0.001. The lookup table depends only on U, n and |Σ|, and not the input. Previously, even for membership queries and U≤ n O(1), the best known data structure with constant query time requires OPT+n/lgn bits of space by Pagh (SIAM J. Comput. 2001) and Pǎtrascu (FOCS 2008); the best known using OPT+n 0.999 space has query time O(lgn); the only known non-trivial data structure with OPT+n 0.001 space has O(lgn) query time and requires a lookup table of size ≥ n 2.99 (!). Our new data structure answers open questions by Pǎtrascu and Thorup.

Towards optimal range medians

In this paper, we introduce a general framework for fine-grained reductions of approximate counting problems to their decision versions. (Thus we use an oracle that decides whether any witness exists to multiplicatively approximate the number of witnesses with minimal overhead.) This mirrors a foundational result of Sipser (STOC 1983) and Stockmeyer (SICOMP 1985) in the polynomial-time setting, and a similar result of Müller (IWPEC 2006) in the FPT setting. Using our framework, we obtain such reductions for some of the most important problems in fine-grained complexity: the Orthogonal Vectors problem, 3SUM, and the Negative-Weight Triangle problem (which is closely related to All-Pairs Shortest Path). While all these problems have simple algorithms over which it is conjectured that no polynomial improvement is possible, our reductions would remain interesting even if these conjectures were proved; they have only polylogarithmic overhead, and can therefore be applied to subpolynomial improvements such as the n3/exp(Θ(√logn))-time algorithm for the Negative-Weight Triangle problem due to Williams (STOC 2014). Our framework is also general enough to apply to versions of the problems for which more efficient algorithms are known. For example, the Orthogonal Vectors problem over GF(m)d for constant m can be solved in time n·poly(d) by a result of Williams and Yu (SODA 2014); our result implies that we can approximately count the number of orthogonal pairs with essentially the same running time. We also provide a fine-grained reduction from approximate #SAT to SAT. Suppose the Strong Exponential Time Hypothesis (SETH) is false, so that for some 1<c<2 and all k there is an O(cn)-time algorithm for #k-SAT. Then we prove that for all k, there is an O((c+o(1))n)-time algorithm for approximate #k-SAT. In particular, our result implies that the Exponential Time Hypothesis (ETH) is equivalent to the seemingly-weaker statement that there is no algorithm to approximate #3-SAT to within a factor of 1+ε in time 2o(n)/ε2 (taking ε > 0 as part of the input). A full version of this paper containing detailed proofs is available at https://arxiv.org/abs/1707.04609.

Article Free Access Share on Efficient searching with linear constraints Authors: Pankaj K. Agarwal Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NC Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NCView Profile , Lars Arge Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NC Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NCView Profile , Jeff Erickson Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NC Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NCView Profile , Paolo G. Franciosa Dipartimento di Informatica e Sistemistica, Università di Roma 'La Sapienza', Via Salaria 113, 00198, Rome Dipartimento di Informatica e Sistemistica, Università di Roma 'La Sapienza', Via Salaria 113, 00198, RomeView Profile , Jeffry Scott Vitter Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NC Center for Geometric Computing, Department of Computer Science, Duke University, Box 90129, Durham, NCView Profile Authors Info & Claims PODS '98: Proceedings of the seventeenth ACM SIGACT-SIGMOD-SIGART symposium on Principles of database systemsMay 1998 Pages 169–178https://doi.org/10.1145/275487.275506Published:01 May 1998Publication History 30citation392DownloadsMetricsTotal Citations30Total Downloads392Last 12 Months16Last 6 weeks1 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteeReaderPDF

We consider preprocessing a set S of n points in the plane that are in convex position into a data structure supporting queries of the following form: given a point q and a directed line l in the plane, report the point of S that is farthest from (or, alternatively, nearest to) the point q subject to being to the left of line l. We present two data structures for this problem. The first data structure uses O(n1+e) space and preprocessing time, and answers queries in O(21/e log n) time. The second data structure uses O(n log3n) space and polynomial preprocessing time, and answers queries in O(log n) time. These are the first solutions to the problem with O(log n) query time and o(n2) space. In the process of developing the second data structure, we develop a new representation of nearest-point and farthest-point Voronoi diagrams of points in convex position. This representation supports insertion of new points in counterclockwise order using only O(log n) amortized pointer changes, subject to supporting O(log n)-time point-location queries, even though every such update may make Θ(n) combinatorial changes to the Voronoi diagram. This data structure is the first demonstration that deterministically and incrementally constructed Voronoi diagrams can be maintained in o(n) pointer changes per operation while keeping O(log n)-time point-location queries.