Latest Articles

## Analysis of a High-Performance TSP Solver on the GPU

Graphical Processing Units have been applied to solve NP-hard problems with no known polynomial time solutions. An example of such a problem is the... (more)

## Graph Bisection with Pareto Optimization

We introduce FlowCutter, a novel algorithm to compute a set of edge cuts or node separators that optimize cut size and balance in the Pareto sense. Our core algorithm heuristically solves the balanced connected st-edge-cut problem, where two given nodes s and t must be separated by removing edges to obtain two connected parts. Using the core... (more)

## Complexity of Coloring Random Graphs: An Experimental Study of the Hardest Region

It is known that the problem of deciding k-colorability of a graph exhibits an easy-hard-easy pattern,—that is, the average-case complexity for backtrack-type algorithms, as a function of k, has a peak. This complexity peak is either at k = χ − 1 or k = χ, where χ is the chromatic number of the graph.... (more)

## Dynamic Merging of Frontiers for Accelerating the Evaluation of Betweenness Centrality

Betweenness Centrality (BC) is a widely used metric of the relevance of a node in a network. The fastest-known algorithm for the evaluation of BC on... (more)

Betweenness is a well-known centrality measure that ranks the nodes according to their participation... (more)

## Computing Critical Nodes in Directed Graphs

We consider the critical node detection problem (CNDP) in directed graphs, which can be defined as follows. Given a directed graph G and a parameter k, we wish to remove a subset S of at most k vertices of G such that the residual graph G∖S has minimum pairwise strong connectivity. This problem is NP-hard, and thus we are interested in... (more)

## An Efficient Algorithm for the 1D Total Visibility-Index Problem and Its Parallelization

Let T be a terrain and P be a set of points on its surface. An important problem in Geographic... (more)

## Computing the Expected Value and Variance of Geometric Measures

Let P be a point set in ℝd, and let M be a function that maps any subset of P to a positive real. We examine the problem of computing the mean... (more)

## I/O-Efficient Generation of Massive Graphs Following the LFR Benchmark

LFR is a popular benchmark graph generator used to evaluate community detection algorithms. We present EM-LFR, the first external memory algorithm... (more)

### JEA joins RCR: Replicated Computational Results Initiative

http://jea.acm.org/rcr_initiative.cfm

### In Memoriam: David S. Johnson

http://dl.acm.org/citation.cfm?id=2907073

The Journal of Experimental Algorithmics (ISSN 1084-6654) is a high-quality journal devoted to the study of discrete algorithms and data structures from an empirical perspective. The journal welcomes original submissions that focus on design, implementation, and performance evaluation through a combination of experimentation and classical techniques.

##### Forthcoming Articles
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Constructing high dimensional kNN-graph using Z-order curve

Although many fast methods exist for constructing kNN-graph for low dimensional data, it is still an open question how to do it efficiently for high dimensional data. We present a new method to construct an approximate kNN-graph for medium to high dimensional data. Our method uses one dimensional mapping with Z-order curve to construct an initial graph and then continues to improve this using neighborhood propagation. Experiments show that the method is faster than the compared methods with four different benchmark data sets, the dimensionality of which ranges from 14 to 544. We also show that errors in the approximate kNN-graph originate more likely from outlier points; and it can be detected during run time which points are likely to have errors in their neighbors.

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Connection Scan Algorithm

We introduce the Connection Scan Algorithm (CSA) to efficiently answer queries to timetable information systems. The input consists, in the simplest setting, of a source position and a desired target position. The output consist is a sequence of vehicles such as trains or buses that a traveler should take to get from the source to the target. We study several problem variations such as the earliest arrival and profile problems. We present algorithm variants that only optimize the arrival time or additionally optimize the number of transfers in the Pareto sense. An advantage of CSA is that is can easily adjust to changes in the timetable, allowing the easy incorporation of known vehicle delays. We additionally introduce the Minimum Expected Arrival Time (MEAT) problem to handle possible, uncertain, future vehicle delays. We present a solution to the MEAT problem that is based upon CSA. Finally, we extend CSA using the multilevel overlay paradigm to answer complex queries on nation-wide integrated timetables with trains and buses.

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Practical Minimum Cut Algorithms

The minimum cut problem for an undirected edge-weighted graph asks us to divide its set of nodes into two blocks while minimizing the weight sum of the cut edges. Here, we introduce a linear-time algorithm to compute near-minimum cuts. Our algorithm is based on cluster contraction using label propagation and Padberg and Rinaldi's contraction heuristics [SIAM Review, 1991]. We give both sequential and shared-memory parallel implementations of our algorithm. Extensive experiments on both real-world and generated instances show that our algorithm finds the optimal cut on nearly all instances significantly faster than other state-of-the-art exact algorithms, and our error rate is lower than that of other heuristic algorithms. In addition, our parallel algorithm runs a factor 7.5x faster on average with 32 threads. We also give a version of our algorithm that first performs random edge contraction. This version achieves a lower running time and better parallel scalability at the expense of a higher error rate.

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BlockQuicksort: Avoiding Branch Mispredictions in Quicksort

It is well-known that Quicksort - which is commonly considered as one of the fastest in-place sorting algorithms - suffers in an essential way from branch mispredictions. We present a novel approach to address this problem by partially decoupling control from data flow: in order to perform the partitioning, we split the input in blocks of constant size; then, all elements in one block are compared with the pivot and the outcomes of the comparisons are stored in a buffer. In a second pass, the respective elements are rearranged. By doing so, we avoid conditional branches based on outcomes of comparisons (except for the final Insertionsort). Moreover, we prove that when sorting $n$ elements the average total number of branch mispredictions is at most $\epsilon n \log n + \Oh(n)$ for some small $\epsilon$ depending on the block size. Our experimental results are promising: when sorting random integer data, we achieve an increase in speed (number of elements sorted per second) of more than 80% over the GCC implementation of Quicksort (C++ std::sort). Also for many other data types and non-random inputs, there is still a significant speedup. Only in few special cases like (almost) sorted inputs, std::sort can beat our implementation. Moreover, on random input permutations, our implementation is even slightly faster than Super Scalar Sample Sort, which uses a linear amount of additional space. Finally, we also apply our approach to Quickselect and we obtain a speed-up of more than 100% over the GCC implementation (C++ std::nth_element).

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