Showing papers by "Alejandro López-Ortiz published in 2005"

An Efficient Bounds Consistency Algorithm for the Global Cardinality Constraint

Abstract: Previous studies have demonstrated that designing special purpose constraint propagators can significantly improve the efficiency of a constraint programming approach. In this paper we present an efficient algorithm for bounds consistency propagation of the generalized cardinality constraint (gcc). Using a variety of benchmark and random problems, we show that on some problems our bounds consistency algorithm can dramatically outperform existing state-of-the-art commercial implementations of constraint propagators for the gcc. We also present a new algorithm for domain consistency propagation of the gcc which improves on the worst-case performance of the best previous algorithm for problems that occur often in applications.

Algorithmic foundations of the internet

Abstract: In this paper we survey the field of Algorithmic Foundations of the Internet, which is a new area within theoretical computer science. We consider six sample topics that illustrate the techniques and challenges in this field.

Sharper upper and lower bounds for an approximation scheme for consensus-pattern

Abstract: We present sharper upper and lower bounds for a known polynomial-time approximation scheme due to Li, Ma and Wang [7] for the Consensus-Pattern problem. This NP-hard problem is an abstraction of motif finding, a common bioinformatics discovery task. The PTAS due to Li et al. is simple, and a preliminary implementation [8] gave reasonable results in practice. However, the previously known bounds on its performance are useless when runtimes are actually manageable. Here, we present much sharper lower and upper bounds on the performance of this algorithm that partially explain why its behavior is so much better in practice than what was previously predicted in theory. We also give specific examples of instances of the problem for which the PTAS performs poorly in practice, and show that the asymptotic performance bound given in the original proof matches the behaviour of a simple variant of the algorithm on a particularly bad instance of the problem.

Fast string sorting using order-preserving compression

Abstract: We give experimental evidence for the benefits of order-preserving compression in sorting algorithms. While, in general, any algorithm might benefit from compressed data because of reduced paging requirements, we identified two natural candidates that would further benefit from order-preserving compression, namely string-oriented sorting algorithms and word-RAM algorithms for keys of bounded length. The word-RAM model has some of the fastest known sorting algorithms in practice. These algorithms are designed for keys of bounded length, usually 32 or 64 bits, which limits their direct applicability for strings. One possibility is to use an order-preserving compression scheme, so that a bounded-key-length algorithm can be applied. For the case of standard algorithms, we took what is considered to be the among the fastest nonword RAM string sorting algorithms, Fast MKQSort, and measured its performance on compressed data. The Fast MKQSort algorithm of Bentley and Sedgewick is optimized to handle text strings. Our experiments show that order-compression techniques results in savings of approximately 15p over the same algorithm on noncompressed data. For the word-RAM, we modified Andersson's sorting algorithm to handle variable-length keys. The resulting algorithm is faster than the standard Unix sort by a factor of 1.5X. Last, we used an order-preserving scheme that is within a constant additive term of the optimal Hu--Tucker, but requires linear time rather than O(mlog m), where m = vΣv is the size of the alphabet.

Search engines and web information retrieval

Abstract: This survey describes the main components of web information retrieval, with emphasis on the algorithmic aspects of web search engine research.

Cross-Stitching Using Little Thread.

Abstract: We consider the problem of cross-stitching a predetermined pattern on a piece of fabric. We show that computing a stitching sequence that minimizes the amount of thread used when cross-stitching a pattern is NPhard. However if the region to be stitched is connected, then the optimal solution can be obtained in linear time.

From linear relaxations to global constraint propagation

Abstract: Recently, many algorithms have been designed to propagate global constraints. Unfortunately, some global constraints, such the AT-MOST-1 constraint and the EXTENDED-GCC are NP-Hard to propagate. Often, these constraints can easily be written as integer linear programs. Using linear relaxations and other techniques developed by the operation research community, we want to efficiently propagate such constraints. We model constraints as integer programs that we relax into linear programs. For each value v in a variable domain dom(x), we create a binary variable xv. The assignment xv = 1 indicates that x = v while xv = 0 indicates that x ≠ v. This is joint work with Emmanuel Hebrard and Toby Walsh.

Algorithmic foundations of the internet : Roundup

Abstract: In this paper we present a short overview of selected topics in the field of Algorithmic Foundations of the Internet, which is a new area within theoretical computer science

Algorithmic foundations of the internet : Foreword

Abstract: During the last 25 years the Internet has grown from being a small academic network connecting a few computer science departments to its present size, connecting more than 285 million computers and serving over 800 million users worldwide [7]