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A compact representation for trips over networks built on self-indexes

This work has been previously published in Information Systems (ISSN: 0306-4379) vol. 28 (November 2018), pages 1-28 and DOI https://doi.org/10.1016/j.is.2018.06.010. The last measured impact factor of that journal is 2.551.Representing the movements of objects (trips) over a network in a compact way while retaining the capability of exploiting such data effectively is an important challenge of real applications. We present a new Compact Trip Representation (CTR) that handles the spatio-temporal data associated with users’ trips over transportation networks. Depending on the network and types of queries, nodes in the network can represent intersections, stops, or even street segments.CTR represents separately sequences of nodes and the time instants when users traverse these nodes. The spatial component is handled with a data structure based on the well-known Compressed Suffix Array, which provides both a compact representation and interesting indexing capabilities. The temporal component is self-indexed with either a Hu–Tucker-shaped Wavelet-Tree or a Wavelet Matrix that solve range-interval queries efficiently. We show how CTR can solve relevant counting-based spatial, temporal, and spatio-temporal queries over large sets of trips. Experimental results show the space requirements (around 50-70% of the space needed by a compact non-indexed baseline) and query efficiency (most queries are solved in <1 ms) of CTR.

Two-Dimensional Block Trees

Brisaboa, N. R.; Gagie, T.; Gomez Brandon, A.; Navarro, G.: «Two-Dimensional Block Trees», en Proceedings of the 2018 Data Compression Conference (DCC 2018), IEEE Computer Society, Snowbird, Utah (Estados Unidos), 2018, pp. 227-236.GGS Class: 2DOI: 10.1109/DCC.2018.00031The Block Tree (BT) is a novel compact data structure designed to compress sequence collections. It obtains compression ratios close to Lempel-Ziv and supports efficient direct access to any substring. The BT divides the text recursively into fixed-size blocks and those appearing earlier are represented with pointers. On repetitive collections, a few blocks can represent all the others, and thus the BT reduces the size by orders of magnitude. In this paper we extend the BT to two dimensions, to exploit repetitiveness in collections of images, graphs, and maps. This two-dimensional Block Tree divides the image regularly into subimages and replaces some of them by pointers to other occurrences thereof. We develop a specific variant aimed at compressing the adjacency matrices of Web graphs, obtaining space reductions of up to 50% compared with the k2-tree, which is the best alternative supporting direct and reverse navigation in the graph.