Helsinki Institute for Information Technology

About: Helsinki Institute for Information Technology is a facility organization based out in Espoo, Finland. It is known for research contribution in the topics: Population & Bayesian network. The organization has 630 authors who have published 1962 publications receiving 63426 citations.

Parallel External Memory Suffix Sorting

Abstract: Suffix sorting (or suffix array construction) is one of the most important tasks in string processing, with dozens of applications, particularly in text indexing and data compression. Some of these applications require the suffix array to be built for large inputs that greatly exceed the size of RAM and so external memory must be used. However, existing approaches for external memory suffix sorting either use debilitatingly large amounts of disk space, or become too slow when the size of the input data is more than a few times bigger than the size of RAM. In this paper we address the latter problem via a non-trivial parallelization of computation. In our experiments, the resulting algorithm is much faster than the best prior external memory algorithms while using very little disk space in addition to what is needed for the input and output. On the way to this result we provide the current fastest (parallel) internal memory algorithm for suffix sorting, which is usually around twice as fast as previous methods, while using around one quarter of the working space.

Information technologies and transitions in the lives of 55-65-year-olds: The case of colliding life interests

Abstract: More and more people entering the stage of retirement at around age 55-65 are healthy, active, and also very computer-literate. This trend is rapidly changing the common image of late-midlife technology users, which rests on the assumption that they find it difficult to embrace new technologies and also that their main interests are health related. Although technology use and lifestyles are changing, however, many other aspects of life remain the same. One of these aspects is that of the transitions, or life changes, that generally take place in these years. Besides retirement, these transitions include changes in health, housing, social interaction, work life, and personal finance. People develop different ways of coping with these transitions, which brings up interesting issues related to the late midlife stage. This paper presents a diary-aided interview study of late middle-age adults (N=24) in Finland and Sweden with a focus on the interplay between technologies and transitions. Transitions were found to play a part in how the life interests of late middle-aged persons are often conflictive, forcing them to choose from among various 'possible selves'. At its best, technology can help alleviate these tensions. This finding is exemplified in the paper's discussion of two design implications associated with particular clashes of interests, related to how daily activities are organized and how contact is maintained with one's friends and family.

Complexity results and algorithms for extension enforcement in abstract argumentation

Abstract: Understanding the dynamics of argumentation frameworks (AFs) is important in the study of argumentation in AI. In this work, we focus on the so-called extension enforcement problem in abstract argumentation. We provide a nearly complete computational complexity map of fixed-argument extension enforcement under various major AF semantics, with results ranging from polynomial-time algorithms to completeness for the second-level of the polynomial hierarchy. Complementing the complexity results, we propose algorithms for NP-hard extension enforcement based on constrained optimization. Going beyond NP, we propose novel counterexample-guided abstraction refinement procedures for the second-level complete problems and present empirical results on a prototype system constituting the first approach to extension enforcement in its generality.

Privacy-preserving 802.11 access-point discovery

Abstract: It is usual for 802.11 WLAN clients to probe actively for access points in order to hasten AP discovery and to find "hidden" APs. These probes reveal the client's list of preferred networks, thus, present a privacy risk: an eavesdropper can infer attributes of the client based on its associations with networks. We propose an access-point discovery protocol that supports fast discovery and hidden networks while also preserving privacy. Our solution is incrementally deployable, efficient, requires only small modifications to current client and AP implementations, interoperates with current networks, and does not change the user experience. We note that our solution is faster than the standard hidden-network discovery protocol based on measurements on a prototype implementation.