67 research outputs found
On coding labeled trees
Trees are probably the most studied class of graphs in Computer Science. In this thesis we study bijective codes that represent labeled trees by means of string of node labels. We contribute to the understanding of their algorithmic tractability, their properties, and their applications.
The thesis is divided into two parts. In the first part we focus on two types of tree codes, namely Prufer-like codes and Transformation codes. We study optimal encoding and decoding algorithms, both in a sequential and in a parallel setting. We propose a unified approach that works for all Prufer-like codes and a more generic scheme based on the transformation of a tree into a functional digraph suitable for all bijective codes. Our results in this area close a variety of open problems.
We also consider possible applications of tree encodings, discussing how to exploit these codes in Genetic Algorithms and in the generation of random trees. Moreover, we introduce a modified version of a known code that, in Genetic Algorithms, outperform all the other known codes.
In the second part of the thesis we focus on two possible generalizations of our work. We first take into account the classes of k-trees and k-arch graphs (both superclasses of trees): we study bijective codes for this classes of graphs and their algorithmic feasibility. Then, we shift our attention to Informative Labeling Schemes. In this context labels are no longer considered as simple unique node identifiers, they rather convey information useful to achieve efficient computations on the tree. We exploit this idea to design a concurrent data structure for the lowest common ancestor problem on dynamic trees.
We also present an experimental comparison between our labeling scheme and the one proposed by Peleg for static trees
XTribe: a web-based social computation platform
In the last few years the Web has progressively acquired the status of an
infrastructure for social computation that allows researchers to coordinate the
cognitive abilities of human agents in on-line communities so to steer the
collective user activity towards predefined goals. This general trend is also
triggering the adoption of web-games as a very interesting laboratory to run
experiments in the social sciences and whenever the contribution of human
beings is crucially required for research purposes. Nowadays, while the number
of on-line users has been steadily growing, there is still a need of
systematization in the approach to the web as a laboratory. In this paper we
present Experimental Tribe (XTribe in short), a novel general purpose web-based
platform for web-gaming and social computation. Ready to use and already
operational, XTribe aims at drastically reducing the effort required to develop
and run web experiments. XTribe has been designed to speed up the
implementation of those general aspects of web experiments that are independent
of the specific experiment content. For example, XTribe takes care of user
management by handling their registration and profiles and in case of
multi-player games, it provides the necessary user grouping functionalities.
XTribe also provides communication facilities to easily achieve both
bidirectional and asynchronous communication. From a practical point of view,
researchers are left with the only task of designing and implementing the game
interface and logic of their experiment, on which they maintain full control.
Moreover, XTribe acts as a repository of different scientific experiments, thus
realizing a sort of showcase that stimulates users' curiosity, enhances their
participation, and helps researchers in recruiting volunteers.Comment: 11 pages, 2 figures, 1 table, 2013 Third International Conference on
Cloud and Green Computing (CGC), Sept. 30 2013-Oct. 2 2013, Karlsruhe,
German
Participatory Patterns in an International Air Quality Monitoring Initiative
The issue of sustainability is at the top of the political and societal
agenda, being considered of extreme importance and urgency. Human individual
action impacts the environment both locally (e.g., local air/water quality,
noise disturbance) and globally (e.g., climate change, resource use). Urban
environments represent a crucial example, with an increasing realization that
the most effective way of producing a change is involving the citizens
themselves in monitoring campaigns (a citizen science bottom-up approach). This
is possible by developing novel technologies and IT infrastructures enabling
large citizen participation. Here, in the wider framework of one of the first
such projects, we show results from an international competition where citizens
were involved in mobile air pollution monitoring using low cost sensing
devices, combined with a web-based game to monitor perceived levels of
pollution. Measures of shift in perceptions over the course of the campaign are
provided, together with insights into participatory patterns emerging from this
study. Interesting effects related to inertia and to direct involvement in
measurement activities rather than indirect information exposure are also
highlighted, indicating that direct involvement can enhance learning and
environmental awareness. In the future, this could result in better adoption of
policies towards decreasing pollution.Comment: 17 pages, 6 figures, 1 supplementary fil
Participatory Patterns in an International Air Quality Monitoring Initiative
The issue of sustainability is at the top of the political and societal agenda, being considered of extreme importance and urgency. Human individual action impacts the environment both locally (e.g., local air/water quality, noise disturbance) and globally (e.g., climate change, resource use). Urban environments represent a crucial example, with an increasing realization that the most effective way of producing a change is involving the citizens themselves in monitoring campaigns (a citizen science bottom-up approach). This is possible by developing novel technologies and IT infrastructures enabling large citizen participation. Here, in the wider framework of one of the first such projects, we show results from an international competition where citizens were involved in mobile air pollution monitoring using low cost sensing devices, combined with a web-based game to monitor perceived levels of pollution. Measures of shift in perceptions over the course of the campaign are provided, together with insights into participatory patterns emerging from this study. Interesting effects related to inertia and to direct involvement in measurement activities rather than indirect information exposure are also highlighted, indicating that direct involvement can enhance learning and environmental awareness. In the future, this could result in better adoption of policies towards decreasing pollution
Unified parallel encoding and decoding algorithms for Dandelion-like codes
The Dandelion-like codes are eight bijections between labeled trees and strings of node labels. The literature contains optimal sequential algorithms for these bijections, but no parallel algorithms have been reported. In this paper the first parallel encoding and decoding algorithms for Dandelion-like codes are presented. Namely, a unique encoding algorithm and a unique decoding algorithm, which when properly parameterized, can be used for all Dandelion-like codes, are designed. These algorithms are optimal in the sequential setting. The encoding algorithm implementation on an EREW PRAM is optimal, while the efficient implementation of the decoding algorithm requires concurrent reading. © 2010 Elsevier Inc. All rights reserved
On the Number of Labeled k-arch Graphs
In this paper we deal with k-arch graphs, a superclass of trees and k-trees. We give a recursive function counting the number of labeled k-arch graphs. Our result relies on a generalization of the well-known Prüfer code for labeled trees. In order to guarantee the generalized code to be a bijection, we characterize the valid code strings. A previous attempt at counting the number of labeled k-arch graphs was made by Lamathe. We point out an error in his work, and prove it by giving a counterexample
Linear-time algorithms for encoding trees as sequences of node labels
In this paper we present O(n)-time algorithms for encoding/decoding n-node labeled trees as sequences of n − 2 node labels. All known encodings of this type are covered, including Prüfer-like codes and the three codes proposed by Picciotto- the happy, blob, and dandelion codes. The algorithms for Picciotto’s codes are of special significance as previous publications describe suboptimal approaches requiring O(n log n) or even O(n 2) time
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