Saverio Caminiti

Awareness and Learning in Participatory Noise Sensing

The development of ICT infrastructures has facilitated the emergence of new paradigms for looking at society and the environment over the last few years. Participatory environmental sensing, i.e. directly involving citizens in environmental monitoring, is one example, which is hoped to encourage learning and enhance awareness of environmental issues. In this paper, an analysis of the behaviour of individuals involved in noise sensing is presented. Citizens have been involved in noise measuring activities through the WideNoise smartphone application. This application has been designed to record both objective (noise samples) and subjective (opinions, feelings) data. The application has been open to be used freely by anyone and has been widely employed worldwide. In addition, several test cases have been organised in European countries. Based on the information submitted by users, an analysis of emerging awareness and learning is performed. The data show that changes in the way the environment is perceived after repeated usage of the application do appear. Specifically, users learn how to recognise different noise levels they are exposed to. Additionally, the subjective data collected indicate an increased user involvement in time and a categorisation effect between pleasant and less pleasant environments.

A unified approach to coding labeled trees

We consider the problem of coding labeled trees by means of strings of node labels and we present a unified approach based on a reduction of both coding and decoding to integer (radix) sorting. Applying this approach to four well-known codes introduced by Prufer [18], Neville [17], and Deo and Micikevicius [5], we close some open problems. With respect to coding, our general sequential algorithm requires optimal linear time, thus solving the problem of optimally computing the second code presented by Neville. The algorithm can be parallelized on the EREW PRAM model, so as to work in O(logn) time using O(n) or O(nlog n) operations, depending on the code. With respect to decoding, the problem of finding an optimal sequential algorithm for the second Neville code was also open, and our general scheme solves it. Furthermore, in a parallel setting our scheme yields the first efficient decoding algorithms for the codes in [5] and [17].

Engineering Tree Labeling Schemes: A Case Study on Least Common Ancestors

We address the problem of labeling the nodes of a tree such that one can determine the identifier of the least common ancestor of any two nodes by looking only at their labels. This problem has application in routing and in distributed computing in peer-to-peer networks. A labeling scheme using i¾?(log2n)-bit labels has been previously presented by Peleg. By engineering this scheme, we obtain a variety of data structures with the same asymptotic performances. We conduct a thorough experimental evaluation of all these data structures. Our results clearly show which variants achieve the best performances in terms of space usage, construction time, and query time.

String Coding of Trees with Locality and Heritability

We consider the problem of coding labelled trees by means of strings of vertex labels and we present a general scheme to define bijective codes based on the transformation of a tree into a functional digraph. Looking at the fields in which codes for labelled trees are utilized, we see that the properties of locality and heritability are required and that codes like the well known Prufer code do not satisfy these properties. We present a general scheme for generating codes based on the construction of functional digraphs. We prove that using this scheme, locality and heritability are satisfied as a direct function of the similarity between the topology of the functional digraph and that of the original tree. Moreover, we also show that the efficiency of our method depends on the transformation of the tree into a functional digraph. Finally we show how it is possible to fit three known codes into our scheme, obtaining maximum efficiency and high locality and heritability.

On coding labeled trees

We consider the problem of coding labeled trees by means of strings of node labels. Different codes have been introduced in the literature by Prufer, Neville, and Deo and Micikevicius. For all of them, we show that both coding and decoding can be reduced to integer (radix) sorting, closing several open problems within a unified framework that can be applied both in a sequential and in a parallel setting. Our sequential coding and decoding schemes require optimal O(n) time when applied to n-node trees, yielding the first linear time decoding algorithm for a code presented by Neville. These schemes can be parallelized on the EREW PRAM model, so as to work in O(logn) time with cost O(n), , or O(nlogn), depending on the code and on the operation: in all cases, they either match or improve the performances of the best ad hoc approaches known so far.

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.

Parallel Algorithms for Dandelion-Like Codes

We consider the class of Dandelion-like codes, i.e., a class of bijective codes for coding labeled trees by means of strings of node labels. In the literature it is possible to find optimal sequential algorithms for codes belonging to this class, but, for the best of our knowledge, no parallel algorithm is reported. In this paper we present the first encoding and decoding parallel algorithms for Dandelion-like codes. Namely, we design a unique encoding algorithm and a unique decoding algorithm that, properly parametrized, can be used for all Dandelion-like codes. 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.

Dynamic programming in faulty memory hierarchies (cache-obliviously)

Random access memories suer from transient errors that lead the logical state of some bits to be read dierently from how they were last written. Due to technological constraints, caches in the memory hierarchy of modern computer platforms appear to be particularly prone to bit flips. Since algorithms implicitly assume data to be stored in reliable memories, they might easily exhibit unpredictable behaviors even in the presence of a small number of faults. In this paper we investigate the design of dynamic programming algorithms in faulty memory hierarchies. Previous works on resilient algorithms considered a one-level faulty memory model and, with respect to dynamic programming, could address only problems with local dependencies. Our improvement upon these works is two-fold: (1) we significantly extend the class of problems that can be solved resiliently via dynamic programming in the presence of faults, settling challenging non-local problems such as all-pairs shortest paths and matrix multiplication; (2) we investigate the connection between resiliency and cache-eciency, providing cache-oblivious implementations that incur an (almost) optimal number of cache misses. Our approach yields the first resilient algorithms that can tolerate faults at any level of the memory hierarchy, while maintaining cacheeciency. All our algorithms are correct with high probability and match the running time and cache misses of their standard non-resilient counterparts while tolerating a large (polynomial) number of faults. Our results also extend to Fast Fourier Transform.

Bijective Linear Time Coding and Decoding for k -Trees

The problem of coding labeled trees has been widely studied in the literature and several bijective codes that realize associations between labeled trees and sequences of labels have been presented. k-trees are one of the most natural and interesting generalizations of trees and there is considerable interest in developing efficient tools to manipulate this class of graphs, since many NP-Complete problems have been shown to be polynomially solvable on k-trees and partial k-trees. In 1970 Rényi and Rényi generalized the Prüfer code, the first bijective code for trees, to a subset of labeled k-trees. Subsequently, non redundant codes that realize bijection between k-trees (or Rényi k-trees) and a well defined set of strings were produced. In this paper we introduce a new bijective code for labeled k-trees which, to the best of our knowledge, produces the first coding and decoding algorithms running in linear time with respect to the size of the k-tree.

A bijective code for k -trees with linear time encoding and decoding

The problem of coding labeled trees has been widely studied in the literature and several bijective codes that realize associations between labeled trees and sequences of labels have been presented. k-trees are one of the most natural and interesting generalizations of trees and there is considerable interest in developing efficient tools to manipulate this class, since many NP-Complete problems have been shown to be polynomially solvable on k-trees and partial k-trees. In 1970 Renyi and Renyi generalized the Prufer code to a subset of labeled k-trees; subsequently, non redundant codes that realize bijection between k-trees (or Renyi k-trees) and a well defined set of strings were produced. In this paper we introduce a new bijective code for labeled k-trees which, to the best of our knowledge, produces the first encoding and decoding algorithms running in linear time with respect to the size of the k-tree.