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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Indexing Compressed Text

He, Meng January 2003 (has links)
As a result of the rapid growth of the volume of electronic data, text compression and indexing techniques are receiving more and more attention. These two issues are usually treated as independent problems, but approaches of combining them have recently attracted the attention of researchers. In this thesis, we review and test some of the more effective and some of the more theoretically interesting techniques. Various compression and indexing techniques are presented, and we also present two compressed text indices. Based on these techniques, we implement an compressed full-text index, so that compressed texts can be indexed to support fast queries without decompressing the whole texts. The experiments show that our index is compact and supports fast search.
2

Indexing Compressed Text

He, Meng January 2003 (has links)
As a result of the rapid growth of the volume of electronic data, text compression and indexing techniques are receiving more and more attention. These two issues are usually treated as independent problems, but approaches of combining them have recently attracted the attention of researchers. In this thesis, we review and test some of the more effective and some of the more theoretically interesting techniques. Various compression and indexing techniques are presented, and we also present two compressed text indices. Based on these techniques, we implement an compressed full-text index, so that compressed texts can be indexed to support fast queries without decompressing the whole texts. The experiments show that our index is compact and supports fast search.
3

Explorations In Searching Compressed Nucleic Acid And Protein Sequence Databases And Their Cooperatively-Compressed Indices

Gardner-Stephen, Paul Mark, paul.gardner-stephen@flinders.edu.au January 2008 (has links)
Nucleic acid and protein databases such as GenBank are growing at a rate that perhaps eclipses even Moore’s Law of increase in computational power. This poses a problem for the biological sciences, which have become increasingly dependant on searching and manipulating these databases. It was once reasonably practical to perform exhaustive searches of these databases, for example using the algorithm described by Smith and Waterman, however it has been many years since this was the case. This has led to the development of a series of search algorithms, such as FASTA, BLAST and BLAT, that are each successively faster, but at similarly successive costs in terms of thoroughness. Attempts have been made to remedy this problem by devising search algorithms that are both fast and thorough. An example is CAFE, which seeks to construct a search system with a sub-linear relationship between search time and database size, and argues that this property must be present for any search system to be successful in the long term. This dissertation explores this notion by seeking to construct a search system that takes advantage of the growing redundancy in databases such as GenBank in order to reduce both the search time and the space required to store the databases and their indices, while preserving or increasing the thoroughness of the search. The result is the creation and implementation of new genomic sequence search and alignment, database compression, and index compression algorithms and systems that make progress toward resolving the problem of reducing search speed and space requirements while improving sensitivity. However, success is tempered by the need for databases with adequate local redundancy, and the computational cost of these algorithms when servicing un-batched queries.
4

Secure Text Communication for the Tiger XS

Hertz, David January 2006 (has links)
The option of communicating via SMS messages can be considered available in all GSM networks. It therefore constitutes a almost universally available method for mobile communication. The Tiger XS, a device for secure communication manufactured by Sectra, is equipped with an encrypted text message transmission system. As the text message service of this device is becoming increasingly popular and as options to connect the Tiger XS to computers or to a keyboard are being researched, the text message service is in need of upgrade. This thesis proposes amendments to the existing protocol structure. It thoroughly examines a number of options for source coding of small text messages and makes recommendations as to implementation of such features. It also suggests security enhancements and introduces a novel form of stegangraphy.
5

Hierarchická komprese / Hierarchical compression

Kreibichová, Lenka January 2011 (has links)
The most of existing text compression methods is based on the same base concept. First the Input text is divided into sequence of text units. These text units cat be single symbols, syllables or words. When compressing large text files, searching for redundancies over longer text units is usually more effective than searching over the shorter ones. But if we choose words as base units we cannot anymore catch redundancies over symbols and syllables. In this paper we propose a new text compression method called Hierarchical compresssion. It constructs hierarchical grammar to store redundancies over syllables, words and upper levels of text. The code of the text then consists of code of this grammer. We proposed a strategy for constructing hierarchical grammar for concrete input text and we proposed an effective way how to encode it. Above mentioned our proposed method is compared with some other common methods of text compression.
6

Optimal Parsing for dictionary text compression / Parsing optimal pour la compression du texte par dictionnaire

Langiu, Alessio 03 April 2012 (has links)
Les algorithmes de compression de données basés sur les dictionnaires incluent une stratégie de parsing pour transformer le texte d'entrée en une séquence de phrases du dictionnaire. Etant donné un texte, un tel processus n'est généralement pas unique et, pour comprimer, il est logique de trouver, parmi les parsing possibles, celui qui minimise le plus le taux de compression finale. C'est ce qu'on appelle le problème du parsing. Un parsing optimal est une stratégie de parsing ou un algorithme de parsing qui résout ce problème en tenant compte de toutes les contraintes d'un algorithme de compression ou d'une classe d'algorithmes de compression homogène. Les contraintes de l'algorithme de compression sont, par exemple, le dictionnaire lui-même, c'est-à-dire l'ensemble dynamique de phrases disponibles, et combien une phrase pèse sur le texte comprimé, c'est-à-dire quelle est la longueur du mot de code qui représente la phrase, appelée aussi le coût du codage d'un pointeur de dictionnaire. En plus de 30 ans d'histoire de la compression de texte par dictionnaire, une grande quantité d'algorithmes, de variantes et d'extensions sont apparus. Cependant, alors qu'une telle approche de la compression du texte est devenue l'une des plus appréciées et utilisées dans presque tous les processus de stockage et de communication, seuls quelques algorithmes de parsing optimaux ont été présentés. Beaucoup d'algorithmes de compression manquent encore d'optimalité pour leur parsing, ou du moins de la preuve de l'optimalité. Cela se produit parce qu'il n'y a pas un modèle général pour le problème de parsing qui inclut tous les algorithmes par dictionnaire et parce que
les parsing optimaux existants travaillent sous des hypothèses trop restrictives. Ce travail focalise sur le problème de parsing et présente à la fois un modèle général pour la compression des textes basée sur les dictionnaires appelé la théorie Dictionary-Symbolwise et un algorithme général de parsing qui a été prouvé être optimal sous certaines hypothèses réalistes. Cet algorithme est appelé Dictionary-Symbolwise Flexible Parsing et couvre pratiquement tous les cas des algorithmes de compression de texte basés sur dictionnaire ainsi que la grande classe de leurs variantes où le texte est décomposé en une séquence de symboles et de phrases du dictionnaire. Dans ce travail, nous avons aussi considéré le cas d'un mélange libre d'un compresseur par dictionnaire et d'un compresseur symbolwise. Notre Dictionary-Symbolwise Flexible Parsing couvre également ce cas-ci. Nous avons bien un algorithme de parsing optimal dans le cas de compression Dictionary-Symbolwise où le dictionnaire est fermé par préfixe et le coût d'encodage des pointeurs du dictionnaire est variable. Le compresseur symbolwise est un compresseur symbolwise classique qui fonctionne en temps linéaire, comme le sont de nombreux codeurs communs à longueur variable. Notre algorithme fonctionne sous l'hypothèse qu'un graphe spécial, qui sera décrit par la suite, soit bien défini. Même si cette condition n'est pas remplie, il est possible d'utiliser la même méthode pour obtenir des parsing presque optimaux. Dans le détail, lorsque le dictionnaire est comme LZ78, nous montrons comment mettre en œuvre notre algorithme en temps linéaire. Lorsque le dictionnaire est comme LZ77 notre algorithme peut être mis en œuvre en temps O (n log 
n) où n est le longueur du texte. Dans les deux cas, la complexité en espace est O (n). Même si l'objectif principal de ce travail est de nature théorique, des résultats expérimentaux seront présentés pour souligner certains effets pratiques de l'optimalité du parsing sur les performances de compression et quelques résultats expérimentaux plus détaillés sont mis dans une annexe appropriée / Dictionary-based compression algorithms include a parsing strategy to transform the input text into a sequence of dictionary phrases. Given a text, such process usually is not unique and, for compression purpose, it makes sense to find one of the possible parsing that minimizes the final compression ratio. This is the parsing problem. An optimal parsing is a parsing strategy or a parsing algorithm that solve the parsing problem taking account of all the constraints of a compression algorithm or of a class of homogeneous compression algorithms. Compression algorithm constrains are, for instance, the dictionary itself, i.e. the dynamic set of available phrases, and how much a phrase weight on the compressed text, i.e. the length of the codeword that represent such phrase also denoted as the cost of a dictionary pointer encoding. In more than 30th years of history of dictionary based text compression, while plenty of algorithms, variants and extensions appeared and while such approach to text compression become one of the most appreciated and utilized in almost all the storage and communication process, only few optimal parsing algorithms was presented. Many compression algorithms still leaks optimality of their parsing or, at least, proof of optimality. This happens because there is not a general model of the parsing problem that includes all the dictionary based algorithms and because the existing optimal parsings work under too restrictive hypothesis. This work focus on the parsing problem and presents both a general model for dictionary based text compression called Dictionary-Symbolwise theory and a general parsing algorithm that is proved to be optimal under some realistic hypothesis. This algorithm is called Dictionary-Symbolwise Flexible Parsing and it covers almost all the cases of dictionary based text compression algorithms together with the large class of their variants where the text is decomposed in a sequence of symbols and dictionary phrases.In this work we further consider the case of a free mixture of a dictionary compressor and a symbolwise compressor. Our Dictionary-Symbolwise Flexible Parsing covers also this case. We have indeed an optimal parsing algorithm in the case of dictionary-symbolwise compression where the dictionary is prefix closed and the cost of encoding dictionary pointer is variable. The symbolwise compressor is any classical one that works in linear time, as many common variable-length encoders do. Our algorithm works under the assumption that a special graph that will be described in the following, is well defined. Even if this condition is not satisfied it is possible to use the same method to obtain almost optimal parses. In detail, when the dictionary is LZ78-like, we show how to implement our algorithm in linear time. When the dictionary is LZ77-like our algorithm can be implemented in time O(n log n). Both have O(n) space complexity. Even if the main aim of this work is of theoretical nature, some experimental results will be introduced to underline some practical effects of the parsing optimality in compression performance and some more detailed experiments are hosted in a devoted appendix
7

Secure Text Communication for the Tiger XS

Hertz, David January 2006 (has links)
<p>The option of communicating via SMS messages can be considered available in all GSM networks. It therefore constitutes a almost universally available method for mobile communication.</p><p>The Tiger XS, a device for secure communication manufactured by Sectra, is equipped with an encrypted text message transmission system. As the text message service of this device is becoming increasingly popular and as options to connect the Tiger XS to computers or to a keyboard are being researched, the text message service is in need of upgrade.</p><p>This thesis proposes amendments to the existing protocol structure. It thoroughly examines a number of options for source coding of small text messages and makes recommendations as to implementation of such features. It also suggests security enhancements and introduces a novel form of stegangraphy.</p>
8

Compression of Endpoint Identifiers in Delay Tolerant Networking

Young, David A. January 2013 (has links)
No description available.
9

Metody sumarizace textových dokumentů / Methods of Text Document Summarization

Pokorný, Lubomír January 2012 (has links)
This thesis deals with one-document summarization of text data. Part of it is devoted to data preparation, mainly to the normalization. Listed are some of the stemming algorithms and it contains also description of lemmatization. The main part is devoted to Luhn"s method for summarization and its extension of use WordNet dictionary. Oswald summarization method is described and applied as well. Designed and implemented application performs automatic generation of abstracts using these methods. A set of experiments where developed, which verified correct functionality of the application and of extension of Luhn"s summarization method too.

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