Alfonso Ortega

The Normalized Compression Distance Is Resistant to Noise

This correspondence studies the influence of noise on the normalized compression distance (NCD), a measure based on the use of compressors to compute the degree of similarity of two files. This influence is approximated by a first order differential equation which gives rise to a complex effect, which explains the fact that the NCD may give values greater than 1, observed by other authors. The model is tested experimentally with good adjustment. Finally, the influence of noise on the clustering of files of different types is explored, finding that the NCD performs well even in the presence of quite high noise levels

Digital Image Scrambling Using 2D Cellular Automata

A digital image scrambling method based on a 2D cellular automaton, specifically the well-known Game of Life, produces an effective image encryption technique.

Grammatical evolution to design fractal curves with a given dimension

Lindenmayer grammars have frequently been applied to represent fractal curves. In this work, the ideas behind grammar evolution are used to automatically generate and evolve Lindenmayer grammars which represent fractal curves with a fractal dimension that approximates a predefined required value. For many dimensions, this is a nontrivial task to be performed manually. The procedure we propose closely parallels biological evolution because it acts through three different levels: a genotype (a vector of integers), a protein-like intermediate level (the Lindenmayer grammar), and a phenotype (the fractal curve). Variation acts at the genotype level, while selection is performed at the phenotype level (by comparing the dimensions of the fractal curves to the desired value).

Audio scrambling technique based on cellular automata

Scrambling is a process that has proved to be very effective in increasing the quality of data hiding, watermarking, and encryption applications. Cellular automata are used in diverse and numerous applications because of their ability to obtain complex global behavior from simple and localized rules. In this paper we apply cellular automata in the field of audio scrambling because of the potential it holds in breaking the correlation between audio samples effectively. We also analyze the effect of using different cellular automata types on audio scrambling and we test different cellular automata rules with different Lambda values. The scrambling degree is measured and the relation between the robustness and the scrambling degree obtained is studied. Experimental results show that the proposed technique is robust to data loss attack where 1/3 of the data is lost and that the algorithm can be applied to music and speech files of different sizes.

Towards the Validation of Plagiarism Detection Tools by Means of Grammar Evolution

Student plagiarism is a major problem in universities worldwide. In this paper, we focus on plagiarism in answers to computer programming assignments, where students mix and/or modify one or more original solutions to obtain counterfeits. Although several software tools have been developed to help the tedious and time consuming task of detecting plagiarism, little has been done to assess their quality, because determining the real authorship of the whole submission corpus is practically impossible for markers. In this paper, we present a grammar evolution technique which generates benchmarks for testing plagiarism detection tools. Given a programming language, our technique generates a set of original solutions to an assignment, together with a set of plagiarisms of the former set which mimic the basic plagiarism techniques performed by students. The authorship of the submission corpus is predefined by the user, providing a base for the assessment and further comparison of copy-catching tools. We give empirical evidence of the suitability of our approach by studying the behavior of one advanced plagiarism detection tool (AC) on four benchmarks coded in APL2, generated with our technique.

Determination of fractal dimensions from equivalent L systems

This paper revises a few existing methods for computing fractal dimensions, underlines their dependency on the graphical properties of the curves, and proposes and discusses a new method, based on the representation of fractals by means of Lindenmayer systems, that makes use of the structure of L systems to compute the fractal dimension. The method is implemented in Prolog, and its limitations and usefulness are discussed.

A simple genetic algorithm for music generation by means of algorithmic information theory

Recent large scale experiments have shown that the normalized information distance, an algorithmic information measure, is among the best similarity metrics for melody classification. This paper proposes the use of this distance as a fitness function which may be used by genetic algorithms to automatically generate music in a given pre-defined style. The minimization of this distance of the generated music to a set of musical guides makes it possible to obtain computer-generated music which recalls the style of a certain human author. The recombination operator plays an important role in this problem and thus several variations are tested to fine tune the genetic algorithm for this application. The superiority of the relative pitch envelope over other music parameters, such as the lengths of the notes, brought us to develop a simplified algorithm that nevertheless obtains interesting results.

A study of the representation of fractal curves by L systems and their equivalences

To represent fractals by means of L systems, a graphic interpretation of the L system is required. Two families of graphic interpretations have been used: turtle graphics and vector graphics. Both are proved to be equivalent for two interesting families of L systems that include many of the fractals in the literature. The equivalence theorems make it possible to start from one L system in one of the families and obtain other systems that represent the same fractal. Sometimes a fractal that has previously been assumed not to be representable by any L system in one of the families can be shown to be representable in this way. Another point shown is the fact that supposed deficiencies in L systems, which have prompted the proposal of extensions, are really deficiencies in the graphic translation scheme.

Automatic generation of benchmarks for plagiarism detection tools using grammatical evolution

Student plagiarism is a mayor problem in universities worldwide. In this paper,we focus on plagiarism in answers to computer programming assignments,where student mix and/or modify one or more original solutions to obtain counterfeits. Although several software tools have been implemented to help the tedious and time consuming task of detecting plagiarism, little has been done to assess their quality, because, in fact, determining the original subset of the whole solutionset is practically impossible for graders. In this article we present a Grammatical Evolution technique which generates benchmarks. Given a programming language, our technique generates a set of original solutions to an assignment, together with a set of plagiarisms of the former set which mimic the way in which students act. The phylogeny of the coded solutions is predefined, providing a base for the evaluationof the performance of copy-catching tools. We give empirical evidence of the suitability of our approach by studying the behavior of one state-of-the-art detection tool (AC) on four benchmarks coded in APL2, generated with this technique.

Distributed simulation of P systems by means of map-reduce: first steps with hadoop and P-lingua

In this paper we face the spatial difficulties inherent to the simulation of P systems on conventional (von Neumann) computers when they are applied to solve real problems. We have extended P Lingua (a well known textual programming language for P systems) to access distributed resources by means of Hadoop (a freely available implementation of the MapReduce paradigm). The current work shows the way that we propose to develop distributed, general purpose simulators for P systems.