13899040
The Regularity of Two-Way Nondeterministic Tree Automata Languages
We establish that regularly extended two-way nondeterministic tree automata with unranked alphabets have the same expressive power as regularly extended nondeterministic tree automata with unranked alphabets alphabets. We obtain this results by establishing regularly extended versions of a congruence on trees and of a congruence on, so called, views. Our motivation for the study of these tree models is the Extensible Markup Language (XML), a metalanguage for defining document grammars. Such grammars have regular sets of right-hand sides for their productions and tree automata provide an alternative and useful modeling tool for them. In particular, we believe that they provide a useful computational model for what we call caterpillar expressions.
29528938
Caterpillars: A Context Specification Technique
1095978
SGML and XML Document Grammars and Exceptions
The Standard Generalized Markup Language (SGML) and the Extensible Markup Language (XML) allow users to define document-type definitions (DTDs), which are essentially extended context-free grammars expressed in a notation that is similar to extended Backus?Naur form. The right-hand side of a production, called a content model, is both an extended and a restricted regular expression. The semantics of content models for SGML DTDs can be modified by exceptions (XML does not allow exceptions). Inclusion exceptions allow named elements to appear anywhere within the content of a content model, and exclusion exceptions preclude named elements from appearing in the content of a content model. We give precise definitions of the semantics of exceptions, and prove that they do not increase the expressive power of SGML DTDs when we restrict DTDs according to accepted SGML practice. We prove the following results:1. Exceptions do not increase the expressive power of extended context-free grammars.2. For each DTD with exceptions, we can obtain a structurally equivalent extended context-free grammar.3. For each DTD with exceptions, we can construct a structurally equivalent DTD when we restrict the DTD to adhere to accepted SGML practice.4. Exceptions are a powerful shorthand notation?eliminating them may cause exponential growth in the size of an extended context-free grammar or of a DTD.
12635352
Planar Strong Visibility
Strong visibility is a generalization of standard visibility, defined with respect to a fixed set of line orientations. We investigate computational properties of this generalized visibility, as well as the related notion of strong convexity, and describe algorithms for the following tasks: 1. Testing the strong visibility of two points in a polygon. 2. Finding the strong convex hull of a point set or polygon. 3. Constructing the strong kernel of a polygon. 4. Identifying the points that are strongly visible from a given point.
6720403
The visual specification of context
We introduce a simple and novel visual technique for the specification of context in structured documents called T-graphs that we base on T-configurations in document trees. The technique is implemented in the current version of DESIGNER. Although we are applying this technique in the specification of context-dependent style sheets for HTML, XML, and SGML documents, it is clear that it can be used in other environments such as query specification for structured documents and for computer program transformations. We compare T-graphs with the context specification techniques found in other style-sheet systems and we also provide examples of context that we can and cannot specify with T-graphs. Although T-graphs are restrictive, they lend themselves to visual construction and modification, our main requirement when we designed this context specification method. We also investigate the time and space complexity of T-graph matching, a necessity for efficient implementation.
3174555
A characterization of Thompson digraphs
A finite-state machine is called a Thompson machine if it can be constructed from an empty-free regular expression using the construction of Thompson as modified by Hopcroft and Ullman. We call the underlying digraph of a Thompson machine a Thompson digraph. We characterize Thompson digraphs and we give an algorithm that generates an equivalent regular expression from a Thompson machine that has size linear in the total number of states and transitions. Although the algorithm is simple, it is novel in that the usual constructions of equivalent regular expressions from finite-state machines produce regular expressions that have size exponential in the size of the given machine, in the worst case. The algorithm provides a tentative first step in the construction of small expressions from finite-state machines.
9788732
On generators and generative capacity of EOL forms
SummaryIn this paper we study the generative capacity of EOL forms from two different points of view. On the one hand, we consider the generative capacity of special EOL forms which one could call ‘linear like’ and ‘context free like’, establishing the existence of a rich variety of non-regular sub-EOL language families. On the other hand, we propose the notion of a ‘generator’ L of a language family ℒ We mean by this that any synchronized EOL system generating L generates — if understood as an EOL form — all languages of ℒ. We characterize the generators of the family of regular languages, and prove that other well known language families do not have generators.
33163861
Finitary and infinitary interpretations of languages
The main result of the paper can be viewed both as a generalization of a classical results concerning colorings of graphs, as well as a very useful over-all theorem from which various specific results concerning grammar and language families can be deduced. Essentially our main result says that, when considering infinitary families obtainable by inverse morphisms, it suffices to consider their finitary parts. Various applications are given, in particular, to grammar forms and context-free languages over infinite alphabets.
2937296
The Wisconsin Wind Tunnel: virtual prototyping of parallel computers
We have developed a new technique for evaluating cache coherent, shared-memory computers. The Wisconsin Wind Tunnel (WWT) runs a parallel shared-memory program on a parallel computer (CM-5) and uses execution-driven, distributed, discrete-event simulation to accurately calculate program execution time. WWT is a virtual prototype that exploits similarities between the system under design (the target) and an existing evaluation platform (the host). The host directly executes all target program instructions and memory references that hit in the target cache. WWT's shared memory uses the CM-5 memory's error-correcting code (ECC) as valid bits for a fine-grained extension of shared virtual memory. Only memory references that miss in the target cache trap to WWT, which simulates a cache-coherence protocol. WWT correctly interleaves target machine events and calculates target program execution time. WWT runs on parallel computers with greater speed and memory capacity than uniprocessors. WWT's simulation time decreases as target system size increases for fixed-size problems and holds roughly constant as the target system and problem scale.
45540830
Image tracking in real-time: a transputer emulation of some early mammalian vision processes
By emulating some of the 'early' vision processes believed to occur in the visual cortex of mammals, dramatic savings can be made in the com­ putational efforts usually associated with image processing. This paper describes the design and implementation of a low-cost visual tracking sys­ tem that uses no special signal processing, vector processing nor floating­ point hardware. Its effectiveness relies only on the parallel replication of fast scaler integer processors with low latency communications. The current implementation uses 10 T425 transputers, processes 544 x 544 byte-pixel images at camera frame rates and provides 100 Hz. tracking feedback signals to the camera pan-and-tilt motors with a latency of no more than one-fifth of a second. The system will track any object that occupies the (majority of the) centre of the field of view moving against any background - no special lighting conditio~s or artificial scenery are required. The design and implementation follow naturally parallel struc­ tures and are expressed at all levels in occam.
3174704
Mechanisms for cooperative shared memory
This paper explores the complexity of implementing directory protocols by examining their <i>mechanisms</i> primitive operations on directories, caches, and network interfaces. We compare the following protocols: <i>Dir</i><sub>1</sub><i>B</i>, <i>Dir</i><sub>4</sub><i>B</i>, <i>Dir</i><sub>4</sub><i>NB</i>, <i>Dir</i><sub>n</sub><i>NB</i>[2], <i>Dir</i><sub>1</sub><i>SW</i>[9] and an improved version of <i>Dir</i><sub>1</sub>SW (<i>Dir</i><sub>1</sub><i>SW</i><sup>+</sup>). The comparison shows that the mechanisms and mechanism sequencing of <i>Dir</i><sub>1</sub><i>SW</i> and <i>Dir</i><sub>1</sub><i>SW</i><sup>+</sup> are simpler than those for other protocols.   We also compare protocol performance by running eight benchmarks on 32 processor systems. Simulations show that <i>Dir</i><sub>1</sub><i>SW</i><sup>+</sup>s performance is comparable to more complex directory protocols. The significant disparity in hardware complexity and the small difference in performance argue that <i>Dir</i><sub>1</sub><i>SW</i><sup>+</sup> may be a more effective use of resources. The small performance difference is attributable to two factors: the low degree of sharing in the benchmarks and Check- In/Check-Out (CICO) directives [9].<br> <br>