Showing papers on "Pushdown automaton published in 1998"

Understanding Java stack inspection

Abstract: Current implementations of Java make security decisions by searching the runtime call stack. These systems have attractive security properties, but they have been criticized as being dependent on specific artifacts of the Java implementation. The paper models the stack inspection algorithm in terms of a well understood logic for access control and demonstrates how stack inspection is a useful tool for expressing and managing complex trust relationships. We show that an access control decision based on stack inspection corresponds to the construction of a proof in the logic, and we present an efficient decision procedure for generating these proofs. By examining the decision procedure, we demonstrate that many statements in the logic are equivalent and can thus be expressed in a simpler form. We show that there are a finite number of such statements, allowing us to represent the security state of the system as a pushdown automaton. We also show that this automaton may be embedded in Java by rewriting all Java classes to pass an additional argument when a procedure is invoked. We call this security passing style and describe its benefits over previous stack inspection systems. Finally, we show how the logic allows us to describe a straightforward design for extending stack inspection across remote procedure calls.

Growing context-sensitive languages and Church-Rosser languages

Abstract: The growing context-sensitive languages (GCSL) are characterized by a nondeterministic machine model, the so-called shrinking two-pushdown automaton (sTPDA). Then the deterministic version of this automaton (sDTPDA) is shown to characterize the class of generalized Church-Rosser languages (GCRL). Finally, we prove that each growing context-sensitive language is accepted in polynomial time by some one-way auxiliary pushdown automaton with a logarithmic space bound (OW-auxPDA[log, poly]). As a consequence the class of (generalized) Church-Rosser languages and the class of context-free languages are incomparable under set inclusion, which verifies a conjecture of Mc-Naughton et al [MNO88].

Deriving Unbounded Petri Nets from Formal Languages

Abstract: We propose decision procedures based on regions for two problems on pure unbounded Petri nets with injective labelling. One problem is to construct nets from incomplete specifications, given by pairs of regular languages that impose respectively upper and lower bounds on the expected behaviours. The second problem is to derive equivalent nets from deterministic pushdown automata, thus exhibiting their hidden concurrency.

Simulation Is Decidable for One-Counter Nets (Extended Abstract)

Abstract: We prove that the simulation preorder is decidable for the class of one-counter nets. A one-counter net consists of a finite-state machine operating on a variable (counter) which ranges over the natural numbers. Each transition can increase or decrease the value of the counter. A transition may not be performed if this implies that the value of the counter becomes negative. The class of one-counter nets is computationally equivalent to the class of Petri nets with one unbounded place, and to the class of pushdown automata where the stack alphabet is restricted to one symbol. To our knowledge, this is the first result in the literature which gives a positive answer to the decidability of simulation preorder between pairs of processes in a class whose elements are neither finite-state nor allow finite partitioning of their state spaces.

Context-free languages over infinite alphabets

Abstract: In this paper we introduce context-free grammars and pushdown automata over infinite alphabets. It is shown that a language is generated by a context-free grammar over an infinite alphabet if and only if it is accepted by a pushdown automaton over an infinite alphabet. Also the generated (accepted) languages possess many of the properties of the ordinary context-free languages: decidability, closure properties, etc.. This provides a substantial evidence for considering context-free grammars and pushdown automata over infinite alphabets as a natural extension of the classical ones.

Principles of logic optimization for a moore microprogrammed automaton

Abstract: The existence of pseudoequivalent states permits minimizing the length of the direct structural table of the Moore automaton and thus reduces the number of terms in the system of automaton memory excitation functions. Automaton logic optimization requires unique identification of the classes of pseudoequivalent states. Method M1 identifies the classesB i ∈ πga without using additional variables and states. However, the application of this method does not always reduce the DST to the corresponding parameter of the equivalent Mealy automaton. Moreover, forR > R 0 the number of feedback parameters in the Moore automaton is greater than in the equivalent Mealy automaton. Method M2 attains the absolute minimum DST length and the absolute minimum number of feedback variables, which are equal to the corresponding parameters of the equivalent Mealy automaton. Moreover, state encoding can be applied that minimizes the number of terms in the microoperation system. However, M2 requires the introduction of a special code converter and thus involves additional hardware costs.

On groups whose word problem is solved by a nested stack automaton

Abstract: Accessible groups for which the language of all words defining the identity is accepted by a certain class of nested stack automata are virtually free

On the Expressiveness of Links in Hypertext Systems

Abstract: In this paper, we study how linking mechanisms contribute to the expressiveness of hypertext systems. For this purpose, we formalize hypertext systems as abstract machines. As the primary benefit of hypertext systems is to be able to read documents non-linearly, their expressiveness is defined in terms of the ability to follow links. Then, we classify hypertext systems according to the power of the underlying automaton. The model allows us to compare embedded versus separate links and simple versus generic links. Then, we investigate history mechanisms, adaptive hypertexts and functional links. Our conclusion is that simple links, whether embedded or separate, generic links and some adaptive links all give hypertext systems the power of finite state automata. The history mechanism confers to them the power of pushdown automata, whereas the general functional links give them Turing completeness.

The neural network pushdown automaton : Architecture, dynamics and training

Abstract: In particular this paper develops a new model, a neural network pushdown automaton (NNPDA), which is a hybrid system that couples a recurrent network to an external stack memory. More importantly, a NNPDA should be capable of learning and recognizing some class of context-free grammars. We explore the capabilities of such a model by inferring automata from sample strings-the problem of grammatical inference.

Complete Formal Systems for Equivalence Problems.

Abstract: We describe four complete and recursively enumerable formal systems S0,D0,H0,B0. Each one of them proves the decidability of some equivalence problem for some class of automata: namely the language equivalence problem for simple automata, the language equivalence problem for deterministic pushdown automata, the function equivalence problem for deterministic pushdown transducers with outputs in an abelian group, the bisimulation equivalence problem for loop-free pushdown automata.

The Head Hierarchy for Oblivious Finite Automata with Polynomial Advice Collapses

Abstract: We show that the hierarchy of classes of languages accepted by finite multi-head automata with oblivious head movements that receive polynomial advice strings collapses to the fifth level. A characterization of nondeterministic logarithmic space with polynomial advice is simplified. In the presence of polynomial advice, the question whether deterministic and nondeterministic logarithmic space are equivalent can be reduced to the question whether simple nondeterministic automata can be simulated deterministically. Polynomial time can be characterized by a one-head device.

Learning Deterministic Finite Automaton with a Recurrent Neural Network

Abstract: We consider the problem of learning a finite automaton with recurrent neural networks from positive evidence. We train an Elman recurrent neural network with a set of sentences in a language and extract a finite automaton by clustering the states of the trained network. We observe that the generalizations beyond the training set, in the language recognized by the extracted automaton, are due to the training regime: the network performs a “loose” minimization of the prefix DFA of the training set, the automaton that has a state for each prefix of the sentences in the set.

Optimal Regular Tree Pattern Matching Using Pushdown Automata

Abstract: We propose a construction that augments the precomputation step of a regular tree pattern matching algorithm to include cost analysis. The matching device generated is a pushdown automaton in contrast with the conventionally generated tree pattern matching automaton. Our technique can handle a larger class of cost augmented regular tree grammars than can be preprocessed by conventional methods, and has been tested on some input problem instances representing instruction sets for processors.

Transforming Lattices into Non-deterministic Automata with Optional Null Arcs

Abstract: The problem of transforming a lattice into a non-deterministic finite state automaton is non-trivial. We present a transformation algorithm which tracks, for each node of an automaton under construction, the larcs which it reflects and the lattice nodes at their origins and extremities. An extension of the algorithm permits the inclusion of null, or epsilon, arcs in the output automaton. The algorithm has been successfully applied to lattices derived from dictionaries, i.e. very large corpora of strings.

Precise estimation of the order of local testability of a deterministic finite automaton

Abstract: A locally testable language L is a language with the property that for some nonnegative integer k, called the order or the level of local testability, whether or not a word u in the language L depends on (1) the prefix and suffix of the word u of length k - 1 and (2) the set of intermediate substrings of length k of the word u. For given k the language is called k-testable. We give necessary and sufficient conditions for the language of an automaton to be k-testable in the terms of the length of paths of a related graph. Some estimations of the upper and of the lower bound of order of testability follow from these results. We improve the upper bound on the order of testability of locally testable deterministic finite automaton with n states to n 2 -n/2+1. This bound is the best possible. We give an answer on the following conjecture of Kim, McNaughton and McCloskey for deterministic finite locally testable automaton with n states: Is the order of local testability no greater than Ω(n 1.5 ) when the alphabet size is two? Our answer is negative. In the case of size two the situation is the same as in general case: the order of local testability is Ω(n 2 ).

Efficient Simulations by Queue Machines

Abstract: The following simulations by machines equipped with a one-way input tape and additional queue storage are shown: Every single-tape Turing machine (no separate input-tape) with time bound t(n) can be simulated by one queue in O(t(n)) time. Every pushdown automaton can be simulated by one queue in time O(n√n). Every deterministic machine with a one-turn pushdown store can be simulated deterministically by one queue in O(n√n) time. Every Turing machine with several multi-dimensional tapes accepting with time bound t(n) can be simulated by two queues in time O(t(n) log2 t(n)). Every deterministic Turing machine with several linear tapes accepting with time bound t(n) can be simulated deterministically by a queue and a pushdown store in O(t(n) log t(n)) time.

A Highly Parallel Finite State Automaton Processor for Biological Pattern Matching.

Abstract: Finite State Automata are useful for string searching problems mostly because they are fast. For very large problems, a software implementation will not be fast enough. I describe here a parallel implementation of a hardware Deterministic Finite State Automaton processor. It can rapidly search a large database for approximately matching strings, as a lter for more detailed processing later. As the most important parts, large Random Access Memory chips, are continually getting cheaper, it should be possible and a ordable to make large arrays of such processors.

Minimal deterministic left-to-right pattern-matching automata

Abstract: We propose a practical technique to compile pattern-matching of prioritised overlapping patterns in equational languages to an minimal deterministic left-to-right matching automaton. First, we present a method to construct a tree matching automaton for such patterns. The automaton obtained allows pattern-matching to be performed without any backtracking. Although this automaton is efficient since it avoids symbol re-examination, it can only achieve this at the cost of increased space requirements. Such space requirements could be minimised by using a dag automaton that shares all the isomorphic subautomata which are duplicated in a tree automaton. We design an efficient method to identify such subautomata and avoid their construction while generating the dag automaton. This is achieved without constructing the tree automaton first.

Automatic Groups and Knuth-Bendix with Infinitely Many Rules

Abstract: It is shown how to use a small finite state automaton in two variables in order to carry out part of the Knuth--Bendix process for rewriting words in a group. The main objective is to provide a substitute for the most space-demanding module of the existing software which attempts to find a shortlex-automatic structure for a group. The two-variable automaton can be used to store an infinite set of rules and to carry out fast reduction of arbitrary words using this infinite set. We introduce a new operation, which we call welding, which applies to an arbitrary finite state automaton. In our context this operation is vital. We point out a small potential improvement in the subset algorithm for making a non-deterministic automaton deterministic.

The Factor Automaton.

Abstract: This paper concerns searching substrings in a string using the factor automaton. The factor automaton is a deterministic finite automaton constructed to accept every substring of the given string. Nondeterministic factor automaton is used to achieve new operations on factor automata for searching in non-constant texts.

Fault-detection experiment with deterministic realizations and a nondeterministic model

Abstract: The algorithmic approach to the design of fault-detection experiments for testing of control devices requires a mathematical model that describes the behavior of the tested device and its reference model. A finite automaton usually provides a suitable mathematical model. The classical problem assumes that the reference model and the tested device behave as a deterministic f'mite automaton (in what follows, we use the term "automaton" for a deterministic f'mite automaton). The class of faults is limited to faults that do not increase the number of automaton states. It is also assumed that the experimenter has direct access to the inputs and outputs of the device being tested. The construction of multiple fault-detection experiments assumes the existence of an input symbol that allows the model automaton to return from any state to some fixed (initial) state. This special symbol remains a reset symbol under all faults [1]. Recent studies use a nondeterministic automaton (nd-automaton) as a reference model for testing. In particular, an ndautomaton is used to construct test cases that check computer network protocols for conformity [2-4]. It has been shown [5] that if the output of the device being tested is observable only on the output of another device, then a fault,detection experiment also can be constructed using a nondeterministic model. The construction of a fault-detection experiment usually focuses mainly on the design of input sequences. The set of admissible output responses is identified with the entire set of model output sequences. In this article we analyze the possibility of reducing the length of the fault-detection experiment by reducing the set of admissible output responses. The article is organized as follows. Section 1 introduces the main def'mitions and notation. Section 2 describes the possibility of reducing the admissible set of output sequences in a fault-detection experiment. Section 3 provides necessary and sufficient conditions for the realization of a nondeterministic automaton by a deterministic automaton. We examine the conditions that permit passing from the nd-automaton P to an nd-automaton P,~ with fewer states and fewer output responses. The set of automata realizing P is identical with the set of automata realizing P~.. We can thus construct a fault~etection experiment for the nd-automaton P based on the nd-automaton P~.. This possibility is illustrated in Section 4.

The Variable Structure Learning Automaton Network

Abstract: Learning automata select an action from a finite set of their available actions and update their strategy on the basis of response received from the random environment using what is known as a reinforcement scheme. As an environment changes, the ordering of the actions with performance criterion may vary. If a learning automaton with a fixed strategy is used in such an environment, it may become less expedient with time and even inexpedient. However, using the learning scheme that has sufficient flexibility to track the better actions makes the performance improved. In this paper, a variable structure learning automaton network with periodic random environment is proposed. The results of some numerical simulations show that our model can be used for tracking some periodic nonstationary environment for which an upper bound on the period is known. © 1999 Scripta Technica, Electr Eng Jpn, 129(1): 39–45, 1999