Borivoj Melichar

Even faster generalized LR parsing

Abstract. We prove a property of generalized LR (GLR) parsing – if the grammar is without right and hidden left recursions, then the number of consecutive reductions between the shifts of two adjacent symbols cannot be greater than a constant. Further, we show that this property can be used for constructing an optimized version of our GLR parser. Compared with a standard GLR parser, our optimized parser reads one symbol on every transition and performs significantly fewer stack operations. Our timings show that, especially for highly ambiguous grammars, our parser is significantly faster than a standard GLR parser.

Implementation of Nondeterministic Finite Automata for Approximate Pattern Matching

There are two ways of using the nondeterministic finite automata (NFA). The first one is the transformation to the equivalent deterministic finite automaton and the second one is the simulation of the run of NFA. In this paper we discuss the second way. We present an overview of the simulation methods that have been found in the approximate string matching. We generalize these simulation methods and form the rules for the usage of these methods.

Approximate String Matching by Finite Automata

Approximate string matching is a sequential problem and therefore it is possible to solve it using finite automata. A nondeterministic finite automaton is constructed for string matching with k mismatches. It is shown, how “dynamic programming” and “shift- and” based algorithms simulate this nondeterministic finite automaton. The corresponding deterministic finite automaton have \(\mathcal{O}(m^{k + 1} )\) states, where m is the length of the pattern and k is the number of mismatches. The time complexity of algorithms based on such deterministic finite automaton is \(\mathcal{O}(n)\), where n is the length of text.

Attribute Evaluation and Parsing

Attribute evaluation during top-down, bottom-up and left-corner parsing strategies is considered. For each of these parsing strategies, restrictions are given for L-attributed grammars that allow deterministic non-backtracking, one-pass evaluators to be used.

Subtree matching by deterministic pushdown automata

Subtree matching is an important problem in Computer Science on which a number of tasks, such as mechanical theorem proving, term-rewriting, symbolic computation and non-procedural programming languages are based on. A systematic approach to the construction of subtree pattern matchers by deterministic pushdown automata, which read subject trees in prefix notation, is presented. The method is analogous to the construction of string pattern matchers: for a given pattern, a nondeterministic pushdown automaton is created and then it is determinised. In addition, it is shown that the size of the resulting deterministic pushdown automata directly corresponds to the size of the existing string pattern matchers based on finite automata.

The Output-Store Formal Translator Directed by LR Parsing

This paper presents a one-pass formal translator that can be constructed for each translation grammar with an LR(k) input grammar. The formal translator is the conventional LR parser whose operations are extended by actions performing both an output and a temporary storing of output symbols. The temporary storing of some symbols before their appending to the output string is implemented by using one synthesized attribute.

A new algorithm for the determinisation of visibly pushdown automata

Visibly pushdown automata are pushdown automata whose pushdown operations are determined by the input symbol, where the input alphabet is partitioned into three parts for push, pop and local pushdown operations. It is well known that nondeterministic visibly pushdown automata can be determinised. In this paper a new algorithm for the determinisation of nondeterministic visibly pushdown automata is presented. The algorithm improves the existing methods and can result in significantly smaller deterministic pushdown automata. This is achieved in a way that only necessary and accessible states and pushdown symbols are computed and constructed during the determinisation.

Syntax Directed Translation with LR Parsing

A simple extension of the usual LR parser construction is made in order to build a translator. The LR parsing algorithm is extended by a facility to do output operations within the action shift and reduce. A class of translation grammars, called R-translation grammars, is introduced as an extension of the class of postfix translation grammars. Transformations called shaking-down and postponing of output symbols are used for transformation of some non-R-translation to R-translation ones.

The Hierarchie of LT-Attributed Grammars

Attribute grammars are a framework for defining semantics of programming languages in a syntax-directed fashion. In this paper, we define attribute grammars, and then illustrate their use for language definition, compiler generation, definite clause grammars, design and specification of algorithms, etc. Our goal is to emphasize its role as a tool for design, formal specification and implementation of practical systems, so our presentation is example rich.