Straight-line grammar
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A straight-line grammar (sometimes abbreviated as SLG) is a formal grammar that generates exactly one string.[1] Consequently, it does not branch (every non-terminal has only one associated production rule) nor loop (if non-terminal A appears in a derivation of B, then B does not appear in a derivation of A).[1]
Areas of usefulness
[edit]Straight-line grammars are widely used in the development of algorithms that execute directly on compressed structures (without prior decompression).[2]: 212
SLGs are of interest in fields like Kolmogorov complexity, Lossless data compression, Structure discovery and Compressed data structures.[clarification needed]
The problem of finding a context-free grammar (equivalently: an SLG) of minimal size that generates a given string is called the smallest grammar problem.[citation needed]
Straight-line grammars (more precisely: straight-line context-free string grammars) can be generalized to Straight-line context-free tree grammars. The latter can be used conveniently to compress trees.[2]: 212
Formal Definition
[edit]A context-free grammar G is an SLG if:
1. for every non-terminal N, there is at most one production rule that has N as its left-hand side, and
2. the directed graph G=<V,E>, defined by V being the set of non-terminals and (A,B) ∈ E whenever B appears at the right-hand side of a production rule for A, is acyclic.
A mathematical definition of the more general formalism of straight-line context-free tree grammars can be found in Lohrey et al.[2]: 215
An SLG in Chomsky normal form is equivalent to a straight-line program.[citation needed]
A list of algorithms using SLGs
[edit]- The Sequitur algorithm constructs a straight-line grammar for a given string.
- The Lempel-Ziv-Welch algorithm creates a context-free grammar in such a deterministic way that it is necessary to store only the start rule of the generated grammar.
- Byte pair encoding
See also
[edit]- Grammar-based code – Lossless data compression algorithm
- Non-recursive grammar - a grammar that does not loop, but may branch; generating a finite rather than a singleton language
References
[edit]- ^ a b Florian Benz and Timo Kötzing, “An effective heuristic for the smallest grammar problem,” Proceedings of the fifteenth annual conference on Genetic and evolutionary computation conference - GECCO ’13, 2013. ISBN 978-1-4503-1963-8 doi:10.1145/2463372.2463441, p. 488
- ^ a b c Markus Lohrey; Sebastian Maneth; Manfred Schmidt-Schauß (2009). "Parameter Reduction in Grammar-Compressed Trees". Proc. FOSSACS (PDF). LNCS. Vol. 5504. Springer. pp. 212–226.