Elsevier

Theoretical Computer Science

Volume 616, 22 February 2016, Pages 18-58
Theoretical Computer Science

Look-ahead removal for total deterministic top-down tree transducers

https://doi.org/10.1016/j.tcs.2015.12.005Get rights and content
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Abstract

Top-down tree transducers are a convenient formalism for describing tree transformations. They can be equipped with regular look-ahead, which allows them to inspect a subtree before processing it. In certain cases, such a look-ahead can be avoided and the transformation can be realized by a transducer without look-ahead. Removing the look-ahead from a transducer, if possible, is technically highly challenging. For a restricted class of transducers with look-ahead, namely those that are total, deterministic, ultralinear, and bounded erasing, we present an algorithm that, for a given transducer from that class, (1) decides whether it is equivalent to a total deterministic transducer without look-ahead, and (2) constructs such a transducer if the answer is positive. For the whole class of total deterministic transducers with look-ahead we present a similar algorithm, which assumes that a so-called difference bound is known for the given transducer. The designer of a transducer can usually also determine a difference bound for it.

Keywords

Tree transducer
Regular look-ahead
Earliest transducer
Ultralinear

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