3-Way Composition of Weighted Finite-State Transducers

TL;DR

This paper introduces a faster 3-way composition algorithm for weighted finite-state transducers, improving efficiency in automata-based applications like speech recognition and string similarity measures.

Contribution

The paper presents a novel 3-way composition algorithm that outperforms standard methods in both theoretical complexity and practical runtime, with standard composition as a special case.

Findings

Significantly faster composition in practice

Theoretical complexity improvements over standard methods

Empirical results confirm efficiency gains

Abstract

Composition of weighted transducers is a fundamental algorithm used in many applications, including for computing complex edit-distances between automata, or string kernels in machine learning, or to combine different components of a speech recognition, speech synthesis, or information extraction system. We present a generalization of the composition of weighted transducers, 3-way composition, which is dramatically faster in practice than the standard composition algorithm when combining more than two transducers. The worst-case complexity of our algorithm for composing three transducers $T_1$, $T_2$, and $T_3$ resulting in $T$, \ignore{depending on the strategy used, is $O(|T|_Q d(T_1) d(T_3) + |T|_E)$ or $(|T|_Q d(T_2) + |T|_E)$,} is $O(|T|_Q \min(d(T_1) d(T_3), d(T_2)) + |T|_E)$, where $|\cdot|_Q$ denotes the number of states, $|\cdot|_E$ the number of transitions, and $d(\cdot)$ the maximum out-degree. As in regular composition, the use of perfect hashing requires a pre-processing step with linear-time expected complexity in the size of the input transducers. In many cases, this approach significantly improves on the complexity of standard composition. Our algorithm also leads to a dramatically faster composition in practice. Furthermore, standard composition can be obtained as a special case of our algorithm. We report the results of several experiments demonstrating this improvement. These theoretical and empirical improvements significantly enhance performance in the applications already mentioned.