A* shortest string decoding for non-idempotent semirings
This work addresses a theoretical and practical challenge in computational linguistics and speech recognition for researchers and engineers working with non-idempotent semirings, though it is incremental as it builds on existing A* and determinization techniques.
The paper tackles the problem of finding the shortest string in weighted finite-state automata over non-idempotent semirings, where traditional shortest path algorithms fail, by proposing an A* search algorithm that uses a heuristic from a deterministic automaton to efficiently compute the result.
The single shortest path algorithm is undefined for weighted finite-state automata over non-idempotent semirings because such semirings do not guarantee the existence of a shortest path. However, in non-idempotent semirings admitting an order satisfying a monotonicity condition (such as the plus-times or log semirings), the notion of shortest string is well-defined. We describe an algorithm which finds the shortest string for a weighted non-deterministic automaton over such semirings using the backwards shortest distance of an equivalent deterministic automaton (DFA) as a heuristic for A* search performed over a companion idempotent semiring, which is proven to return the shortest string. While there may be exponentially more states in the DFA, this algorithm needs to visit only a small fraction of them if determinization is performed "on the fly".