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(2019-07-18,
MichaelRiley
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---+ !OpenGrm SFST Glossary #BackoffDef $ *backoff-complete FST*: a canonical FST for which each state =s= that has a failure transition to a state =s'= and another transition with a label =x= then there is also a transition labeled with =x= from =s'=. #CanonicalDef $ *canonical FST*: an FST for which: * the states are sorted by input label * there may be [[SFstGlossary#FailureTransitionDef][failure transitions]] but * there is at most one such transition per state * there are no failure-transition (and/or epsilon-transition) cycles * no assumption is made of general determinism or what transitions must be present on failure (unlike in a [[http://www.opengrm.org/twiki/bin/view/GRM/NGramModelFormat][canonical n-gram model]]). * there may be [[FST.FstGlossary#EpsilonDef][epsilon transitions]]<sup>1</sup> but they are treated by failure transitions as regular symbols with each instance behaving as if it is uniquely labeled (i.e, they are not constrained by failure transitions). #FailureTransitionDef $ *faliure transition*: specially (_phi_) labeled transitions that are taken only when no immediate match is possible at a given state #NormalizedDef $ *normalized FST*: a canonical FST for which the weights of the paths into the future from each state sum to Weight::One()<sup>2</sup> --- <sup>1</sup>When the =phi_label= is not 0. <br><sup>2</sup>Computation is done using the log semiring ([[http://www.openfst.org/twiki/bin/view/FST/FstAdvancedUsage#Weights][Log64Weight]]), appropriate for negative log probabilities. The input weight type is converted to this type internally if needed (with conversion done using a =WeightConvert= functor, pre-defined for common weight types like =TropicalWeight= and =LogWeight=).
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Topic revision: r2 - 2019-07-18
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MichaelRiley
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