// itf/transition-information.h
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// Copyright 2021 NVIDIA (author: Daniel Galvez)
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// See ../../COPYING for clarification regarding multiple authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// THIS CODE IS PROVIDED *AS IS* BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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// KIND, EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED
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// WARRANTIES OR CONDITIONS OF TITLE, FITNESS FOR A PARTICULAR PURPOSE,
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// MERCHANTABLITY OR NON-INFRINGEMENT.
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// See the Apache 2 License for the specific language governing permissions and
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// limitations under the License.
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#ifndef KALDI_ITF_TRANSITION_INFORMATION_H_
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#define KALDI_ITF_TRANSITION_INFORMATION_H_
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#include <stdint.h>
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#include <vector>
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namespace kaldi {
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/**
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* Class that abstracts out TransitionModel's methods originally used
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* in the lat/ directory. By instantiating a subclass of this abstract
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* class other than TransitionModel, you can use kaldi's lattice tools
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* without a dependency on the hmm/ and tree/ directories. For
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* example, you can consider creating a subclass that implements these
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* via extracting information from Eesen's CTC T.fst object.
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*
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* TransitionId values must be contiguous, and starting from 1, rather
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* than 0, since 0 corresponds to epsilon in OpenFST.
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*/
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class TransitionInformation {
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public:
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virtual ~TransitionInformation() {};
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/**
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* Returns true if trans_id1 and trans_id2 can correspond to the
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* same phone when trans_id1 immediately precedes trans_id2 (i.e.,
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* trans_id1 occurss at timestep t, and trans_id2 ocurs at timestep
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* 2) (possibly with epsilons between trans_id1 and trans_id2) OR
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* trans_id1 ocurs before trans_id2, with some number of
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* trans_id_{k} values, all of which fulfill
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* TransitionIdsEquivalent(trans_id1, trans_id_{k})
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*
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* If trans_id1 == trans_id2, it must be the case that
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* TransitionIdsEquivalent(trans_id1, trans_id2) == true
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*/
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virtual bool TransitionIdsEquivalent(int32_t trans_id1, int32_t trans_id2) const = 0;
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/**
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* Returns true if this trans_id corresponds to the start of a
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* phone.
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*/
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virtual bool TransitionIdIsStartOfPhone(int32_t trans_id) const = 0;
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/**
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* Phone is a historical term, and should be understood in a wider
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* sense that also includes graphemes, word pieces, etc.: any
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* minimal entity in your problem domain which is represented by a
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* sequence of transitions with a PDF assigned to each of them by
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* the model. In this sense, Token is a better word. Since
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* TransitionInformation was added to subsume TransitionModel, we
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* did not want to change the call site of every
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* TransitionModel::TransitionIdToPhone to
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* TransitionInformation::TransitionIdToToken.
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*/
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virtual int32_t TransitionIdToPhone(int32_t trans_id) const = 0;
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/**
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* Returns true if the destination of any edge with this trans_id
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* as its ilabel is a final state (or if a final state is
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* epsilon-reachable from its destination state).
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*/
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virtual bool IsFinal(int32_t trans_id) const = 0;
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/**
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* Returns true if *all* of the FST edge labeled by this trans_id
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* have the same start and end states.
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*/
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virtual bool IsSelfLoop(int32_t trans_id) const = 0;
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int32_t TransitionIdToPdf(int32_t trans_id) const {
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return TransitionIdToPdfArray()[trans_id];
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}
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/**
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* Returns the contiguous array that backs calls to
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* TransitionIdToPdf().
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*
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* Ideally, this would return a std::span, but it doesn't because
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* kaldi doesn't support C++20 at the time this interface was
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* written.
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*/
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virtual const std::vector<int32_t>& TransitionIdToPdfArray() const = 0;
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int32_t NumTransitionIds() const {
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return TransitionIdToPdfArray().size() - 1;
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}
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/**
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* Return the number of distinct outputs from
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* TransitionIdToPdf(). Another way to look at this is as the number
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* of outputs over which your acoustic model does a softmax.
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*/
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virtual int32_t NumPdfs() const = 0;
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};
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} // namespace kaldi
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#endif // KALDI_TRANSITION_INFORMATION_H_
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