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107 lines
3.1 KiB
107 lines
3.1 KiB
// Copyright 2005-2024 Google LLC
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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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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an 'AS IS' BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// See www.openfst.org for extensive documentation on this weighted
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// finite-state transducer library.
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//
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// Topological sort of FSTs.
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#ifndef FST_TOPSORT_H_
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#define FST_TOPSORT_H_
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#include <vector>
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#include <fst/dfs-visit.h>
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#include <fst/fst.h>
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#include <fst/mutable-fst.h>
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#include <fst/properties.h>
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#include <fst/statesort.h>
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namespace fst {
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// DFS visitor class to return topological ordering.
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template <class Arc>
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class TopOrderVisitor {
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public:
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using StateId = typename Arc::StateId;
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// If acyclic, order[i] gives the topological position of StateId i;
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// otherwise it is unchanged. acyclic_ will be true iff the FST has no
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// cycles. The caller retains ownership of the state order vector.
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TopOrderVisitor(std::vector<StateId> *order, bool *acyclic)
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: order_(order), acyclic_(acyclic) {}
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void InitVisit(const Fst<Arc> &fst) {
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finish_.clear();
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*acyclic_ = true;
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}
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constexpr bool InitState(StateId, StateId) const { return true; }
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constexpr bool TreeArc(StateId, const Arc &) const { return true; }
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bool BackArc(StateId, const Arc &) { return (*acyclic_ = false); }
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constexpr bool ForwardOrCrossArc(StateId, const Arc &) const { return true; }
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void FinishState(StateId s, StateId, const Arc *) { finish_.push_back(s); }
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void FinishVisit() {
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if (*acyclic_) {
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order_->clear();
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for (StateId s = 0; s < finish_.size(); ++s) {
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order_->push_back(kNoStateId);
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}
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for (StateId s = 0; s < finish_.size(); ++s) {
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(*order_)[finish_[finish_.size() - s - 1]] = s;
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}
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}
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}
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private:
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std::vector<StateId> *order_;
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bool *acyclic_;
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// States in finish-time order.
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std::vector<StateId> finish_;
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};
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// Topologically sorts its input if acyclic, modifying it. Otherwise, the input
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// is unchanged. When sorted, all transitions are from lower to higher state
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// IDs.
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//
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// Complexity:
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//
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// Time: O(V + E)
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// Space: O(V + E)
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//
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// where V is the number of states and E is the number of arcs.
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template <class Arc>
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bool TopSort(MutableFst<Arc> *fst) {
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std::vector<typename Arc::StateId> order;
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bool acyclic;
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TopOrderVisitor<Arc> top_order_visitor(&order, &acyclic);
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DfsVisit(*fst, &top_order_visitor);
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if (acyclic) {
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StateSort(fst, order);
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fst->SetProperties(kAcyclic | kInitialAcyclic | kTopSorted,
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kAcyclic | kInitialAcyclic | kTopSorted);
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} else {
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fst->SetProperties(kCyclic | kNotTopSorted, kCyclic | kNotTopSorted);
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}
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return acyclic;
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}
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} // namespace fst
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#endif // FST_TOPSORT_H_
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