// 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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// Reverses an MPDT.
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#ifndef FST_EXTENSIONS_MPDT_REVERSE_H_
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#define FST_EXTENSIONS_MPDT_REVERSE_H_
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#include <limits>
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#include <vector>
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#include <fst/mutable-fst.h>
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#include <fst/relabel.h>
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#include <fst/reverse.h>
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namespace fst {
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// Reverses a multi-stack pushdown transducer (MPDT) encoded as an FST.
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template <class Arc, class RevArc>
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void Reverse(
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const Fst<Arc> &ifst,
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const std::vector<std::pair<typename Arc::Label, typename Arc::Label>>
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&parens,
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std::vector<typename Arc::Label> *assignments, MutableFst<RevArc> *ofst) {
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using Label = typename Arc::Label;
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// Reverses FST component.
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Reverse(ifst, ofst);
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// Exchanges open and close parenthesis pairs.
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std::vector<std::pair<Label, Label>> relabel_pairs;
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relabel_pairs.reserve(2 * parens.size());
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for (const auto &pair : parens) {
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relabel_pairs.emplace_back(pair.first, pair.second);
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relabel_pairs.emplace_back(pair.second, pair.first);
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}
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Relabel(ofst, relabel_pairs, relabel_pairs);
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// Computes new bounds for the stack assignments.
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Label max_level = -1;
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Label min_level = std::numeric_limits<Label>::max();
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for (const auto assignment : *assignments) {
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if (assignment < min_level) {
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min_level = assignment;
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} else if (assignment > max_level) {
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max_level = assignment;
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}
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}
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// Actually reverses stack assignments.
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for (auto &assignment : *assignments) {
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assignment = (max_level - assignment) + min_level;
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}
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}
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} // namespace fst
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#endif // FST_EXTENSIONS_MPDT_REVERSE_H_
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