1 // See www.openfst.org for extensive documentation on this weighted
2 // finite-state transducer library.
4 // Functions and classes to determine the equivalence of two FSTs.
6 #ifndef FST_EQUIVALENT_H_
7 #define FST_EQUIVALENT_H_
11 #include <unordered_map>
16 #include <fst/encode.h>
18 #include <fst/union-find.h>
19 #include <fst/vector-fst.h>
25 // Traits-like struct holding utility functions/typedefs/constants for
26 // the equivalence algorithm.
28 // Encoding device: in order to make the statesets of the two acceptors
29 // disjoint, we map Arc::StateId on the type MappedId. The states of
30 // the first acceptor are mapped on odd numbers (s -> 2s + 1), and
31 // those of the second one on even numbers (s -> 2s + 2). The number 0
32 // is reserved for an implicit (non-final) dead state (required for
33 // the correct treatment of non-coaccessible states; kNoStateId is mapped to
34 // kDeadState for both acceptors). The union-find algorithm operates on the
37 struct EquivalenceUtil {
38 using StateId = typename Arc::StateId;
39 using Weight = typename Arc::Weight;
41 using MappedId = StateId; // ID for an equivalence class.
43 // MappedId for an implicit dead state.
44 static constexpr MappedId kDeadState = 0;
46 // MappedId for lookup failure.
47 static constexpr MappedId kInvalidId = -1;
49 // Maps state ID to the representative of the corresponding
50 // equivalence class. The parameter 'which_fst' takes the values 1
51 // and 2, identifying the input FST.
52 static MappedId MapState(StateId s, int32 which_fst) {
53 return (kNoStateId == s) ? kDeadState
54 : (static_cast<MappedId>(s) << 1) + which_fst;
57 // Maps set ID to State ID.
58 static StateId UnMapState(MappedId id) {
59 return static_cast<StateId>((--id) >> 1);
62 // Convenience function: checks if state with MappedId s is final in
64 static bool IsFinal(const Fst<Arc> &fa, MappedId s) {
65 return (kDeadState == s) ? false
66 : (fa.Final(UnMapState(s)) != Weight::Zero());
68 // Convenience function: returns the representative of ID in sets,
69 // creating a new set if needed.
70 static MappedId FindSet(UnionFind<MappedId> *sets, MappedId id) {
71 const auto repr = sets->FindSet(id);
72 if (repr != kInvalidId) {
83 typename EquivalenceUtil<Arc>::MappedId EquivalenceUtil<Arc>::kDeadState;
87 typename EquivalenceUtil<Arc>::MappedId EquivalenceUtil<Arc>::kInvalidId;
89 } // namespace internal
91 // Equivalence checking algorithm: determines if the two FSTs fst1 and fst2
92 // are equivalent. The input FSTs must be deterministic input-side epsilon-free
93 // acceptors, unweighted or with weights over a left semiring. Two acceptors are
94 // considered equivalent if they accept exactly the same set of strings (with
97 // The algorithm (cf. Aho, Hopcroft and Ullman, "The Design and Analysis of
98 // Computer Programs") successively constructs sets of states that can be
99 // reached by the same prefixes, starting with a set containing the start states
100 // of both acceptors. A disjoint tree forest (the union-find algorithm) is used
101 // to represent the sets of states. The algorithm returns false if one of the
102 // constructed sets contains both final and non-final states. Returns an
103 // optional error value (useful when FLAGS_error_fatal = false).
107 // Quasi-linear, i.e., O(n G(n)), where
109 // n = |S1| + |S2| is the number of states in both acceptors
111 // G(n) is a very slowly growing function that can be approximated
112 // by 4 by all practical purposes.
114 bool Equivalent(const Fst<Arc> &fst1, const Fst<Arc> &fst2,
115 double delta = kDelta, bool *error = nullptr) {
116 using Weight = typename Arc::Weight;
117 if (error) *error = false;
118 // Check that the symbol table are compatible.
119 if (!CompatSymbols(fst1.InputSymbols(), fst2.InputSymbols()) ||
120 !CompatSymbols(fst1.OutputSymbols(), fst2.OutputSymbols())) {
121 FSTERROR() << "Equivalent: Input/output symbol tables of 1st argument "
122 << "do not match input/output symbol tables of 2nd argument";
123 if (error) *error = true;
126 // Check properties first.
127 static constexpr auto props = kNoEpsilons | kIDeterministic | kAcceptor;
128 if (fst1.Properties(props, true) != props) {
129 FSTERROR() << "Equivalent: 1st argument not an"
130 << " epsilon-free deterministic acceptor";
131 if (error) *error = true;
134 if (fst2.Properties(props, true) != props) {
135 FSTERROR() << "Equivalent: 2nd argument not an"
136 << " epsilon-free deterministic acceptor";
137 if (error) *error = true;
140 if ((fst1.Properties(kUnweighted, true) != kUnweighted) ||
141 (fst2.Properties(kUnweighted, true) != kUnweighted)) {
142 VectorFst<Arc> efst1(fst1);
143 VectorFst<Arc> efst2(fst2);
144 Push(&efst1, REWEIGHT_TO_INITIAL, delta);
145 Push(&efst2, REWEIGHT_TO_INITIAL, delta);
146 ArcMap(&efst1, QuantizeMapper<Arc>(delta));
147 ArcMap(&efst2, QuantizeMapper<Arc>(delta));
148 EncodeMapper<Arc> mapper(kEncodeWeights | kEncodeLabels, ENCODE);
149 ArcMap(&efst1, &mapper);
150 ArcMap(&efst2, &mapper);
151 return Equivalent(efst1, efst2);
153 using Util = internal::EquivalenceUtil<Arc>;
154 using MappedId = typename Util::MappedId;
155 enum { FST1 = 1, FST2 = 2 }; // Required by Util::MapState(...)
156 auto s1 = Util::MapState(fst1.Start(), FST1);
157 auto s2 = Util::MapState(fst2.Start(), FST2);
158 // The union-find structure.
159 UnionFind<MappedId> eq_classes(1000, Util::kInvalidId);
160 // Initializes the union-find structure.
161 eq_classes.MakeSet(s1);
162 eq_classes.MakeSet(s2);
163 // Data structure for the (partial) acceptor transition function of fst1 and
164 // fst2: input labels mapped to pairs of MappedIds representing destination
165 // states of the corresponding arcs in fst1 and fst2, respectively.
166 using Label2StatePairMap =
167 std::unordered_map<typename Arc::Label, std::pair<MappedId, MappedId>>;
168 Label2StatePairMap arc_pairs;
169 // Pairs of MappedId's to be processed, organized in a queue.
170 std::deque<std::pair<MappedId, MappedId>> q;
172 // Returns early if the start states differ w.r.t. finality.
173 if (Util::IsFinal(fst1, s1) != Util::IsFinal(fst2, s2)) ret = false;
174 // Main loop: explores the two acceptors in a breadth-first manner, updating
175 // the equivalence relation on the statesets. Loop invariant: each block of
176 // the states contains either final states only or non-final states only.
177 for (q.push_back(std::make_pair(s1, s2)); ret && !q.empty(); q.pop_front()) {
178 s1 = q.front().first;
179 s2 = q.front().second;
180 // Representatives of the equivalence classes of s1/s2.
181 const auto rep1 = Util::FindSet(&eq_classes, s1);
182 const auto rep2 = Util::FindSet(&eq_classes, s2);
184 eq_classes.Union(rep1, rep2);
186 // Copies outgoing arcs starting at s1 into the hash-table.
187 if (Util::kDeadState != s1) {
188 ArcIterator<Fst<Arc>> arc_iter(fst1, Util::UnMapState(s1));
189 for (; !arc_iter.Done(); arc_iter.Next()) {
190 const auto &arc = arc_iter.Value();
191 // Zero-weight arcs are treated as if they did not exist.
192 if (arc.weight != Weight::Zero()) {
193 arc_pairs[arc.ilabel].first = Util::MapState(arc.nextstate, FST1);
197 // Copies outgoing arcs starting at s2 into the hashtable.
198 if (Util::kDeadState != s2) {
199 ArcIterator<Fst<Arc>> arc_iter(fst2, Util::UnMapState(s2));
200 for (; !arc_iter.Done(); arc_iter.Next()) {
201 const auto &arc = arc_iter.Value();
202 // Zero-weight arcs are treated as if they did not exist.
203 if (arc.weight != Weight::Zero()) {
204 arc_pairs[arc.ilabel].second = Util::MapState(arc.nextstate, FST2);
208 // Iterates through the hashtable and process pairs of target states.
209 for (const auto &arc_iter : arc_pairs) {
210 const auto &pair = arc_iter.second;
211 if (Util::IsFinal(fst1, pair.first) !=
212 Util::IsFinal(fst2, pair.second)) {
213 // Detected inconsistency: return false.
221 if (fst1.Properties(kError, false) || fst2.Properties(kError, false)) {
222 if (error) *error = true;
230 #endif // FST_EQUIVALENT_H_