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// dfs-visit.h
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
//
// \file
// Depth-first search visitation
#ifndef FST_LIB_DFS_VISIT_H__
#define FST_LIB_DFS_VISIT_H__
#include <stack>
#include "fst/lib/arcfilter.h"
#include "fst/lib/expanded-fst.h"
namespace fst {
// Visitor Interface - class determines actions taken during a Dfs.
// If any of the boolean member functions return false, the DFS is
// aborted by first calling FinishState() on all currently grey states
// and then calling FinishVisit().
//
// template <class Arc>
// class Visitor {
// public:
// typedef typename Arc::StateId StateId;
//
// Visitor(T *return_data);
// // Invoked before DFS visit
// void InitVisit(const Fst<Arc> &fst);
// // Invoked when state discovered (2nd arg is DFS tree root)
// bool InitState(StateId s, StateId root);
// // Invoked when tree arc examined (to white/undiscovered state)
// bool TreeArc(StateId s, const Arc &a);
// // Invoked when back arc examined (to grey/unfinished state)
// bool BackArc(StateId s, const Arc &a);
// // Invoked when forward or cross arc examined (to black/finished state)
// bool ForwardOrCrossArc(StateId s, const Arc &a);
// // Invoked when state finished (PARENT is kNoStateID and ARC == NULL
// // when S is tree root)
// void FinishState(StateId s, StateId parent, const Arc *parent_arc);
// // Invoked after DFS visit
// void FinishVisit();
// };
// An Fst state's DFS status
const int kDfsWhite = 0; // Undiscovered
const int kDfsGrey = 1; // Discovered & unfinished
const int kDfsBlack = 2; // Finished
// An Fst state's DFS stack state
template <class Arc>
struct DfsState {
typedef typename Arc::StateId StateId;
DfsState(const Fst<Arc> &fst, StateId s): state_id(s), arc_iter(fst, s) {}
StateId state_id; // Fst state ...
ArcIterator< Fst<Arc> > arc_iter; // and its corresponding arcs
};
// Performs depth-first visitation. Visitor class argument determines actions
// and contains any return data. ArcFilter determines arcs that are considered.
template <class Arc, class V, class ArcFilter>
void DfsVisit(const Fst<Arc> &fst, V *visitor, ArcFilter filter) {
typedef typename Arc::StateId StateId;
visitor->InitVisit(fst);
StateId start = fst.Start();
if (start == kNoStateId) {
visitor->FinishVisit();
return;
}
vector<char> state_color; // Fst state DFS status
stack<DfsState<Arc> *> state_stack; // DFS execution stack
StateId nstates = CountStates(fst);
while ((StateId)state_color.size() < nstates)
state_color.push_back(kDfsWhite);
// Continue DFS while true
bool dfs = true;
// Iterate over trees in DFS forest.
for (StateId root = start; dfs && root < nstates;) {
state_color[root] = kDfsGrey;
state_stack.push(new DfsState<Arc>(fst, root));
dfs = visitor->InitState(root, root);
while (!state_stack.empty()) {
DfsState<Arc> *dfs_state = state_stack.top();
StateId s = dfs_state->state_id;
ArcIterator< Fst<Arc> > &aiter = dfs_state->arc_iter;
if (!dfs || aiter.Done()) {
state_color[s] = kDfsBlack;
delete dfs_state;
state_stack.pop();
if (!state_stack.empty()) {
DfsState<Arc> *parent_state = state_stack.top();
StateId p = parent_state->state_id;
ArcIterator< Fst<Arc> > &piter = parent_state->arc_iter;
visitor->FinishState(s, p, &piter.Value());
piter.Next();
} else {
visitor->FinishState(s, kNoStateId, 0);
}
continue;
}
const Arc &arc = aiter.Value();
if (!filter(arc)) {
aiter.Next();
continue;
}
int next_color = state_color[arc.nextstate];
switch (next_color) {
default:
case kDfsWhite:
dfs = visitor->TreeArc(s, arc);
if (!dfs) break;
state_color[arc.nextstate] = kDfsGrey;
state_stack.push(new DfsState<Arc>(fst, arc.nextstate));
dfs = visitor->InitState(arc.nextstate, root);
break;
case kDfsGrey:
dfs = visitor->BackArc(s, arc);
aiter.Next();
break;
case kDfsBlack:
dfs = visitor->ForwardOrCrossArc(s, arc);
aiter.Next();
break;
}
}
// Find next tree root
for (root = root == start ? 0 : root + 1;
root < nstates && state_color[root] != kDfsWhite;
++root);
}
visitor->FinishVisit();
}
template <class Arc, class V>
void DfsVisit(const Fst<Arc> &fst, V *visitor) {
DfsVisit(fst, visitor, AnyArcFilter<Arc>());
}
} // namespace fst
#endif // FST_LIB_DFS_VISIT_H__
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