1// Copyright 2006 The RE2 Authors. All Rights Reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5#ifndef RE2_WALKER_INL_H_
6#define RE2_WALKER_INL_H_
7
8// Helper class for traversing Regexps without recursion.
9// Clients should declare their own subclasses that override
10// the PreVisit and PostVisit methods, which are called before
11// and after visiting the subexpressions.
12
13// Not quite the Visitor pattern, because (among other things)
14// the Visitor pattern is recursive.
15
16#include <stack>
17
18#include "util/logging.h"
19#include "re2/regexp.h"
20
21namespace re2 {
22
23template<typename T> struct WalkState;
24
25template<typename T> class Regexp::Walker {
26 public:
27 Walker();
28 virtual ~Walker();
29
30 // Virtual method called before visiting re's children.
31 // PreVisit passes ownership of its return value to its caller.
32 // The Arg* that PreVisit returns will be passed to PostVisit as pre_arg
33 // and passed to the child PreVisits and PostVisits as parent_arg.
34 // At the top-most Regexp, parent_arg is arg passed to walk.
35 // If PreVisit sets *stop to true, the walk does not recurse
36 // into the children. Instead it behaves as though the return
37 // value from PreVisit is the return value from PostVisit.
38 // The default PreVisit returns parent_arg.
39 virtual T PreVisit(Regexp* re, T parent_arg, bool* stop);
40
41 // Virtual method called after visiting re's children.
42 // The pre_arg is the T that PreVisit returned.
43 // The child_args is a vector of the T that the child PostVisits returned.
44 // PostVisit takes ownership of pre_arg.
45 // PostVisit takes ownership of the Ts
46 // in *child_args, but not the vector itself.
47 // PostVisit passes ownership of its return value
48 // to its caller.
49 // The default PostVisit simply returns pre_arg.
50 virtual T PostVisit(Regexp* re, T parent_arg, T pre_arg,
51 T* child_args, int nchild_args);
52
53 // Virtual method called to copy a T,
54 // when Walk notices that more than one child is the same re.
55 virtual T Copy(T arg);
56
57 // Virtual method called to do a "quick visit" of the re,
58 // but not its children. Only called once the visit budget
59 // has been used up and we're trying to abort the walk
60 // as quickly as possible. Should return a value that
61 // makes sense for the parent PostVisits still to be run.
62 // This function is (hopefully) only called by
63 // WalkExponential, but must be implemented by all clients,
64 // just in case.
65 virtual T ShortVisit(Regexp* re, T parent_arg) = 0;
66
67 // Walks over a regular expression.
68 // Top_arg is passed as parent_arg to PreVisit and PostVisit of re.
69 // Returns the T returned by PostVisit on re.
70 T Walk(Regexp* re, T top_arg);
71
72 // Like Walk, but doesn't use Copy. This can lead to
73 // exponential runtimes on cross-linked Regexps like the
74 // ones generated by Simplify. To help limit this,
75 // at most max_visits nodes will be visited and then
76 // the walk will be cut off early.
77 // If the walk *is* cut off early, ShortVisit(re)
78 // will be called on regexps that cannot be fully
79 // visited rather than calling PreVisit/PostVisit.
80 T WalkExponential(Regexp* re, T top_arg, int max_visits);
81
82 // Clears the stack. Should never be necessary, since
83 // Walk always enters and exits with an empty stack.
84 // Logs DFATAL if stack is not already clear.
85 void Reset();
86
87 // Returns whether walk was cut off.
88 bool stopped_early() { return stopped_early_; }
89
90 private:
91 // Walk state for the entire traversal.
92 std::stack<WalkState<T> >* stack_;
93 bool stopped_early_;
94 int max_visits_;
95
96 T WalkInternal(Regexp* re, T top_arg, bool use_copy);
97
98 Walker(const Walker&) = delete;
99 Walker& operator=(const Walker&) = delete;
100};
101
102template<typename T> T Regexp::Walker<T>::PreVisit(Regexp* re,
103 T parent_arg,
104 bool* stop) {
105 return parent_arg;
106}
107
108template<typename T> T Regexp::Walker<T>::PostVisit(Regexp* re,
109 T parent_arg,
110 T pre_arg,
111 T* child_args,
112 int nchild_args) {
113 return pre_arg;
114}
115
116template<typename T> T Regexp::Walker<T>::Copy(T arg) {
117 return arg;
118}
119
120// State about a single level in the traversal.
121template<typename T> struct WalkState {
122 WalkState<T>(Regexp* re, T parent)
123 : re(re),
124 n(-1),
125 parent_arg(parent),
126 child_args(NULL) { }
127
128 Regexp* re; // The regexp
129 int n; // The index of the next child to process; -1 means need to PreVisit
130 T parent_arg; // Accumulated arguments.
131 T pre_arg;
132 T child_arg; // One-element buffer for child_args.
133 T* child_args;
134};
135
136template<typename T> Regexp::Walker<T>::Walker() {
137 stack_ = new std::stack<WalkState<T> >;
138 stopped_early_ = false;
139}
140
141template<typename T> Regexp::Walker<T>::~Walker() {
142 Reset();
143 delete stack_;
144}
145
146// Clears the stack. Should never be necessary, since
147// Walk always enters and exits with an empty stack.
148// Logs DFATAL if stack is not already clear.
149template<typename T> void Regexp::Walker<T>::Reset() {
150 if (stack_ && stack_->size() > 0) {
151 LOG(DFATAL) << "Stack not empty.";
152 while (stack_->size() > 0) {
153 delete[] stack_->top().child_args;
154 stack_->pop();
155 }
156 }
157}
158
159template<typename T> T Regexp::Walker<T>::WalkInternal(Regexp* re, T top_arg,
160 bool use_copy) {
161 Reset();
162
163 if (re == NULL) {
164 LOG(DFATAL) << "Walk NULL";
165 return top_arg;
166 }
167
168 stack_->push(WalkState<T>(re, top_arg));
169
170 WalkState<T>* s;
171 for (;;) {
172 T t;
173 s = &stack_->top();
174 Regexp* re = s->re;
175 switch (s->n) {
176 case -1: {
177 if (--max_visits_ < 0) {
178 stopped_early_ = true;
179 t = ShortVisit(re, s->parent_arg);
180 break;
181 }
182 bool stop = false;
183 s->pre_arg = PreVisit(re, s->parent_arg, &stop);
184 if (stop) {
185 t = s->pre_arg;
186 break;
187 }
188 s->n = 0;
189 s->child_args = NULL;
190 if (re->nsub_ == 1)
191 s->child_args = &s->child_arg;
192 else if (re->nsub_ > 1)
193 s->child_args = new T[re->nsub_];
194 FALLTHROUGH_INTENDED;
195 }
196 default: {
197 if (re->nsub_ > 0) {
198 Regexp** sub = re->sub();
199 if (s->n < re->nsub_) {
200 if (use_copy && s->n > 0 && sub[s->n - 1] == sub[s->n]) {
201 s->child_args[s->n] = Copy(s->child_args[s->n - 1]);
202 s->n++;
203 } else {
204 stack_->push(WalkState<T>(sub[s->n], s->pre_arg));
205 }
206 continue;
207 }
208 }
209
210 t = PostVisit(re, s->parent_arg, s->pre_arg, s->child_args, s->n);
211 if (re->nsub_ > 1)
212 delete[] s->child_args;
213 break;
214 }
215 }
216
217 // We've finished stack_->top().
218 // Update next guy down.
219 stack_->pop();
220 if (stack_->size() == 0)
221 return t;
222 s = &stack_->top();
223 if (s->child_args != NULL)
224 s->child_args[s->n] = t;
225 else
226 s->child_arg = t;
227 s->n++;
228 }
229}
230
231template<typename T> T Regexp::Walker<T>::Walk(Regexp* re, T top_arg) {
232 // Without the exponential walking behavior,
233 // this budget should be more than enough for any
234 // regexp, and yet not enough to get us in trouble
235 // as far as CPU time.
236 max_visits_ = 1000000;
237 return WalkInternal(re, top_arg, true);
238}
239
240template<typename T> T Regexp::Walker<T>::WalkExponential(Regexp* re, T top_arg,
241 int max_visits) {
242 max_visits_ = max_visits;
243 return WalkInternal(re, top_arg, false);
244}
245
246} // namespace re2
247
248#endif // RE2_WALKER_INL_H_
249