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 "absl/base/macros.h" |
19 | #include "util/logging.h" |
20 | #include "re2/regexp.h" |
21 | |
22 | namespace re2 { |
23 | |
24 | template<typename T> struct WalkState; |
25 | |
26 | template<typename T> class Regexp::Walker { |
27 | public: |
28 | Walker(); |
29 | virtual ~Walker(); |
30 | |
31 | // Virtual method called before visiting re's children. |
32 | // PreVisit passes ownership of its return value to its caller. |
33 | // The Arg* that PreVisit returns will be passed to PostVisit as pre_arg |
34 | // and passed to the child PreVisits and PostVisits as parent_arg. |
35 | // At the top-most Regexp, parent_arg is arg passed to walk. |
36 | // If PreVisit sets *stop to true, the walk does not recurse |
37 | // into the children. Instead it behaves as though the return |
38 | // value from PreVisit is the return value from PostVisit. |
39 | // The default PreVisit returns parent_arg. |
40 | virtual T PreVisit(Regexp* re, T parent_arg, bool* stop); |
41 | |
42 | // Virtual method called after visiting re's children. |
43 | // The pre_arg is the T that PreVisit returned. |
44 | // The child_args is a vector of the T that the child PostVisits returned. |
45 | // PostVisit takes ownership of pre_arg. |
46 | // PostVisit takes ownership of the Ts |
47 | // in *child_args, but not the vector itself. |
48 | // PostVisit passes ownership of its return value |
49 | // to its caller. |
50 | // The default PostVisit simply returns pre_arg. |
51 | virtual T PostVisit(Regexp* re, T parent_arg, T pre_arg, |
52 | T* child_args, int nchild_args); |
53 | |
54 | // Virtual method called to copy a T, |
55 | // when Walk notices that more than one child is the same re. |
56 | virtual T Copy(T arg); |
57 | |
58 | // Virtual method called to do a "quick visit" of the re, |
59 | // but not its children. Only called once the visit budget |
60 | // has been used up and we're trying to abort the walk |
61 | // as quickly as possible. Should return a value that |
62 | // makes sense for the parent PostVisits still to be run. |
63 | // This function is (hopefully) only called by |
64 | // WalkExponential, but must be implemented by all clients, |
65 | // just in case. |
66 | virtual T ShortVisit(Regexp* re, T parent_arg) = 0; |
67 | |
68 | // Walks over a regular expression. |
69 | // Top_arg is passed as parent_arg to PreVisit and PostVisit of re. |
70 | // Returns the T returned by PostVisit on re. |
71 | T Walk(Regexp* re, T top_arg); |
72 | |
73 | // Like Walk, but doesn't use Copy. This can lead to |
74 | // exponential runtimes on cross-linked Regexps like the |
75 | // ones generated by Simplify. To help limit this, |
76 | // at most max_visits nodes will be visited and then |
77 | // the walk will be cut off early. |
78 | // If the walk *is* cut off early, ShortVisit(re) |
79 | // will be called on regexps that cannot be fully |
80 | // visited rather than calling PreVisit/PostVisit. |
81 | T WalkExponential(Regexp* re, T top_arg, int max_visits); |
82 | |
83 | // Clears the stack. Should never be necessary, since |
84 | // Walk always enters and exits with an empty stack. |
85 | // Logs DFATAL if stack is not already clear. |
86 | void Reset(); |
87 | |
88 | // Returns whether walk was cut off. |
89 | bool stopped_early() { return stopped_early_; } |
90 | |
91 | private: |
92 | // Walk state for the entire traversal. |
93 | std::stack<WalkState<T>> stack_; |
94 | bool stopped_early_; |
95 | int max_visits_; |
96 | |
97 | T WalkInternal(Regexp* re, T top_arg, bool use_copy); |
98 | |
99 | Walker(const Walker&) = delete; |
100 | Walker& operator=(const Walker&) = delete; |
101 | }; |
102 | |
103 | template<typename T> T Regexp::Walker<T>::PreVisit(Regexp* re, |
104 | T parent_arg, |
105 | bool* stop) { |
106 | return parent_arg; |
107 | } |
108 | |
109 | template<typename T> T Regexp::Walker<T>::PostVisit(Regexp* re, |
110 | T parent_arg, |
111 | T pre_arg, |
112 | T* child_args, |
113 | int nchild_args) { |
114 | return pre_arg; |
115 | } |
116 | |
117 | template<typename T> T Regexp::Walker<T>::Copy(T arg) { |
118 | return arg; |
119 | } |
120 | |
121 | // State about a single level in the traversal. |
122 | template<typename T> struct WalkState { |
123 | WalkState(Regexp* re, T parent) |
124 | : re(re), |
125 | n(-1), |
126 | parent_arg(parent), |
127 | child_args(NULL) { } |
128 | |
129 | Regexp* re; // The regexp |
130 | int n; // The index of the next child to process; -1 means need to PreVisit |
131 | T parent_arg; // Accumulated arguments. |
132 | T pre_arg; |
133 | T child_arg; // One-element buffer for child_args. |
134 | T* child_args; |
135 | }; |
136 | |
137 | template<typename T> Regexp::Walker<T>::Walker() { |
138 | stopped_early_ = false; |
139 | } |
140 | |
141 | template<typename T> Regexp::Walker<T>::~Walker() { |
142 | Reset(); |
143 | } |
144 | |
145 | // Clears the stack. Should never be necessary, since |
146 | // Walk always enters and exits with an empty stack. |
147 | // Logs DFATAL if stack is not already clear. |
148 | template<typename T> void Regexp::Walker<T>::Reset() { |
149 | if (!stack_.empty()) { |
150 | LOG(DFATAL) << "Stack not empty." ; |
151 | while (!stack_.empty()) { |
152 | if (stack_.top().re->nsub_ > 1) |
153 | delete[] stack_.top().child_args; |
154 | stack_.pop(); |
155 | } |
156 | } |
157 | } |
158 | |
159 | template<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 | 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 | ABSL_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_.empty()) |
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 | |
231 | template<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 | |
240 | template<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 | |