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// Copyright 2006 The RE2 Authors. All Rights Reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
#ifndef RE2_WALKER_INL_H_
#define RE2_WALKER_INL_H_
// Helper class for traversing Regexps without recursion.
// Clients should declare their own subclasses that override
// the PreVisit and PostVisit methods, which are called before
// and after visiting the subexpressions.
// Not quite the Visitor pattern, because (among other things)
// the Visitor pattern is recursive.
#include <stack>
#include "absl/base/macros.h"
#include "util/logging.h"
#include "re2/regexp.h"
namespace re2 {
template<typename T> struct WalkState;
template<typename T> class Regexp::Walker {
virtual ~Walker();
// Virtual method called before visiting re's children.
// PreVisit passes ownership of its return value to its caller.
// The Arg* that PreVisit returns will be passed to PostVisit as pre_arg
// and passed to the child PreVisits and PostVisits as parent_arg.
// At the top-most Regexp, parent_arg is arg passed to walk.
// If PreVisit sets *stop to true, the walk does not recurse
// into the children. Instead it behaves as though the return
// value from PreVisit is the return value from PostVisit.
// The default PreVisit returns parent_arg.
virtual T PreVisit(Regexp* re, T parent_arg, bool* stop);
// Virtual method called after visiting re's children.
// The pre_arg is the T that PreVisit returned.
// The child_args is a vector of the T that the child PostVisits returned.
// PostVisit takes ownership of pre_arg.
// PostVisit takes ownership of the Ts
// in *child_args, but not the vector itself.
// PostVisit passes ownership of its return value
// to its caller.
// The default PostVisit simply returns pre_arg.
virtual T PostVisit(Regexp* re, T parent_arg, T pre_arg,
T* child_args, int nchild_args);
// Virtual method called to copy a T,
// when Walk notices that more than one child is the same re.
virtual T Copy(T arg);
// Virtual method called to do a "quick visit" of the re,
// but not its children. Only called once the visit budget
// has been used up and we're trying to abort the walk
// as quickly as possible. Should return a value that
// makes sense for the parent PostVisits still to be run.
// This function is (hopefully) only called by
// WalkExponential, but must be implemented by all clients,
// just in case.
virtual T ShortVisit(Regexp* re, T parent_arg) = 0;
// Walks over a regular expression.
// Top_arg is passed as parent_arg to PreVisit and PostVisit of re.
// Returns the T returned by PostVisit on re.
T Walk(Regexp* re, T top_arg);
// Like Walk, but doesn't use Copy. This can lead to
// exponential runtimes on cross-linked Regexps like the
// ones generated by Simplify. To help limit this,
// at most max_visits nodes will be visited and then
// the walk will be cut off early.
// If the walk *is* cut off early, ShortVisit(re)
// will be called on regexps that cannot be fully
// visited rather than calling PreVisit/PostVisit.
T WalkExponential(Regexp* re, T top_arg, int max_visits);
// Clears the stack. Should never be necessary, since
// Walk always enters and exits with an empty stack.
// Logs DFATAL if stack is not already clear.
void Reset();
// Returns whether walk was cut off.
bool stopped_early() { return stopped_early_; }
// Walk state for the entire traversal.
std::stack<WalkState<T>> stack_;
bool stopped_early_;
int max_visits_;
T WalkInternal(Regexp* re, T top_arg, bool use_copy);
Walker(const Walker&) = delete;
Walker& operator=(const Walker&) = delete;
template<typename T> T Regexp::Walker<T>::PreVisit(Regexp* re,
T parent_arg,
bool* stop) {
return parent_arg;
template<typename T> T Regexp::Walker<T>::PostVisit(Regexp* re,
T parent_arg,
T pre_arg,
T* child_args,
int nchild_args) {
return pre_arg;
template<typename T> T Regexp::Walker<T>::Copy(T arg) {
return arg;
// State about a single level in the traversal.
template<typename T> struct WalkState {
WalkState(Regexp* re, T parent)
: re(re),
child_args(NULL) { }
Regexp* re; // The regexp
int n; // The index of the next child to process; -1 means need to PreVisit
T parent_arg; // Accumulated arguments.
T pre_arg;
T child_arg; // One-element buffer for child_args.
T* child_args;
template<typename T> Regexp::Walker<T>::Walker() {
stopped_early_ = false;
template<typename T> Regexp::Walker<T>::~Walker() {
// Clears the stack. Should never be necessary, since
// Walk always enters and exits with an empty stack.
// Logs DFATAL if stack is not already clear.
template<typename T> void Regexp::Walker<T>::Reset() {
if (!stack_.empty()) {
LOG(DFATAL) << "Stack not empty.";
while (!stack_.empty()) {
if (>nsub_ > 1)
template<typename T> T Regexp::Walker<T>::WalkInternal(Regexp* re, T top_arg,
bool use_copy) {
if (re == NULL) {
return top_arg;
stack_.push(WalkState<T>(re, top_arg));
WalkState<T>* s;
for (;;) {
T t;
s = &;
re = s->re;
switch (s->n) {
case -1: {
if (--max_visits_ < 0) {
stopped_early_ = true;
t = ShortVisit(re, s->parent_arg);
bool stop = false;
s->pre_arg = PreVisit(re, s->parent_arg, &stop);
if (stop) {
t = s->pre_arg;
s->n = 0;
s->child_args = NULL;
if (re->nsub_ == 1)
s->child_args = &s->child_arg;
else if (re->nsub_ > 1)
s->child_args = new T[re->nsub_];
default: {
if (re->nsub_ > 0) {
Regexp** sub = re->sub();
if (s->n < re->nsub_) {
if (use_copy && s->n > 0 && sub[s->n - 1] == sub[s->n]) {
s->child_args[s->n] = Copy(s->child_args[s->n - 1]);
} else {
stack_.push(WalkState<T>(sub[s->n], s->pre_arg));
t = PostVisit(re, s->parent_arg, s->pre_arg, s->child_args, s->n);
if (re->nsub_ > 1)
delete[] s->child_args;
// We've finished
// Update next guy down.
if (stack_.empty())
return t;
s = &;
if (s->child_args != NULL)
s->child_args[s->n] = t;
s->child_arg = t;
template<typename T> T Regexp::Walker<T>::Walk(Regexp* re, T top_arg) {
// Without the exponential walking behavior,
// this budget should be more than enough for any
// regexp, and yet not enough to get us in trouble
// as far as CPU time.
max_visits_ = 1000000;
return WalkInternal(re, top_arg, true);
template<typename T> T Regexp::Walker<T>::WalkExponential(Regexp* re, T top_arg,
int max_visits) {
max_visits_ = max_visits;
return WalkInternal(re, top_arg, false);
} // namespace re2
#endif // RE2_WALKER_INL_H_