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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.
+
+// 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.
+
+#ifndef RE2_WALKER_INL_H__
+#define RE2_WALKER_INL_H__
+
+#include "re2/regexp.h"
+
+namespace re2 {
+
+template<typename T> struct WalkState;
+
+template<typename T> class Regexp::Walker {
+ public:
+  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_; }
+
+ private:
+  // Walk state for the entire traversal.
+  stack<WalkState<T> >* stack_;
+  bool stopped_early_;
+  int max_visits_;
+
+  T WalkInternal(Regexp* re, T top_arg, bool use_copy);
+
+  DISALLOW_EVIL_CONSTRUCTORS(Walker);
+};
+
+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<T>(Regexp* re, T parent)
+    : re(re),
+      n(-1),
+      parent_arg(parent),
+      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() {
+  stack_ = new stack<WalkState<T> >;
+  stopped_early_ = false;
+}
+
+template<typename T> Regexp::Walker<T>::~Walker() {
+  Reset();
+  delete stack_;
+}
+
+// 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_ && stack_->size() > 0) {
+    LOG(DFATAL) << "Stack not empty.";
+    while (stack_->size() > 0) {
+      delete stack_->top().child_args;
+      stack_->pop();
+    }
+  }
+}
+
+template<typename T> T Regexp::Walker<T>::WalkInternal(Regexp* re, T top_arg,
+                                                       bool use_copy) {
+  Reset();
+
+  if (re == NULL) {
+    LOG(DFATAL) << "Walk NULL";
+    return top_arg;
+  }
+
+  stack_->push(WalkState<T>(re, top_arg));
+
+  WalkState<T>* s;
+  for (;;) {
+    T t;
+    s = &stack_->top();
+    Regexp* re = s->re;
+    switch (s->n) {
+      case -1: {
+        if (--max_visits_ < 0) {
+          stopped_early_ = true;
+          t = ShortVisit(re, s->parent_arg);
+          break;
+        }
+        bool stop = false;
+        s->pre_arg = PreVisit(re, s->parent_arg, &stop);
+        if (stop) {
+          t = s->pre_arg;
+          break;
+        }
+        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_];
+        // Fall through.
+      }
+      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]);
+              s->n++;
+            } else {
+              stack_->push(WalkState<T>(sub[s->n], s->pre_arg));
+            }
+            continue;
+          }
+        }
+
+        t = PostVisit(re, s->parent_arg, s->pre_arg, s->child_args, s->n);
+        if (re->nsub_ > 1)
+          delete[] s->child_args;
+        break;
+      }
+    }
+
+    // We've finished stack_->top().
+    // Update next guy down.
+    stack_->pop();
+    if (stack_->size() == 0)
+      return t;
+    s = &stack_->top();
+    if (s->child_args != NULL)
+      s->child_args[s->n] = t;
+    else
+      s->child_arg = t;
+    s->n++;
+  }
+}
+
+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__