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 //===- llvm/ADT/DepthFirstIterator.h - Depth First iterator -----*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// This file builds on the ADT/GraphTraits.h file to build generic depth
 /// first graph iterator.  This file exposes the following functions/types:
 ///
 /// df_begin/df_end/df_iterator
 ///   * Normal depth-first iteration - visit a node and then all of its
 ///     children.
 ///
 /// idf_begin/idf_end/idf_iterator
 ///   * Depth-first iteration on the 'inverse' graph.
 ///
 /// df_ext_begin/df_ext_end/df_ext_iterator
 ///   * Normal depth-first iteration - visit a node and then all of its
 ///     children. This iterator stores the 'visited' set in an external set,
 ///     which allows it to be more efficient, and allows external clients to
 ///     use the set for other purposes.
 ///
 /// idf_ext_begin/idf_ext_end/idf_ext_iterator
 ///   * Depth-first iteration on the 'inverse' graph.
 ///     This iterator stores the 'visited' set in an external set, which
 ///     allows it to be more efficient, and allows external clients to use
 ///     the set for other purposes.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_DEPTHFIRSTITERATOR_H
 #define LLVM_ADT_DEPTHFIRSTITERATOR_H
 
 #include "llvm/ADT/GraphTraits.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/iterator_range.h"
 #include <iterator>
 #include <optional>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
 namespace llvm {
 
 // df_iterator_storage - A private class which is used to figure out where to
 // store the visited set.
 template<class SetType, bool External>   // Non-external set
 class df_iterator_storage {
 public:
   SetType Visited;
 };
 
 template<class SetType>
 class df_iterator_storage<SetType, true> {
 public:
   df_iterator_storage(SetType &VSet) : Visited(VSet) {}
   df_iterator_storage(const df_iterator_storage &S) : Visited(S.Visited) {}
 
   SetType &Visited;
 };
 
 // The visited stated for the iteration is a simple set augmented with
 // one more method, completed, which is invoked when all children of a
 // node have been processed. It is intended to distinguish of back and
 // cross edges in the spanning tree but is not used in the common case.
 template <typename NodeRef, unsigned SmallSize=8>
 struct df_iterator_default_set : public SmallPtrSet<NodeRef, SmallSize> {
   using BaseSet = SmallPtrSet<NodeRef, SmallSize>;
   using iterator = typename BaseSet::iterator;
 
   std::pair<iterator,bool> insert(NodeRef N) { return BaseSet::insert(N); }
   template <typename IterT>
   void insert(IterT Begin, IterT End) { BaseSet::insert(Begin,End); }
 
   void completed(NodeRef) {}
 };
 
 // Generic Depth First Iterator
 template <class GraphT,
           class SetType =
               df_iterator_default_set<typename GraphTraits<GraphT>::NodeRef>,
           bool ExtStorage = false, class GT = GraphTraits<GraphT>>
 class df_iterator : public df_iterator_storage<SetType, ExtStorage> {
 public:
   // When External storage is used we are not multi-pass safe.
   using iterator_category =
       std::conditional_t<ExtStorage, std::input_iterator_tag,
                          std::forward_iterator_tag>;
   using value_type = typename GT::NodeRef;
   using difference_type = std::ptrdiff_t;
   using pointer = value_type *;
   using reference = const value_type &;
 
 private:
   using NodeRef = typename GT::NodeRef;
   using ChildItTy = typename GT::ChildIteratorType;
 
   // First element is node reference, second is the 'next child' to visit.
   // The second child is initialized lazily to pick up graph changes during the
   // DFS.
   using StackElement = std::pair<NodeRef, std::optional<ChildItTy>>;
 
   // VisitStack - Used to maintain the ordering.  Top = current block
   std::vector<StackElement> VisitStack;
 
   inline df_iterator(NodeRef Node) {
     this->Visited.insert(Node);
     VisitStack.push_back(StackElement(Node, std::nullopt));
   }
 
   inline df_iterator() = default; // End is when stack is empty
 
   inline df_iterator(NodeRef Node, SetType &S)
       : df_iterator_storage<SetType, ExtStorage>(S) {
     if (this->Visited.insert(Node).second)
       VisitStack.push_back(StackElement(Node, std::nullopt));
   }
 
   inline df_iterator(SetType &S)
     : df_iterator_storage<SetType, ExtStorage>(S) {
     // End is when stack is empty
   }
 
   inline void toNext() {
     do {
       NodeRef Node = VisitStack.back().first;
       std::optional<ChildItTy> &Opt = VisitStack.back().second;
 
       if (!Opt)
         Opt.emplace(GT::child_begin(Node));
 
       // Notice that we directly mutate *Opt here, so that
       // VisitStack.back().second actually gets updated as the iterator
       // increases.
       while (*Opt != GT::child_end(Node)) {
         NodeRef Next = *(*Opt)++;
         // Has our next sibling been visited?
         if (this->Visited.insert(Next).second) {
           // No, do it now.
           VisitStack.push_back(StackElement(Next, std::nullopt));
           return;
         }
       }
       this->Visited.completed(Node);
 
       // Oops, ran out of successors... go up a level on the stack.
       VisitStack.pop_back();
     } while (!VisitStack.empty());
   }
 
 public:
   // Provide static begin and end methods as our public "constructors"
   static df_iterator begin(const GraphT &G) {
     return df_iterator(GT::getEntryNode(G));
   }
   static df_iterator end(const GraphT &G) { return df_iterator(); }
 
   // Static begin and end methods as our public ctors for external iterators
   static df_iterator begin(const GraphT &G, SetType &S) {
     return df_iterator(GT::getEntryNode(G), S);
   }
   static df_iterator end(const GraphT &G, SetType &S) { return df_iterator(S); }
 
   bool operator==(const df_iterator &x) const {
     return VisitStack == x.VisitStack;
   }
   bool operator!=(const df_iterator &x) const { return !(*this == x); }
 
   reference operator*() const { return VisitStack.back().first; }
 
   // This is a nonstandard operator-> that dereferences the pointer an extra
   // time... so that you can actually call methods ON the Node, because
   // the contained type is a pointer.  This allows BBIt->getTerminator() f.e.
   //
   NodeRef operator->() const { return **this; }
 
   df_iterator &operator++() { // Preincrement
     toNext();
     return *this;
   }
 
   /// Skips all children of the current node and traverses to next node
   ///
   /// Note: This function takes care of incrementing the iterator. If you
   /// always increment and call this function, you risk walking off the end.
   df_iterator &skipChildren() {
     VisitStack.pop_back();
     if (!VisitStack.empty())
       toNext();
     return *this;
   }
 
   df_iterator operator++(int) { // Postincrement
     df_iterator tmp = *this;
     ++*this;
     return tmp;
   }
 
   // nodeVisited - return true if this iterator has already visited the
   // specified node.  This is public, and will probably be used to iterate over
   // nodes that a depth first iteration did not find: ie unreachable nodes.
   //
   bool nodeVisited(NodeRef Node) const {
     return this->Visited.contains(Node);
   }
 
   /// getPathLength - Return the length of the path from the entry node to the
   /// current node, counting both nodes.
   unsigned getPathLength() const { return VisitStack.size(); }
 
   /// getPath - Return the n'th node in the path from the entry node to the
   /// current node.
   NodeRef getPath(unsigned n) const { return VisitStack[n].first; }
 };
 
 // Provide global constructors that automatically figure out correct types...
 //
 template <class T>
 df_iterator<T> df_begin(const T& G) {
   return df_iterator<T>::begin(G);
 }
 
 template <class T>
 df_iterator<T> df_end(const T& G) {
   return df_iterator<T>::end(G);
 }
 
 // Provide an accessor method to use them in range-based patterns.
 template <class T>
 iterator_range<df_iterator<T>> depth_first(const T& G) {
   return make_range(df_begin(G), df_end(G));
 }
 
 // Provide global definitions of external depth first iterators...
 template <class T, class SetTy = df_iterator_default_set<typename GraphTraits<T>::NodeRef>>
 struct df_ext_iterator : public df_iterator<T, SetTy, true> {
   df_ext_iterator(const df_iterator<T, SetTy, true> &V)
     : df_iterator<T, SetTy, true>(V) {}
 };
 
 template <class T, class SetTy>
 df_ext_iterator<T, SetTy> df_ext_begin(const T& G, SetTy &S) {
   return df_ext_iterator<T, SetTy>::begin(G, S);
 }
 
 template <class T, class SetTy>
 df_ext_iterator<T, SetTy> df_ext_end(const T& G, SetTy &S) {
   return df_ext_iterator<T, SetTy>::end(G, S);
 }
 
 template <class T, class SetTy>
 iterator_range<df_ext_iterator<T, SetTy>> depth_first_ext(const T& G,
                                                           SetTy &S) {
   return make_range(df_ext_begin(G, S), df_ext_end(G, S));
 }
 
 // Provide global definitions of inverse depth first iterators...
 template <class T,
           class SetTy =
               df_iterator_default_set<typename GraphTraits<T>::NodeRef>,
           bool External = false>
 struct idf_iterator : public df_iterator<Inverse<T>, SetTy, External> {
   idf_iterator(const df_iterator<Inverse<T>, SetTy, External> &V)
     : df_iterator<Inverse<T>, SetTy, External>(V) {}
 };
 
 template <class T>
 idf_iterator<T> idf_begin(const T& G) {
   return idf_iterator<T>::begin(Inverse<T>(G));
 }
 
 template <class T>
 idf_iterator<T> idf_end(const T& G){
   return idf_iterator<T>::end(Inverse<T>(G));
 }
 
 // Provide an accessor method to use them in range-based patterns.
 template <class T>
 iterator_range<idf_iterator<T>> inverse_depth_first(const T& G) {
   return make_range(idf_begin(G), idf_end(G));
 }
 
 // Provide global definitions of external inverse depth first iterators...
 template <class T, class SetTy = df_iterator_default_set<typename GraphTraits<T>::NodeRef>>
 struct idf_ext_iterator : public idf_iterator<T, SetTy, true> {
   idf_ext_iterator(const idf_iterator<T, SetTy, true> &V)
     : idf_iterator<T, SetTy, true>(V) {}
   idf_ext_iterator(const df_iterator<Inverse<T>, SetTy, true> &V)
     : idf_iterator<T, SetTy, true>(V) {}
 };
 
 template <class T, class SetTy>
 idf_ext_iterator<T, SetTy> idf_ext_begin(const T& G, SetTy &S) {
   return idf_ext_iterator<T, SetTy>::begin(Inverse<T>(G), S);
 }
 
 template <class T, class SetTy>
 idf_ext_iterator<T, SetTy> idf_ext_end(const T& G, SetTy &S) {
   return idf_ext_iterator<T, SetTy>::end(Inverse<T>(G), S);
 }
 
 template <class T, class SetTy>
 iterator_range<idf_ext_iterator<T, SetTy>> inverse_depth_first_ext(const T& G,
                                                                    SetTy &S) {
   return make_range(idf_ext_begin(G, S), idf_ext_end(G, S));
 }
 
 } // end namespace llvm
 
 #endif // LLVM_ADT_DEPTHFIRSTITERATOR_H

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