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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 <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:

  using iterator_category = std::forward_iterator_tag;

  using value_type = typename GT::NodeRef;

  using difference_type = std::ptrdiff_t;

  using pointer = value_type *;

  using reference = 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); }

  const NodeRef &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