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//===- llvm/ADT/BreadthFirstIterator.h - Breadth 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 a generic breadth
/// first graph iterator. This file exposes the following functions/types:
///
/// bf_begin/bf_end/bf_iterator
/// * Normal breadth-first iteration - visit a graph level-by-level.
///
//===----------------------------------------------------------------------===//
#ifndef LLVM_ADT_BREADTHFIRSTITERATOR_H
#define LLVM_ADT_BREADTHFIRSTITERATOR_H
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/iterator_range.h"
#include <iterator>
#include <optional>
#include <queue>
#include <utility>
namespace llvm {
// bf_iterator_storage - A private class which is used to figure out where to
// store the visited set. We only provide a non-external variant for now.
template <class SetType> class bf_iterator_storage {
public:
SetType Visited;
};
// The visited state for the iteration is a simple set.
template <typename NodeRef, unsigned SmallSize = 8>
using bf_iterator_default_set = SmallPtrSet<NodeRef, SmallSize>;
// Generic Breadth first search iterator.
template <class GraphT,
class SetType =
bf_iterator_default_set<typename GraphTraits<GraphT>::NodeRef>,
class GT = GraphTraits<GraphT>>
class bf_iterator : public bf_iterator_storage<SetType> {
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 = const value_type &;
private:
using NodeRef = typename GT::NodeRef;
using ChildItTy = typename GT::ChildIteratorType;
// First element is the node reference, second is the next child to visit.
using QueueElement = std::pair<NodeRef, std::optional<ChildItTy>>;
// Visit queue - used to maintain BFS ordering.
// std::optional<> because we need markers for levels.
std::queue<std::optional<QueueElement>> VisitQueue;
// Current level.
unsigned Level = 0;
inline bf_iterator(NodeRef Node) {
this->Visited.insert(Node);
Level = 0;
// Also, insert a dummy node as marker.
VisitQueue.push(QueueElement(Node, std::nullopt));
VisitQueue.push(std::nullopt);
}
inline bf_iterator() = default;
inline void toNext() {
std::optional<QueueElement> Head = VisitQueue.front();
QueueElement H = *Head;
NodeRef Node = H.first;
std::optional<ChildItTy> &ChildIt = H.second;
if (!ChildIt)
ChildIt.emplace(GT::child_begin(Node));
while (*ChildIt != GT::child_end(Node)) {
NodeRef Next = *(*ChildIt)++;
// Already visited?
if (this->Visited.insert(Next).second)
VisitQueue.push(QueueElement(Next, std::nullopt));
}
VisitQueue.pop();
// Go to the next element skipping markers if needed.
if (!VisitQueue.empty()) {
Head = VisitQueue.front();
if (Head != std::nullopt)
return;
Level += 1;
VisitQueue.pop();
// Don't push another marker if this is the last
// element.
if (!VisitQueue.empty())
VisitQueue.push(std::nullopt);
}
}
public:
// Provide static begin and end methods as our public "constructors"
static bf_iterator begin(const GraphT &G) {
return bf_iterator(GT::getEntryNode(G));
}
static bf_iterator end(const GraphT &G) { return bf_iterator(); }
bool operator==(const bf_iterator &RHS) const {
return VisitQueue == RHS.VisitQueue;
}
bool operator!=(const bf_iterator &RHS) const { return !(*this == RHS); }
reference operator*() const { return VisitQueue.front()->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.
NodeRef operator->() const { return **this; }
bf_iterator &operator++() { // Pre-increment
toNext();
return *this;
}
bf_iterator operator++(int) { // Post-increment
bf_iterator ItCopy = *this;
++*this;
return ItCopy;
}
unsigned getLevel() const { return Level; }
};
// Provide global constructors that automatically figure out correct types.
template <class T> bf_iterator<T> bf_begin(const T &G) {
return bf_iterator<T>::begin(G);
}
template <class T> bf_iterator<T> bf_end(const T &G) {
return bf_iterator<T>::end(G);
}
// Provide an accessor method to use them in range-based patterns.
template <class T> iterator_range<bf_iterator<T>> breadth_first(const T &G) {
return make_range(bf_begin(G), bf_end(G));
}
} // end namespace llvm
#endif // LLVM_ADT_BREADTHFIRSTITERATOR_H
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