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+//===- iterator.h - Utilities for using and defining iterators --*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_ITERATOR_H
+#define LLVM_ADT_ITERATOR_H
+
+#include "llvm/ADT/iterator_range.h"
+#include <algorithm>
+#include <cstddef>
+#include <iterator>
+#include <type_traits>
+#include <utility>
+
+namespace llvm {
+
+/// CRTP base class which implements the entire standard iterator facade
+/// in terms of a minimal subset of the interface.
+///
+/// Use this when it is reasonable to implement most of the iterator
+/// functionality in terms of a core subset. If you need special behavior or
+/// there are performance implications for this, you may want to override the
+/// relevant members instead.
+///
+/// Note, one abstraction that this does *not* provide is implementing
+/// subtraction in terms of addition by negating the difference. Negation isn't
+/// always information preserving, and I can see very reasonable iterator
+/// designs where this doesn't work well. It doesn't really force much added
+/// boilerplate anyways.
+///
+/// Another abstraction that this doesn't provide is implementing increment in
+/// terms of addition of one. These aren't equivalent for all iterator
+/// categories, and respecting that adds a lot of complexity for little gain.
+///
+/// Classes wishing to use `iterator_facade_base` should implement the following
+/// methods:
+///
+/// Forward Iterators:
+///   (All of the following methods)
+///   - DerivedT &operator=(const DerivedT &R);
+///   - bool operator==(const DerivedT &R) const;
+///   - const T &operator*() const;
+///   - T &operator*();
+///   - DerivedT &operator++();
+///
+/// Bidirectional Iterators:
+///   (All methods of forward iterators, plus the following)
+///   - DerivedT &operator--();
+///
+/// Random-access Iterators:
+///   (All methods of bidirectional iterators excluding the following)
+///   - DerivedT &operator++();
+///   - DerivedT &operator--();
+///   (and plus the following)
+///   - bool operator<(const DerivedT &RHS) const;
+///   - DifferenceTypeT operator-(const DerivedT &R) const;
+///   - DerivedT &operator+=(DifferenceTypeT N);
+///   - DerivedT &operator-=(DifferenceTypeT N);
+///
+template <typename DerivedT, typename IteratorCategoryT, typename T,
+          typename DifferenceTypeT = std::ptrdiff_t, typename PointerT = T *,
+          typename ReferenceT = T &>
+class iterator_facade_base
+    : public std::iterator<IteratorCategoryT, T, DifferenceTypeT, PointerT,
+                           ReferenceT> {
+protected:
+  enum {
+    IsRandomAccess = std::is_base_of<std::random_access_iterator_tag,
+                                     IteratorCategoryT>::value,
+    IsBidirectional = std::is_base_of<std::bidirectional_iterator_tag,
+                                      IteratorCategoryT>::value,
+  };
+
+  /// A proxy object for computing a reference via indirecting a copy of an
+  /// iterator. This is used in APIs which need to produce a reference via
+  /// indirection but for which the iterator object might be a temporary. The
+  /// proxy preserves the iterator internally and exposes the indirected
+  /// reference via a conversion operator.
+  class ReferenceProxy {
+    friend iterator_facade_base;
+
+    DerivedT I;
+
+    ReferenceProxy(DerivedT I) : I(std::move(I)) {}
+
+  public:
+    operator ReferenceT() const { return *I; }
+  };
+
+public:
+  DerivedT operator+(DifferenceTypeT n) const {
+    static_assert(std::is_base_of<iterator_facade_base, DerivedT>::value,
+                  "Must pass the derived type to this template!");
+    static_assert(
+        IsRandomAccess,
+        "The '+' operator is only defined for random access iterators.");
+    DerivedT tmp = *static_cast<const DerivedT *>(this);
+    tmp += n;
+    return tmp;
+  }
+  friend DerivedT operator+(DifferenceTypeT n, const DerivedT &i) {
+    static_assert(
+        IsRandomAccess,
+        "The '+' operator is only defined for random access iterators.");
+    return i + n;
+  }
+  DerivedT operator-(DifferenceTypeT n) const {
+    static_assert(
+        IsRandomAccess,
+        "The '-' operator is only defined for random access iterators.");
+    DerivedT tmp = *static_cast<const DerivedT *>(this);
+    tmp -= n;
+    return tmp;
+  }
+
+  DerivedT &operator++() {
+    static_assert(std::is_base_of<iterator_facade_base, DerivedT>::value,
+                  "Must pass the derived type to this template!");
+    return static_cast<DerivedT *>(this)->operator+=(1);
+  }
+  DerivedT operator++(int) {
+    DerivedT tmp = *static_cast<DerivedT *>(this);
+    ++*static_cast<DerivedT *>(this);
+    return tmp;
+  }
+  DerivedT &operator--() {
+    static_assert(
+        IsBidirectional,
+        "The decrement operator is only defined for bidirectional iterators.");
+    return static_cast<DerivedT *>(this)->operator-=(1);
+  }
+  DerivedT operator--(int) {
+    static_assert(
+        IsBidirectional,
+        "The decrement operator is only defined for bidirectional iterators.");
+    DerivedT tmp = *static_cast<DerivedT *>(this);
+    --*static_cast<DerivedT *>(this);
+    return tmp;
+  }
+
+  bool operator!=(const DerivedT &RHS) const {
+    return !static_cast<const DerivedT *>(this)->operator==(RHS);
+  }
+
+  bool operator>(const DerivedT &RHS) const {
+    static_assert(
+        IsRandomAccess,
+        "Relational operators are only defined for random access iterators.");
+    return !static_cast<const DerivedT *>(this)->operator<(RHS) &&
+           !static_cast<const DerivedT *>(this)->operator==(RHS);
+  }
+  bool operator<=(const DerivedT &RHS) const {
+    static_assert(
+        IsRandomAccess,
+        "Relational operators are only defined for random access iterators.");
+    return !static_cast<const DerivedT *>(this)->operator>(RHS);
+  }
+  bool operator>=(const DerivedT &RHS) const {
+    static_assert(
+        IsRandomAccess,
+        "Relational operators are only defined for random access iterators.");
+    return !static_cast<const DerivedT *>(this)->operator<(RHS);
+  }
+
+  PointerT operator->() { return &static_cast<DerivedT *>(this)->operator*(); }
+  PointerT operator->() const {
+    return &static_cast<const DerivedT *>(this)->operator*();
+  }
+  ReferenceProxy operator[](DifferenceTypeT n) {
+    static_assert(IsRandomAccess,
+                  "Subscripting is only defined for random access iterators.");
+    return ReferenceProxy(static_cast<DerivedT *>(this)->operator+(n));
+  }
+  ReferenceProxy operator[](DifferenceTypeT n) const {
+    static_assert(IsRandomAccess,
+                  "Subscripting is only defined for random access iterators.");
+    return ReferenceProxy(static_cast<const DerivedT *>(this)->operator+(n));
+  }
+};
+
+/// CRTP base class for adapting an iterator to a different type.
+///
+/// This class can be used through CRTP to adapt one iterator into another.
+/// Typically this is done through providing in the derived class a custom \c
+/// operator* implementation. Other methods can be overridden as well.
+template <
+    typename DerivedT, typename WrappedIteratorT,
+    typename IteratorCategoryT =
+        typename std::iterator_traits<WrappedIteratorT>::iterator_category,
+    typename T = typename std::iterator_traits<WrappedIteratorT>::value_type,
+    typename DifferenceTypeT =
+        typename std::iterator_traits<WrappedIteratorT>::difference_type,
+    typename PointerT = typename std::conditional<
+        std::is_same<T, typename std::iterator_traits<
+                            WrappedIteratorT>::value_type>::value,
+        typename std::iterator_traits<WrappedIteratorT>::pointer, T *>::type,
+    typename ReferenceT = typename std::conditional<
+        std::is_same<T, typename std::iterator_traits<
+                            WrappedIteratorT>::value_type>::value,
+        typename std::iterator_traits<WrappedIteratorT>::reference, T &>::type>
+class iterator_adaptor_base
+    : public iterator_facade_base<DerivedT, IteratorCategoryT, T,
+                                  DifferenceTypeT, PointerT, ReferenceT> {
+  using BaseT = typename iterator_adaptor_base::iterator_facade_base;
+
+protected:
+  WrappedIteratorT I;
+
+  iterator_adaptor_base() = default;
+
+  explicit iterator_adaptor_base(WrappedIteratorT u) : I(std::move(u)) {
+    static_assert(std::is_base_of<iterator_adaptor_base, DerivedT>::value,
+                  "Must pass the derived type to this template!");
+  }
+
+  const WrappedIteratorT &wrapped() const { return I; }
+
+public:
+  using difference_type = DifferenceTypeT;
+
+  DerivedT &operator+=(difference_type n) {
+    static_assert(
+        BaseT::IsRandomAccess,
+        "The '+=' operator is only defined for random access iterators.");
+    I += n;
+    return *static_cast<DerivedT *>(this);
+  }
+  DerivedT &operator-=(difference_type n) {
+    static_assert(
+        BaseT::IsRandomAccess,
+        "The '-=' operator is only defined for random access iterators.");
+    I -= n;
+    return *static_cast<DerivedT *>(this);
+  }
+  using BaseT::operator-;
+  difference_type operator-(const DerivedT &RHS) const {
+    static_assert(
+        BaseT::IsRandomAccess,
+        "The '-' operator is only defined for random access iterators.");
+    return I - RHS.I;
+  }
+
+  // We have to explicitly provide ++ and -- rather than letting the facade
+  // forward to += because WrappedIteratorT might not support +=.
+  using BaseT::operator++;
+  DerivedT &operator++() {
+    ++I;
+    return *static_cast<DerivedT *>(this);
+  }
+  using BaseT::operator--;
+  DerivedT &operator--() {
+    static_assert(
+        BaseT::IsBidirectional,
+        "The decrement operator is only defined for bidirectional iterators.");
+    --I;
+    return *static_cast<DerivedT *>(this);
+  }
+
+  bool operator==(const DerivedT &RHS) const { return I == RHS.I; }
+  bool operator<(const DerivedT &RHS) const {
+    static_assert(
+        BaseT::IsRandomAccess,
+        "Relational operators are only defined for random access iterators.");
+    return I < RHS.I;
+  }
+
+  ReferenceT operator*() const { return *I; }
+};
+
+/// An iterator type that allows iterating over the pointees via some
+/// other iterator.
+///
+/// The typical usage of this is to expose a type that iterates over Ts, but
+/// which is implemented with some iterator over T*s:
+///
+/// \code
+///   using iterator = pointee_iterator<SmallVectorImpl<T *>::iterator>;
+/// \endcode
+template <typename WrappedIteratorT,
+          typename T = typename std::remove_reference<
+              decltype(**std::declval<WrappedIteratorT>())>::type>
+struct pointee_iterator
+    : iterator_adaptor_base<
+          pointee_iterator<WrappedIteratorT, T>, WrappedIteratorT,
+          typename std::iterator_traits<WrappedIteratorT>::iterator_category,
+          T> {
+  pointee_iterator() = default;
+  template <typename U>
+  pointee_iterator(U &&u)
+      : pointee_iterator::iterator_adaptor_base(std::forward<U &&>(u)) {}
+
+  T &operator*() const { return **this->I; }
+};
+
+template <typename RangeT, typename WrappedIteratorT =
+                               decltype(std::begin(std::declval<RangeT>()))>
+iterator_range<pointee_iterator<WrappedIteratorT>>
+make_pointee_range(RangeT &&Range) {
+  using PointeeIteratorT = pointee_iterator<WrappedIteratorT>;
+  return make_range(PointeeIteratorT(std::begin(std::forward<RangeT>(Range))),
+                    PointeeIteratorT(std::end(std::forward<RangeT>(Range))));
+}
+
+template <typename WrappedIteratorT,
+          typename T = decltype(&*std::declval<WrappedIteratorT>())>
+class pointer_iterator
+    : public iterator_adaptor_base<
+          pointer_iterator<WrappedIteratorT, T>, WrappedIteratorT,
+          typename std::iterator_traits<WrappedIteratorT>::iterator_category,
+          T> {
+  mutable T Ptr;
+
+public:
+  pointer_iterator() = default;
+
+  explicit pointer_iterator(WrappedIteratorT u)
+      : pointer_iterator::iterator_adaptor_base(std::move(u)) {}
+
+  T &operator*() { return Ptr = &*this->I; }
+  const T &operator*() const { return Ptr = &*this->I; }
+};
+
+template <typename RangeT, typename WrappedIteratorT =
+                               decltype(std::begin(std::declval<RangeT>()))>
+iterator_range<pointer_iterator<WrappedIteratorT>>
+make_pointer_range(RangeT &&Range) {
+  using PointerIteratorT = pointer_iterator<WrappedIteratorT>;
+  return make_range(PointerIteratorT(std::begin(std::forward<RangeT>(Range))),
+                    PointerIteratorT(std::end(std::forward<RangeT>(Range))));
+}
+
+// Wrapper iterator over iterator ItType, adding DataRef to the type of ItType,
+// to create NodeRef = std::pair<InnerTypeOfItType, DataRef>.
+template <typename ItType, typename NodeRef, typename DataRef>
+class WrappedPairNodeDataIterator
+    : public iterator_adaptor_base<
+          WrappedPairNodeDataIterator<ItType, NodeRef, DataRef>, ItType,
+          typename std::iterator_traits<ItType>::iterator_category, NodeRef,
+          std::ptrdiff_t, NodeRef *, NodeRef &> {
+  using BaseT = iterator_adaptor_base<
+      WrappedPairNodeDataIterator, ItType,
+      typename std::iterator_traits<ItType>::iterator_category, NodeRef,
+      std::ptrdiff_t, NodeRef *, NodeRef &>;
+
+  const DataRef DR;
+  mutable NodeRef NR;
+
+public:
+  WrappedPairNodeDataIterator(ItType Begin, const DataRef DR)
+      : BaseT(Begin), DR(DR) {
+    NR.first = DR;
+  }
+
+  NodeRef &operator*() const {
+    NR.second = *this->I;
+    return NR;
+  }
+};
+
+} // end namespace llvm
+
+#endif // LLVM_ADT_ITERATOR_H