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+// Copyright (c) 2018-Present Red Hat Inc.  All rights reserved.
+//
+// Copyright (c) 2011-2018, Facebook, Inc.  All rights reserved.
+// This source code is licensed under both the GPLv2 and Apache 2.0 License
+
+#ifndef CEPH_AUTOVECTOR_H
+#define CEPH_AUTOVECTOR_H
+
+#include <algorithm>
+#include <cassert>
+#include <initializer_list>
+#include <iterator>
+#include <stdexcept>
+#include <vector>
+
+#include "include/ceph_assert.h"
+
+// A vector that leverages pre-allocated stack-based array to achieve better
+// performance for array with small amount of items.
+//
+// The interface resembles that of vector, but with less features since we aim
+// to solve the problem that we have in hand, rather than implementing a
+// full-fledged generic container.
+//
+// Currently we don't support:
+//  * reserve()/shrink_to_fit()
+//     If used correctly, in most cases, people should not touch the
+//     underlying vector at all.
+//  * random insert()/erase(), please only use push_back()/pop_back().
+//  * No move/swap operations. Each autovector instance has a
+//     stack-allocated array and if we want support move/swap operations, we
+//     need to copy the arrays other than just swapping the pointers. In this
+//     case we'll just explicitly forbid these operations since they may
+//     lead users to make false assumption by thinking they are inexpensive
+//     operations.
+//
+// Naming style of public methods almost follows that of the STL's.
+namespace ceph {
+
+template <class T, size_t kSize = 8>
+class autovector {
+ public:
+  // General STL-style container member types.
+  typedef T value_type;
+  typedef typename std::vector<T>::difference_type difference_type;
+  typedef typename std::vector<T>::size_type size_type;
+  typedef value_type& reference;
+  typedef const value_type& const_reference;
+  typedef value_type* pointer;
+  typedef const value_type* const_pointer;
+
+  // This class is the base for regular/const iterator
+  template <class TAutoVector, class TValueType>
+  class iterator_impl {
+   public:
+    // -- iterator traits
+    typedef iterator_impl<TAutoVector, TValueType> self_type;
+    typedef TValueType value_type;
+    typedef TValueType& reference;
+    typedef TValueType* pointer;
+    typedef typename TAutoVector::difference_type difference_type;
+    typedef std::random_access_iterator_tag iterator_category;
+
+    iterator_impl(TAutoVector* vect, size_t index)
+        : vect_(vect), index_(index) {};
+    iterator_impl(const iterator_impl&) = default;
+    ~iterator_impl() {}
+    iterator_impl& operator=(const iterator_impl&) = default;
+
+    // -- Advancement
+    // ++iterator
+    self_type& operator++() {
+      ++index_;
+      return *this;
+    }
+
+    // iterator++
+    self_type operator++(int) {
+      auto old = *this;
+      ++index_;
+      return old;
+    }
+
+    // --iterator
+    self_type& operator--() {
+      --index_;
+      return *this;
+    }
+
+    // iterator--
+    self_type operator--(int) {
+      auto old = *this;
+      --index_;
+      return old;
+    }
+
+    self_type operator-(difference_type len) const {
+      return self_type(vect_, index_ - len);
+    }
+
+    difference_type operator-(const self_type& other) const {
+      ceph_assert(vect_ == other.vect_);
+      return index_ - other.index_;
+    }
+
+    self_type operator+(difference_type len) const {
+      return self_type(vect_, index_ + len);
+    }
+
+    self_type& operator+=(difference_type len) {
+      index_ += len;
+      return *this;
+    }
+
+    self_type& operator-=(difference_type len) {
+      index_ -= len;
+      return *this;
+    }
+
+    // -- Reference
+    reference operator*() {
+      ceph_assert(vect_->size() >= index_);
+      return (*vect_)[index_];
+    }
+
+    const_reference operator*() const {
+      ceph_assert(vect_->size() >= index_);
+      return (*vect_)[index_];
+    }
+
+    pointer operator->() {
+      ceph_assert(vect_->size() >= index_);
+      return &(*vect_)[index_];
+    }
+
+    const_pointer operator->() const {
+      ceph_assert(vect_->size() >= index_);
+      return &(*vect_)[index_];
+    }
+
+
+    // -- Logical Operators
+    bool operator==(const self_type& other) const {
+      ceph_assert(vect_ == other.vect_);
+      return index_ == other.index_;
+    }
+
+    bool operator!=(const self_type& other) const { return !(*this == other); }
+
+    bool operator>(const self_type& other) const {
+      ceph_assert(vect_ == other.vect_);
+      return index_ > other.index_;
+    }
+
+    bool operator<(const self_type& other) const {
+      ceph_assert(vect_ == other.vect_);
+      return index_ < other.index_;
+    }
+
+    bool operator>=(const self_type& other) const {
+      ceph_assert(vect_ == other.vect_);
+      return index_ >= other.index_;
+    }
+
+    bool operator<=(const self_type& other) const {
+      ceph_assert(vect_ == other.vect_);
+      return index_ <= other.index_;
+    }
+
+   private:
+    TAutoVector* vect_ = nullptr;
+    size_t index_ = 0;
+  };
+
+  typedef iterator_impl<autovector, value_type> iterator;
+  typedef iterator_impl<const autovector, const value_type> const_iterator;
+  typedef std::reverse_iterator<iterator> reverse_iterator;
+  typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
+
+  autovector() = default;
+
+  autovector(std::initializer_list<T> init_list) {
+    for (const T& item : init_list) {
+      push_back(item);
+    }
+  }
+
+  ~autovector() = default;
+
+  // -- Immutable operations
+  // Indicate if all data resides in in-stack data structure.
+  bool only_in_stack() const {
+    // If no element was inserted at all, the vector's capacity will be `0`.
+    return vect_.capacity() == 0;
+  }
+
+  size_type size() const { return num_stack_items_ + vect_.size(); }
+
+  // resize does not guarantee anything about the contents of the newly
+  // available elements
+  void resize(size_type n) {
+    if (n > kSize) {
+      vect_.resize(n - kSize);
+      num_stack_items_ = kSize;
+    } else {
+      vect_.clear();
+      num_stack_items_ = n;
+    }
+  }
+
+  bool empty() const { return size() == 0; }
+
+  const_reference operator[](size_type n) const {
+    ceph_assert(n < size());
+    return n < kSize ? values_[n] : vect_[n - kSize];
+  }
+
+  reference operator[](size_type n) {
+    ceph_assert(n < size());
+    return n < kSize ? values_[n] : vect_[n - kSize];
+  }
+
+  const_reference at(size_type n) const {
+    ceph_assert(n < size());
+    return (*this)[n];
+  }
+
+  reference at(size_type n) {
+    ceph_assert(n < size());
+    return (*this)[n];
+  }
+
+  reference front() {
+    ceph_assert(!empty());
+    return *begin();
+  }
+
+  const_reference front() const {
+    ceph_assert(!empty());
+    return *begin();
+  }
+
+  reference back() {
+    ceph_assert(!empty());
+    return *(end() - 1);
+  }
+
+  const_reference back() const {
+    ceph_assert(!empty());
+    return *(end() - 1);
+  }
+
+  // -- Mutable Operations
+  void push_back(T&& item) {
+    if (num_stack_items_ < kSize) {
+      values_[num_stack_items_++] = std::move(item);
+    } else {
+      vect_.push_back(item);
+    }
+  }
+
+  void push_back(const T& item) {
+    if (num_stack_items_ < kSize) {
+      values_[num_stack_items_++] = item;
+    } else {
+      vect_.push_back(item);
+    }
+  }
+
+  template <class... Args>
+  void emplace_back(Args&&... args) {
+    push_back(value_type(args...));
+  }
+
+  void pop_back() {
+    ceph_assert(!empty());
+    if (!vect_.empty()) {
+      vect_.pop_back();
+    } else {
+      --num_stack_items_;
+    }
+  }
+
+  void clear() {
+    num_stack_items_ = 0;
+    vect_.clear();
+  }
+
+  // -- Copy and Assignment
+  autovector& assign(const autovector& other);
+
+  autovector(const autovector& other) { assign(other); }
+
+  autovector& operator=(const autovector& other) { return assign(other); }
+
+  // -- Iterator Operations
+  iterator begin() { return iterator(this, 0); }
+
+  const_iterator begin() const { return const_iterator(this, 0); }
+
+  iterator end() { return iterator(this, this->size()); }
+
+  const_iterator end() const { return const_iterator(this, this->size()); }
+
+  reverse_iterator rbegin() { return reverse_iterator(end()); }
+
+  const_reverse_iterator rbegin() const {
+    return const_reverse_iterator(end());
+  }
+
+  reverse_iterator rend() { return reverse_iterator(begin()); }
+
+  const_reverse_iterator rend() const {
+    return const_reverse_iterator(begin());
+  }
+
+ private:
+  size_type num_stack_items_ = 0;  // current number of items
+  value_type values_[kSize];       // the first `kSize` items
+  // used only if there are more than `kSize` items.
+  std::vector<T> vect_;
+};
+
+template <class T, size_t kSize>
+autovector<T, kSize>& autovector<T, kSize>::assign(const autovector& other) {
+  // copy the internal vector
+  vect_.assign(other.vect_.begin(), other.vect_.end());
+
+  // copy array
+  num_stack_items_ = other.num_stack_items_;
+  std::copy(other.values_, other.values_ + num_stack_items_, values_);
+
+  return *this;
+}
+}  // namespace ceph 
+#endif // CEPH_AUTOVECTOR_H