autovector.h
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <algorithm>
#include <cassert>
#include <initializer_list>
#include <iterator>
#include <stdexcept>
#include <vector>
#include "rocksdb/rocksdb_namespace.h"
namespace ROCKSDB_NAMESPACE {
#ifdef ROCKSDB_LITE
template <class T, size_t kSize = 8>
class autovector : public std::vector<T> {
using std::vector<T>::vector;
public:
autovector() {
// Make sure the initial vector has space for kSize elements
std::vector<T>::reserve(kSize);
}
};
#else
// 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.
template <class T, size_t kSize = 8>
class autovector {
public:
// General STL-style container member types.
using value_type = T;
using difference_type = typename std::vector<T>::difference_type;
using size_type = typename std::vector<T>::size_type;
using reference = value_type&;
using const_reference = const value_type&;
using pointer = value_type*;
using const_pointer = const value_type*;
// This class is the base for regular/const iterator
template <class TAutoVector, class TValueType>
class iterator_impl {
public:
// -- iterator traits
using self_type = iterator_impl<TAutoVector, TValueType>;
using value_type = TValueType;
using reference = TValueType&;
using pointer = TValueType*;
using difference_type = typename TAutoVector::difference_type;
using iterator_category = std::random_access_iterator_tag;
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 {
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*() const {
assert(vect_->size() >= index_);
return (*vect_)[index_];
}
pointer operator->() const {
assert(vect_->size() >= index_);
return &(*vect_)[index_];
}
reference operator[](difference_type len) const {
return *(*this + len);
}
// -- Logical Operators
bool operator==(const self_type& other) const {
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 {
assert(vect_ == other.vect_);
return index_ > other.index_;
}
bool operator<(const self_type& other) const {
assert(vect_ == other.vect_);
return index_ < other.index_;
}
bool operator>=(const self_type& other) const {
assert(vect_ == other.vect_);
return index_ >= other.index_;
}
bool operator<=(const self_type& other) const {
assert(vect_ == other.vect_);
return index_ <= other.index_;
}
private:
TAutoVector* vect_ = nullptr;
size_t index_ = 0;
};
using iterator = iterator_impl<autovector, value_type>;
using const_iterator = iterator_impl<const autovector, const value_type>;
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
autovector() : values_(reinterpret_cast<pointer>(buf_)) {}
autovector(std::initializer_list<T> init_list)
: values_(reinterpret_cast<pointer>(buf_)) {
for (const T& item : init_list) {
push_back(item);
}
}
~autovector() { clear(); }
// -- 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);
while (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_++])) value_type();
}
num_stack_items_ = kSize;
} else {
vect_.clear();
while (num_stack_items_ < n) {
new ((void*)(&values_[num_stack_items_++])) value_type();
}
while (num_stack_items_ > n) {
values_[--num_stack_items_].~value_type();
}
}
}
bool empty() const { return size() == 0; }
const_reference operator[](size_type n) const {
assert(n < size());
if (n < kSize) {
return values_[n];
}
return vect_[n - kSize];
}
reference operator[](size_type n) {
assert(n < size());
if (n < kSize) {
return values_[n];
}
return vect_[n - kSize];
}
const_reference at(size_type n) const {
assert(n < size());
return (*this)[n];
}
reference at(size_type n) {
assert(n < size());
return (*this)[n];
}
reference front() {
assert(!empty());
return *begin();
}
const_reference front() const {
assert(!empty());
return *begin();
}
reference back() {
assert(!empty());
return *(end() - 1);
}
const_reference back() const {
assert(!empty());
return *(end() - 1);
}
// -- Mutable Operations
void push_back(T&& item) {
if (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_])) value_type();
values_[num_stack_items_++] = std::move(item);
} else {
vect_.push_back(item);
}
}
void push_back(const T& item) {
if (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_])) value_type();
values_[num_stack_items_++] = item;
} else {
vect_.push_back(item);
}
}
template <class... Args>
void emplace_back(Args&&... args) {
if (num_stack_items_ < kSize) {
new ((void*)(&values_[num_stack_items_++]))
value_type(std::forward<Args>(args)...);
} else {
vect_.emplace_back(std::forward<Args>(args)...);
}
}
void pop_back() {
assert(!empty());
if (!vect_.empty()) {
vect_.pop_back();
} else {
values_[--num_stack_items_].~value_type();
}
}
void clear() {
while (num_stack_items_ > 0) {
values_[--num_stack_items_].~value_type();
}
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
alignas(alignof(
value_type)) char buf_[kSize *
sizeof(value_type)]; // the first `kSize` items
pointer values_;
// 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) {
values_ = reinterpret_cast<pointer>(buf_);
// 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;
}
#endif // ROCKSDB_LITE
} // namespace ROCKSDB_NAMESPACE