// Copyright 2017 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_CONTAINERS_CIRCULAR_DEQUE_H_
#define BASE_CONTAINERS_CIRCULAR_DEQUE_H_
#include <algorithm>
#include <cstddef>
#include <iterator>
#include <utility>
#include "base/check.h"
#include "base/containers/span.h"
#include "base/containers/vector_buffer.h"
#include "base/dcheck_is_on.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/numerics/checked_math.h"
#include "base/numerics/safe_conversions.h"
#include "base/types/cxx23_from_range.h"
#if DCHECK_IS_ON()
#include <ostream>
#endif
// base::circular_deque is similar to std::deque. Unlike std::deque, the
// storage is provided in a flat circular buffer conceptually similar to a
// vector. The beginning and end will wrap around as necessary so that
// pushes and pops will be constant time as long as a capacity expansion is
// not required.
//
// The API should be identical to std::deque with the following differences:
//
// - ITERATORS ARE NOT STABLE. Mutating the container will invalidate all
// iterators.
//
// - Insertions may resize the vector and so are not constant time (std::deque
// guarantees constant time for insertions at the ends).
//
// - Container-wide comparisons are not implemented. If you want to compare
// two containers, use an algorithm so the expensive iteration is explicit.
//
// If you want a similar container with only a queue API, use base::queue in
// base/containers/queue.h.
//
// Constructors:
// circular_deque();
// circular_deque(size_t count);
// circular_deque(size_t count, const T& value);
// circular_deque(InputIterator first, InputIterator last);
// circular_deque(base::from_range_t, Range range);
// circular_deque(const circular_deque&);
// circular_deque(circular_deque&&);
// circular_deque(std::initializer_list<value_type>);
//
// Assignment functions:
// circular_deque& operator=(const circular_deque&);
// circular_deque& operator=(circular_deque&&);
// circular_deque& operator=(std::initializer_list<T>);
// void assign(size_t count, const T& value);
// void assign(InputIterator first, InputIterator last);
// void assign(std::initializer_list<T> value);
// void assign_range(Range range);
//
// Random accessors:
// T& at(size_t);
// const T& at(size_t) const;
// T& operator[](size_t);
// const T& operator[](size_t) const;
//
// End accessors:
// T& front();
// const T& front() const;
// T& back();
// const T& back() const;
//
// Iterator functions:
// iterator begin();
// const_iterator begin() const;
// const_iterator cbegin() const;
// iterator end();
// const_iterator end() const;
// const_iterator cend() const;
// reverse_iterator rbegin();
// const_reverse_iterator rbegin() const;
// const_reverse_iterator crbegin() const;
// reverse_iterator rend();
// const_reverse_iterator rend() const;
// const_reverse_iterator crend() const;
//
// Memory management:
// void reserve(size_t); // SEE IMPLEMENTATION FOR SOME GOTCHAS.
// size_t capacity() const;
// void shrink_to_fit();
//
// Size management:
// void clear();
// bool empty() const;
// size_t size() const;
// void resize(size_t);
// void resize(size_t count, const T& value);
//
// Positional insert and erase:
// void insert(const_iterator pos, size_type count, const T& value);
// void insert(const_iterator pos,
// InputIterator first, InputIterator last);
// iterator insert(const_iterator pos, const T& value);
// iterator insert(const_iterator pos, T&& value);
// iterator emplace(const_iterator pos, Args&&... args);
// iterator erase(const_iterator pos);
// iterator erase(const_iterator first, const_iterator last);
//
// End insert and erase:
// void push_front(const T&);
// void push_front(T&&);
// void push_back(const T&);
// void push_back(T&&);
// T& emplace_front(Args&&...);
// T& emplace_back(Args&&...);
// void pop_front();
// void pop_back();
//
// General:
// void swap(circular_deque&);
namespace base {
template <class T>
class circular_deque;
namespace internal {
// Start allocating nonempty buffers with this many entries. This is the
// external capacity so the internal buffer will be one larger (= 4) which is
// more even for the allocator. See the descriptions of internal vs. external
// capacity on the comment above the buffer_ variable below.
constexpr size_t kCircularBufferInitialCapacity = 3;
template <typename T>
class circular_deque_const_iterator {
public:
using difference_type = ptrdiff_t;
using value_type = T;
using pointer = const T*;
using reference = const T&;
using iterator_category = std::random_access_iterator_tag;
circular_deque_const_iterator() = default;
// Dereferencing.
const T& operator*() const {
CHECK_NE(index_, end_);
CheckUnstableUsage();
CheckValidIndex(index_);
// SAFETY: Increment() and Decrement() and Add() operations ensure that
// `index_` stays inside [begin_, end_] (while supporting wrap around for
// the structure. This maintains that `index_` always points at a
// valid position for the `buffer_`. We also CHECK above that `index_` is
// not `end_` making it a valid pointer to dereference.
return UNSAFE_BUFFERS(buffer_[index_]);
}
const T* operator->() const {
CHECK_NE(index_, end_);
CheckUnstableUsage();
CheckValidIndex(index_);
// SAFETY: Increment() and Decrement() and Add() operations ensure that
// `index_` stays inside [begin_, end_] while supporting wrap around for
// the structure. This maintains that `index_` always points at a
// valid position for the `buffer_`. We also CHECK above that `index_` is
// not `end_` making it a valid pointer to dereference.
return &UNSAFE_BUFFERS(buffer_[index_]);
}
const value_type& operator[](difference_type i) const { return *(*this + i); }
// Increment and decrement.
circular_deque_const_iterator& operator++() {
Increment();
return *this;
}
circular_deque_const_iterator operator++(int) {
circular_deque_const_iterator ret = *this;
Increment();
return ret;
}
circular_deque_const_iterator& operator--() {
Decrement();
return *this;
}
circular_deque_const_iterator operator--(int) {
circular_deque_const_iterator ret = *this;
Decrement();
return ret;
}
// Random access mutation.
friend circular_deque_const_iterator operator+(
const circular_deque_const_iterator& iter,
difference_type offset) {
circular_deque_const_iterator ret = iter;
ret.Add(offset);
return ret;
}
circular_deque_const_iterator& operator+=(difference_type offset) {
Add(offset);
return *this;
}
friend circular_deque_const_iterator operator-(
const circular_deque_const_iterator& iter,
difference_type offset) {
circular_deque_const_iterator ret = iter;
ret.Add(-offset);
return ret;
}
circular_deque_const_iterator& operator-=(difference_type offset) {
Add(-offset);
return *this;
}
friend std::ptrdiff_t operator-(const circular_deque_const_iterator& lhs,
const circular_deque_const_iterator& rhs) {
lhs.CheckComparable(rhs);
return static_cast<std::ptrdiff_t>(lhs.OffsetFromBegin() -
rhs.OffsetFromBegin());
}
// Comparisons.
friend bool operator==(const circular_deque_const_iterator& lhs,
const circular_deque_const_iterator& rhs) {
lhs.CheckComparable(rhs);
return lhs.index_ == rhs.index_;
}
friend std::strong_ordering operator<=>(
const circular_deque_const_iterator& lhs,
const circular_deque_const_iterator& rhs) {
lhs.CheckComparable(rhs);
// The order is based on the position of the element in the circular_dequeue
// rather than `index_` at which the element is stored in the ring buffer.
return lhs.OffsetFromBegin() <=> rhs.OffsetFromBegin();
}
protected:
friend class circular_deque<T>;
circular_deque_const_iterator(const circular_deque<T>* parent, size_t index)
: buffer_(parent->buffer_.data()),
cap_(parent->buffer_.capacity()),
begin_(parent->begin_),
end_(parent->end_),
index_(index) {
if (begin_ <= end_) {
CHECK_GE(index_, begin_);
CHECK_LE(index_, end_);
} else if (index_ >= begin_) {
CHECK(index_ < cap_);
} else {
CHECK(index_ <= end_);
}
#if DCHECK_IS_ON()
parent_deque_ = parent;
created_generation_ = parent->generation_;
#endif // DCHECK_IS_ON()
}
// Returns the offset from the beginning index of the buffer to the current
// item.
size_t OffsetFromBegin() const {
if (index_ >= begin_) {
return index_ - begin_; // On the same side as begin.
}
return cap_ - begin_ + index_;
}
// The size of the deque, ie. the number of elements in it.
size_t Size() const {
if (begin_ <= end_) {
return end_ - begin_;
}
return cap_ - begin_ + end_;
}
// Most uses will be ++ and -- so use a simplified implementation.
void Increment() {
CheckUnstableUsage();
CheckValidIndex(index_);
CHECK_NE(index_, end_);
index_++;
if (index_ == cap_) {
index_ = 0u;
}
}
void Decrement() {
CheckUnstableUsage();
CheckValidIndexOrEnd(index_);
CHECK_NE(index_, begin_);
if (index_ == 0u) {
index_ = cap_ - 1u;
} else {
index_--;
}
}
void Add(difference_type delta) {
CheckUnstableUsage();
#if DCHECK_IS_ON()
if (delta <= 0) {
CheckValidIndexOrEnd(index_);
} else {
CheckValidIndex(index_);
}
#endif
// It should be valid to add 0 to any iterator, even if the container is
// empty and the iterator points to end(). The modulo below will divide
// by 0 if the buffer capacity is empty, so it's important to check for
// this case explicitly.
if (delta == 0) {
return;
}
const auto offset_from_begin =
// The max allocation size is PTRDIFF_MAX, so this value can't be larger
// than fits in ptrdiff_t.
static_cast<difference_type>(OffsetFromBegin());
const auto deque_size =
// The max allocation size is PTRDIFF_MAX, so this value can't be larger
// than fits in ptrdiff_t.
static_cast<difference_type>(Size());
if (delta >= 0) {
// Check `offset_from_begin + delta <= deque_size` without overflowing.
CHECK_LE(delta, deque_size - offset_from_begin);
} else {
// Check `offset_from_begin + delta >= 0` without overflowing. We avoid
// negating a negative `delta` which can overflow. Instead negate the
// positive number which can not.
CHECK_GE(delta, -offset_from_begin) << offset_from_begin;
}
const auto new_offset =
// The above checks verify that `offset_from_begin + delta` is in the
// range [0, deque_size] and does not overflow, so it also fits in
// `size_t`.
static_cast<size_t>(offset_from_begin + delta);
index_ = (new_offset + begin_) % cap_;
}
#if DCHECK_IS_ON()
void CheckValidIndexOrEnd(size_t index) const {
parent_deque_->CheckValidIndexOrEnd(index_);
}
void CheckValidIndex(size_t index) const {
parent_deque_->CheckValidIndex(index_);
}
void CheckUnstableUsage() const {
DCHECK(parent_deque_);
// Since circular_deque doesn't guarantee stability, any attempt to
// dereference this iterator after a mutation (i.e. the generation doesn't
// match the original) in the container is illegal.
DCHECK_EQ(created_generation_, parent_deque_->generation_)
<< "circular_deque iterator dereferenced after mutation.";
}
void CheckComparable(const circular_deque_const_iterator& other) const {
DCHECK_EQ(parent_deque_, other.parent_deque_);
// Since circular_deque doesn't guarantee stability, two iterators that
// are compared must have been generated without mutating the container.
// If this fires, the container was mutated between generating the two
// iterators being compared.
DCHECK_EQ(created_generation_, other.created_generation_);
}
#else
inline void CheckUnstableUsage() const {}
inline void CheckComparable(const circular_deque_const_iterator&) const {}
void CheckValidIndexOrEnd(size_t index) const {}
void CheckValidIndex(size_t index) const {}
#endif // DCHECK_IS_ON()
// `buffer_` is not a raw_ptr<...> for performance reasons: Usually
// on-stack pointer, pointing back to the collection being iterated, owned by
// object that iterates over it. Additionally this is supported by the
// analysis of sampling profiler data and tab_search:top100:2020.
RAW_PTR_EXCLUSION const T* buffer_ = nullptr;
size_t cap_ = 0u;
size_t begin_ = 0u;
size_t end_ = 0u;
size_t index_ = 0u;
#if DCHECK_IS_ON()
RAW_PTR_EXCLUSION const circular_deque<T>* parent_deque_ = nullptr;
// The generation of the parent deque when this iterator was created. The
// container will update the generation for every modification so we can
// test if the container was modified by comparing them.
uint64_t created_generation_ = 0u;
#endif // DCHECK_IS_ON()
};
template <typename T>
class circular_deque_iterator : public circular_deque_const_iterator<T> {
using base = circular_deque_const_iterator<T>;
public:
friend class circular_deque<T>;
using difference_type = std::ptrdiff_t;
using value_type = T;
using pointer = T*;
using reference = T&;
using iterator_category = std::random_access_iterator_tag;
// Expose the base class' constructor.
circular_deque_iterator() : circular_deque_const_iterator<T>() {}
// Dereferencing.
T& operator*() const { return const_cast<T&>(base::operator*()); }
T* operator->() const { return const_cast<T*>(base::operator->()); }
T& operator[](difference_type i) {
return const_cast<T&>(base::operator[](i));
}
// Random access mutation.
friend circular_deque_iterator operator+(const circular_deque_iterator& iter,
difference_type offset) {
circular_deque_iterator ret = iter;
ret.Add(offset);
return ret;
}
circular_deque_iterator& operator+=(difference_type offset) {
base::Add(offset);
return *this;
}
friend circular_deque_iterator operator-(const circular_deque_iterator& iter,
difference_type offset) {
circular_deque_iterator ret = iter;
ret.Add(-offset);
return ret;
}
circular_deque_iterator& operator-=(difference_type offset) {
base::Add(-offset);
return *this;
}
// Increment and decrement.
circular_deque_iterator& operator++() {
base::Increment();
return *this;
}
circular_deque_iterator operator++(int) {
circular_deque_iterator ret = *this;
base::Increment();
return ret;
}
circular_deque_iterator& operator--() {
base::Decrement();
return *this;
}
circular_deque_iterator operator--(int) {
circular_deque_iterator ret = *this;
base::Decrement();
return ret;
}
private:
circular_deque_iterator(const circular_deque<T>* parent, size_t index)
: circular_deque_const_iterator<T>(parent, index) {}
};
} // namespace internal
template <typename T>
class circular_deque {
private:
using VectorBuffer = internal::VectorBuffer<T>;
public:
using value_type = T;
using size_type = size_t;
using difference_type = std::ptrdiff_t;
using reference = value_type&;
using const_reference = const value_type&;
using pointer = value_type*;
using const_pointer = const value_type*;
using iterator = internal::circular_deque_iterator<T>;
using const_iterator = internal::circular_deque_const_iterator<T>;
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
// ---------------------------------------------------------------------------
// Constructor
// Constructs an empty deque.
constexpr circular_deque() = default;
// Constructs with `count` copies of a default-constructed T.
explicit circular_deque(size_type count) { resize(count); }
// Constructs with `count` copies of `value`.
circular_deque(size_type count, const T& value) { resize(count, value); }
// Construct a deque by constructing its elements from each element in
// `[first, last)`.
//
// Prefer using the `from_range_t` constructor, which builds a deque from a
// range, instead of from problematic iterator pairs.
//
// # Safety
// The `first` and `last` iterators must be from the same container, with
// `first <= last`.
template <class InputIterator>
requires(std::input_iterator<InputIterator>)
UNSAFE_BUFFER_USAGE circular_deque(InputIterator first, InputIterator last)
: circular_deque() {
// SAFETY: The caller is responsible for giving iterator from the same
// container.
UNSAFE_BUFFERS(assign(first, last));
}
// Constructs a deque from the elements in a range (a container or span),
// typically by copy-constructing if the range also holds objects of type
// `T`.
//
// Example:
// ```
// int values[] = {1, 3};
// circular_deque<int> deq(base::from_range, values);
// ```
template <typename Range>
requires(std::ranges::input_range<Range>)
circular_deque(base::from_range_t, Range&& value) : circular_deque() {
assign_range(std::forward<Range>(value));
}
// Copy/move.
circular_deque(const circular_deque& other) : buffer_(other.size() + 1) {
assign_range(other);
}
circular_deque(circular_deque&& other) noexcept
: buffer_(std::move(other.buffer_)),
begin_(std::exchange(other.begin_, 0u)),
end_(std::exchange(other.end_, 0u)) {}
circular_deque(std::initializer_list<value_type> init) { assign(init); }
~circular_deque() { DestructRange(begin_, end_); }
// ---------------------------------------------------------------------------
// Assignments.
//
// All of these may invalidate iterators and references.
circular_deque& operator=(const circular_deque& other) {
if (&other == this) {
return *this;
}
reserve(other.size());
assign_range(other);
return *this;
}
circular_deque& operator=(circular_deque&& other) noexcept {
if (&other == this) {
return *this;
}
// We're about to overwrite the buffer, so don't free it in clear to
// avoid doing it twice.
ClearRetainCapacity();
buffer_ = std::move(other.buffer_);
begin_ = std::exchange(other.begin_, 0u);
end_ = std::exchange(other.end_, 0u);
IncrementGeneration();
return *this;
}
circular_deque& operator=(std::initializer_list<value_type> ilist) {
reserve(ilist.size());
assign_range(ilist);
return *this;
}
void assign(size_type count, const value_type& value) {
ClearRetainCapacity();
reserve(count);
for (size_t i = 0; i < count; i++) {
emplace_back(value);
}
IncrementGeneration();
}
// Constructs and appends new elements into the container from each element in
// `[first, last)`, typically by copy-constructing if the iterators are also
// over objects of type `T`.
//
// # Safety
// Requires that `first` and `last` are valid iterators into a container, with
// `first <= last`.
template <typename InputIterator>
requires(std::forward_iterator<InputIterator>)
UNSAFE_BUFFER_USAGE void assign(InputIterator first, InputIterator last) {
// Possible future enhancement, dispatch on iterator tag type. For forward
// iterators we can use std::difference to preallocate the space required
// and only do one copy.
ClearRetainCapacity();
// SAFETY: Pointers are iterators, so `first` may be a pointer. We require
// the caller to provide valid pointers such that `last` is for the same
// allocation and `first <= last`, and we've checked in the loop condition
// that `first != last` so incrementing will stay a valid pointer for the
// allocation.
for (; first != last; UNSAFE_BUFFERS(++first)) {
emplace_back(*first);
}
IncrementGeneration();
}
// Copies and appends new elements into the container from each element in
// the initializer list.
void assign(std::initializer_list<value_type> value) { assign_range(value); }
// Constructs and appends new elements into the container from each element in
// a range (a container or span), typically by copy-constructing if
// the range also holds objects of type `T`.
template <typename Range>
requires(std::ranges::input_range<Range>)
void assign_range(Range&& range) {
reserve(std::ranges::distance(range));
// SAFETY: begin() and end() produce iterators from the same container with
// begin <= end.
UNSAFE_BUFFERS(assign(std::ranges::begin(range), std::ranges::end(range)));
}
// ---------------------------------------------------------------------------
// Accessors.
//
// Since this class assumes no exceptions, at() and operator[] are equivalent.
const value_type& at(size_type i) const {
CHECK_LT(i, size());
size_t right_size = buffer_.capacity() - begin_;
if (begin_ <= end_ || i < right_size) {
return buffer_[begin_ + i];
}
return buffer_[i - right_size];
}
value_type& at(size_type i) {
return const_cast<value_type&>(std::as_const(*this).at(i));
}
const value_type& operator[](size_type i) const { return at(i); }
value_type& operator[](size_type i) { return at(i); }
value_type& front() {
CHECK(!empty());
return buffer_[begin_];
}
const value_type& front() const {
CHECK(!empty());
return buffer_[begin_];
}
value_type& back() {
CHECK(!empty());
return *(end() - 1);
}
const value_type& back() const {
CHECK(!empty());
return *(end() - 1);
}
// ---------------------------------------------------------------------------
// Iterators.
iterator begin() { return iterator(this, begin_); }
const_iterator begin() const { return const_iterator(this, begin_); }
const_iterator cbegin() const { return const_iterator(this, begin_); }
iterator end() { return iterator(this, end_); }
const_iterator end() const { return const_iterator(this, end_); }
const_iterator cend() const { return const_iterator(this, end_); }
reverse_iterator rbegin() { return reverse_iterator(end()); }
const_reverse_iterator rbegin() const {
return const_reverse_iterator(end());
}
const_reverse_iterator crbegin() const { return rbegin(); }
reverse_iterator rend() { return reverse_iterator(begin()); }
const_reverse_iterator rend() const {
return const_reverse_iterator(begin());
}
const_reverse_iterator crend() const { return rend(); }
// ---------------------------------------------------------------------------
// Memory management.
// IMPORTANT NOTE ON reserve(...): This class implements auto-shrinking of
// the buffer when elements are deleted and there is "too much" wasted space.
// So if you call reserve() with a large size in anticipation of pushing many
// elements, but pop an element before the queue is full, the capacity you
// reserved may be lost.
//
// As a result, it's only worthwhile to call reserve() when you're adding
// many things at once with no intermediate operations.
void reserve(size_type new_capacity) {
if (new_capacity > capacity()) {
SetCapacityTo(new_capacity);
}
}
size_type capacity() const {
// One item is wasted to indicate end().
return buffer_.capacity() == 0 ? 0 : buffer_.capacity() - 1;
}
void shrink_to_fit() {
if (empty()) {
// Optimize empty case to really delete everything if there was
// something.
if (buffer_.capacity()) {
buffer_ = VectorBuffer();
}
} else {
SetCapacityTo(size());
}
}
// ---------------------------------------------------------------------------
// Size management.
// This will additionally reset the capacity() to 0.
void clear() {
// This can't resize(0) because that requires a default constructor to
// compile, which not all contained classes may implement.
ClearRetainCapacity();
buffer_ = VectorBuffer();
}
bool empty() const { return begin_ == end_; }
size_type size() const {
if (begin_ <= end_) {
return end_ - begin_;
}
return buffer_.capacity() - begin_ + end_;
}
// When reducing size, the elements are deleted from the end. When expanding
// size, elements are added to the end with |value| or the default
// constructed version. Even when using resize(count) to shrink, a default
// constructor is required for the code to compile, even though it will not
// be called.
//
// There are two versions rather than using a default value to avoid
// creating a temporary when shrinking (when it's not needed). Plus if
// the default constructor is desired when expanding usually just calling it
// for each element is faster than making a default-constructed temporary and
// copying it.
void resize(size_type count) {
// SEE BELOW VERSION if you change this. The code is mostly the same.
if (count > size()) {
// This could be slighly more efficient but expanding a queue with
// identical elements is unusual and the extra computations of emplacing
// one-by-one will typically be small relative to calling the constructor
// for every item.
ExpandCapacityIfNecessary(count - size());
while (size() < count) {
emplace_back();
}
} else if (count < size()) {
size_t new_end = (begin_ + count) % buffer_.capacity();
DestructRange(new_end, end_);
end_ = new_end;
ShrinkCapacityIfNecessary();
}
IncrementGeneration();
}
void resize(size_type count, const value_type& value) {
// SEE ABOVE VERSION if you change this. The code is mostly the same.
if (count > size()) {
ExpandCapacityIfNecessary(count - size());
while (size() < count) {
emplace_back(value);
}
} else if (count < size()) {
size_t new_end = (begin_ + count) % buffer_.capacity();
DestructRange(new_end, end_);
end_ = new_end;
ShrinkCapacityIfNecessary();
}
IncrementGeneration();
}
// ---------------------------------------------------------------------------
// Insert and erase.
//
// Insertion and deletion in the middle is O(n) and invalidates all existing
// iterators.
//
// The implementation of insert isn't optimized as much as it could be. If
// the insertion requires that the buffer be grown, it will first be grown
// and everything moved, and then the items will be inserted, potentially
// moving some items twice. This simplifies the implemntation substantially
// and means less generated templatized code. Since this is an uncommon
// operation for deques, and already relatively slow, it doesn't seem worth
// the benefit to optimize this.
void insert(const_iterator pos, size_type count, const T& value) {
ValidateIterator(pos);
// Optimize insert at the beginning.
if (pos == begin()) {
ExpandCapacityIfNecessary(count);
for (size_t i = 0; i < count; i++) {
push_front(value);
}
return;
}
CHECK_LT(pos.index_, buffer_.capacity());
iterator insert_cur(this, pos.index_);
iterator insert_end;
MakeRoomFor(count, &insert_cur, &insert_end);
while (insert_cur < insert_end) {
std::construct_at(buffer_.get_at(insert_cur.index_), value);
++insert_cur;
}
IncrementGeneration();
}
template <class InputIterator>
requires(std::forward_iterator<InputIterator>)
void insert(const_iterator pos, InputIterator first, InputIterator last) {
ValidateIterator(pos);
const size_t inserted_items =
checked_cast<size_t>(std::distance(first, last));
if (inserted_items == 0u) {
return; // Can divide by 0 when doing modulo below, so return early.
}
// Make a hole to copy the items into.
iterator insert_cur;
iterator insert_end;
if (pos == begin()) {
// Optimize insert at the beginning, nothing needs to be shifted and the
// hole is the |inserted_items| block immediately before |begin_|.
ExpandCapacityIfNecessary(inserted_items);
const size_t old_begin = begin_;
begin_ = (old_begin + buffer_.capacity() - inserted_items) %
buffer_.capacity();
insert_cur = begin();
insert_end = iterator(this, old_begin);
} else {
CHECK_LT(pos.index_, buffer_.capacity());
insert_cur = iterator(this, pos.index_);
MakeRoomFor(inserted_items, &insert_cur, &insert_end);
}
// Copy the items.
while (insert_cur < insert_end) {
std::construct_at(buffer_.get_at(insert_cur.index_), *first);
++insert_cur;
// SAFETY: The input iterator may be a pointer, in which case we will
// produce UB if `first` is incremented past `last`. We use checked_cast
// of std::distance to an unsigned value above, which ensures that `last
// >= first`. Then we need that `insert_end - insert_cur <= last - first`:
// - If inserting at the start, pos == begin() and `insert_cur` is
// positioned at `begin_ - (last - first)`, and `insert_end` is
// positioned at `begin_` so we have
// `insert_end - insert_cur == last - first`.
// - If inserting elsewhere, `MakeRoomFor(last - first, ...)` returns an
// iterator
// pair with distance of `last - first`, so we have
// `insert_end - insert_cur == last - first`.
UNSAFE_BUFFERS(++first);
}
IncrementGeneration();
}
// These all return an iterator to the inserted item. Existing iterators will
// be invalidated.
iterator insert(const_iterator pos, const T& value) {
return emplace(pos, value);
}
iterator insert(const_iterator pos, T&& value) {
return emplace(pos, std::move(value));
}
template <class... Args>
iterator emplace(const_iterator pos, Args&&... args) {
ValidateIterator(pos);
// Optimize insert at beginning which doesn't require shifting.
if (pos == cbegin()) {
emplace_front(std::forward<Args>(args)...);
return begin();
}
// Do this before we make the new iterators we return.
IncrementGeneration();
CHECK_LT(pos.index_, buffer_.capacity());
iterator insert_begin(this, pos.index_);
iterator insert_end;
MakeRoomFor(1, &insert_begin, &insert_end);
std::construct_at(buffer_.get_at(insert_begin.index_),
std::forward<Args>(args)...);
return insert_begin;
}
// Calling erase() won't automatically resize the buffer smaller like resize
// or the pop functions. Erase is slow and relatively uncommon, and for
// normal deque usage a pop will normally be done on a regular basis that
// will prevent excessive buffer usage over long periods of time. It's not
// worth having the extra code for every template instantiation of erase()
// to resize capacity downward to a new buffer.
iterator erase(const_iterator pos) { return erase(pos, pos + 1); }
iterator erase(const_iterator pos_begin, const_iterator pos_end) {
ValidateIterator(pos_begin);
ValidateIterator(pos_end);
IncrementGeneration();
if (pos_begin.index_ == pos_end.index_) {
// Nothing deleted. Need to return early to avoid falling through to
// moving items on top of themselves.
return iterator(this, pos_begin.index_);
}
// First, call the destructor on the deleted items.
DestructRange(pos_begin.index_, pos_end.index_);
if (pos_begin.index_ == begin_) {
// This deletion is from the beginning. Nothing needs to be copied, only
// begin_ needs to be updated.
begin_ = pos_end.index_;
return iterator(this, pos_end.index_);
}
// In an erase operation, the shifted items all move logically to the left,
// so move them from left-to-right.
//
// The elements are being moved to memory where the T objects were
// previously destroyed.
//
// TODO(danakj): We could skip destruction and do MoveAssignRange here, for
// the elements that are being replaced.
size_t move_src = pos_end.index_;
const size_t move_src_end = end_;
size_t move_dest = pos_begin.index_;
const size_t cap = buffer_.capacity();
while (move_src != move_src_end) {
VectorBuffer::MoveConstructRange(buffer_.subspan(move_src, 1u),
buffer_.subspan(move_dest, 1u));
move_src = (move_src + 1u) % cap;
move_dest = (move_dest + 1u) % cap;
}
end_ = move_dest;
// Since we did not reallocate and only changed things after the erase
// element(s), the input iterator's index points to the thing following the
// deletion.
return iterator(this, pos_begin.index_);
}
// ---------------------------------------------------------------------------
// Begin/end operations.
void push_front(const T& value) { emplace_front(value); }
void push_front(T&& value) { emplace_front(std::move(value)); }
void push_back(const T& value) { emplace_back(value); }
void push_back(T&& value) { emplace_back(std::move(value)); }
template <class... Args>
reference emplace_front(Args&&... args) {
ExpandCapacityIfNecessary(1);
if (begin_ == 0) {
begin_ = buffer_.capacity() - 1;
} else {
begin_--;
}
IncrementGeneration();
std::construct_at(buffer_.get_at(begin_), std::forward<Args>(args)...);
return front();
}
template <class... Args>
reference emplace_back(Args&&... args) {
ExpandCapacityIfNecessary(1);
std::construct_at(buffer_.get_at(end_), std::forward<Args>(args)...);
if (end_ == buffer_.capacity() - 1) {
end_ = 0;
} else {
end_++;
}
IncrementGeneration();
return back();
}
void pop_front() {
CHECK(!empty());
DestructRange(begin_, begin_ + 1u);
begin_++;
if (begin_ == buffer_.capacity()) {
begin_ = 0;
}
ShrinkCapacityIfNecessary();
// Technically popping will not invalidate any iterators since the
// underlying buffer will be stable. But in the future we may want to add a
// feature that resizes the buffer smaller if there is too much wasted
// space. This ensures we can make such a change safely.
IncrementGeneration();
}
void pop_back() {
CHECK(!empty());
if (end_ == 0) {
end_ = buffer_.capacity() - 1;
} else {
end_--;
}
DestructRange(end_, end_ + 1u);
ShrinkCapacityIfNecessary();
// See pop_front comment about why this is here.
IncrementGeneration();
}
// ---------------------------------------------------------------------------
// General operations.
void swap(circular_deque& other) {
std::swap(buffer_, other.buffer_);
std::swap(begin_, other.begin_);
std::swap(end_, other.end_);
IncrementGeneration();
}
friend void swap(circular_deque& lhs, circular_deque& rhs) { lhs.swap(rhs); }
private:
friend internal::circular_deque_iterator<T>;
friend internal::circular_deque_const_iterator<T>;
// Moves the items in the given circular buffer to the current one. The source
// is moved from so will become invalid. The destination buffer must have
// already been allocated with enough size.
//
// # Safety
// `from_begin` and `from_end` must be less-than and less-than-or-equal-to the
// capacity of `from_buf` respectively, with `from_begin <= from_end`, or
// Undefined Behaviour may result.
UNSAFE_BUFFER_USAGE static void MoveBuffer(VectorBuffer& from_buf,
size_t from_begin,
size_t from_end,
VectorBuffer& to_buf,
size_t* to_begin,
size_t* to_end) {
*to_begin = 0;
if (from_begin < from_end) {
// Contiguous.
VectorBuffer::MoveConstructRange(
from_buf.subspan(from_begin, from_end - from_begin),
to_buf.subspan(0u, from_end - from_begin));
*to_end = from_end - from_begin;
} else if (from_begin > from_end) {
// Discontiguous, copy the right side to the beginning of the new buffer.
span<T> right_side = from_buf.subspan(from_begin);
VectorBuffer::MoveConstructRange(right_side,
to_buf.subspan(0u, right_side.size()));
// Append the left side.
span<T> left_side = from_buf.subspan(0u, from_end);
VectorBuffer::MoveConstructRange(
left_side, to_buf.subspan(right_side.size(), left_side.size()));
*to_end = left_side.size() + right_side.size();
} else {
// No items.
*to_end = 0;
}
}
// Expands the buffer size. This assumes the size is larger than the
// number of elements in the vector (it won't call delete on anything).
void SetCapacityTo(size_t new_capacity) {
// Use the capacity + 1 as the internal buffer size to differentiate
// empty and full (see definition of buffer_ below).
VectorBuffer new_buffer(new_capacity + 1u);
// SAFETY: This class maintains an invariant that `begin_` and `end_` are
// less than `buffer_`'s capacity.
UNSAFE_BUFFERS(
MoveBuffer(buffer_, begin_, end_, new_buffer, &begin_, &end_));
buffer_ = std::move(new_buffer);
}
void ExpandCapacityIfNecessary(size_t additional_elts) {
const size_t cur_size = size();
const size_t cur_capacity = capacity();
// Protect against overflow when adding `additional_elts`, and exceeding the
// max allocation size.
CHECK_LE(additional_elts, PTRDIFF_MAX - cur_size);
size_t min_new_capacity = cur_size + additional_elts;
if (cur_capacity >= min_new_capacity) {
return; // Already enough room.
}
min_new_capacity =
std::max(min_new_capacity, internal::kCircularBufferInitialCapacity);
// std::vector always grows by at least 50%. blink::Deque grows by at least
// 25%. We expect queue workloads to generally stay at a similar size and
// grow less than a vector might, so use 25%.
SetCapacityTo(std::max(min_new_capacity, cur_capacity + cur_capacity / 4u));
}
void ShrinkCapacityIfNecessary() {
// Don't auto-shrink below this size.
if (capacity() <= internal::kCircularBufferInitialCapacity) {
return;
}
// Shrink when 100% of the size() is wasted.
size_t sz = size();
size_t empty_spaces = capacity() - sz;
if (empty_spaces < sz) {
return;
}
// Leave 1/4 the size as free capacity, not going below the initial
// capacity.
size_t new_capacity =
std::max(internal::kCircularBufferInitialCapacity, sz + sz / 4);
if (new_capacity < capacity()) {
// Count extra item to convert to internal capacity.
SetCapacityTo(new_capacity);
}
}
// Backend for clear() but does not resize the internal buffer.
void ClearRetainCapacity() {
// This can't resize(0) because that requires a default constructor to
// compile, which not all contained classes may implement.
// SAFETY: This class maintains an invariant that `begin_` and `end_` are
// less than `buffer_`'s capacity. `new_end` is computed modulo the capacity
// so it is in range.
DestructRange(begin_, end_);
begin_ = 0;
end_ = 0;
IncrementGeneration();
}
// Calls destructors for the given begin->end indices. The indices may wrap
// around. The buffer is not resized, and the begin_ and end_ members are
// not changed.
void DestructRange(size_t begin, size_t end) {
if (end == begin) {
return;
} else if (end > begin) {
VectorBuffer::DestructRange(buffer_.subspan(begin, end - begin));
} else {
VectorBuffer::DestructRange(buffer_.subspan(begin));
VectorBuffer::DestructRange(buffer_.subspan(0u, end));
}
}
// Makes room for |count| items starting at |*insert_begin|. Since iterators
// are not stable across buffer resizes, |*insert_begin| will be updated to
// point to the beginning of the newly opened position in the new array (it's
// in/out), and the end of the newly opened position (it's out-only).
void MakeRoomFor(size_t count, iterator* insert_begin, iterator* insert_end) {
if (count == 0) {
*insert_end = *insert_begin;
return;
}
// The offset from the beginning will be stable across reallocations.
size_t begin_offset = insert_begin->OffsetFromBegin();
ExpandCapacityIfNecessary(count);
// Update the new end and prepare the iterators for copying. The newly
// used space contains uninitialized memory.
const size_t cap = buffer_.capacity();
size_t src = end_;
end_ = (end_ + count) % cap;
size_t dest = end_;
*insert_begin = iterator(this, (begin_ + begin_offset) % cap);
*insert_end = iterator(this, (insert_begin->index_ + count) % cap);
// Move the elements. This will always involve shifting logically to the
// right, so move in a right-to-left order.
while (true) {
if (src == insert_begin->index_) {
break;
}
src = (src + cap - 1u) % cap;
dest = (dest + cap - 1u) % cap;
VectorBuffer::MoveConstructRange(buffer_.subspan(src, 1u),
buffer_.subspan(dest, 1u));
}
}
#if DCHECK_IS_ON()
// Asserts the given index is dereferencable. The index is an index into the
// buffer, not an index used by operator[] or at() which will be offsets from
// begin.
void CheckValidIndex(size_t i) const {
if (begin_ <= end_) {
DCHECK(i >= begin_ && i < end_);
} else {
DCHECK((i >= begin_ && i < buffer_.capacity()) || i < end_);
}
}
// Asserts the given index is either dereferencable or points to end().
void CheckValidIndexOrEnd(size_t i) const {
if (i != end_) {
CheckValidIndex(i);
}
}
void ValidateIterator(const const_iterator& i) const {
DCHECK(i.parent_deque_ == this);
i.CheckUnstableUsage();
}
// See generation_ below.
void IncrementGeneration() { generation_++; }
#else
// No-op versions of these functions for release builds.
void CheckValidIndex(size_t) const {}
void CheckValidIndexOrEnd(size_t) const {}
void ValidateIterator(const const_iterator& i) const {}
void IncrementGeneration() {}
#endif
// Danger, the buffer_.capacity() is the "internal capacity" which is
// capacity() + 1 since there is an extra item to indicate the end. Otherwise
// being completely empty and completely full are indistinguishable (begin ==
// end). We could add a separate flag to avoid it, but that adds significant
// extra complexity since every computation will have to check for it. Always
// keeping one extra unused element in the buffer makes iterator computations
// much simpler.
//
// Container internal code will want to use buffer_.capacity() for offset
// computations rather than capacity().
VectorBuffer buffer_;
size_type begin_ = 0;
size_type end_ = 0;
#if DCHECK_IS_ON()
// Incremented every time a modification is made that could affect iterator
// invalidations.
uint64_t generation_ = 0;
#endif
};
// Implementations of base::Erase[If] (see base/stl_util.h).
template <class T, class Value>
size_t Erase(circular_deque<T>& container, const Value& value) {
auto removed = std::ranges::remove(container, value);
size_t num_removed = removed.size();
container.erase(removed.begin(), removed.end());
return num_removed;
}
template <class T, class Predicate>
size_t EraseIf(circular_deque<T>& container, Predicate pred) {
auto removed = std::ranges::remove_if(container, pred);
size_t num_removed = removed.size();
container.erase(removed.begin(), removed.end());
return num_removed;
}
} // namespace base
#endif // BASE_CONTAINERS_CIRCULAR_DEQUE_H_