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util: add VecDeque

This is an STL-like container that interface-wise looks like std::deque, but
is backed by a (fixed size, with vector-like capacity/reserve) circular buffer.
This commit is contained in:
Pieter Wuille 2024-02-07 14:38:52 -05:00
parent 1040a1fc80
commit 62fd24af6a
2 changed files with 317 additions and 0 deletions

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@ -333,6 +333,7 @@ BITCOIN_CORE_H = \
util/translation.h \
util/types.h \
util/ui_change_type.h \
util/vecdeque.h \
util/vector.h \
validation.h \
validationinterface.h \

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src/util/vecdeque.h Normal file
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@ -0,0 +1,316 @@
// Copyright (c) The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#ifndef BITCOIN_UTIL_VECDEQUE_H
#define BITCOIN_UTIL_VECDEQUE_H
#include <util/check.h>
#include <cstring>
#include <memory>
/** Data structure largely mimicking std::deque, but using single preallocated ring buffer.
*
* - More efficient and better memory locality than std::deque.
* - Most operations ({push_,pop_,emplace_,}{front,back}(), operator[], ...) are O(1),
* unless reallocation is needed (in which case they are O(n)).
* - Supports reserve(), capacity(), shrink_to_fit() like vectors.
* - No iterator support.
* - Data is not stored in a single contiguous block, so no data().
*/
template<typename T>
class VecDeque
{
/** Pointer to allocated memory. Can contain constructed and uninitialized T objects. */
T* m_buffer{nullptr};
/** m_buffer + m_offset points to first object in queue. m_offset = 0 if m_capacity is 0;
* otherwise 0 <= m_offset < m_capacity. */
size_t m_offset{0};
/** Number of objects in the container. 0 <= m_size <= m_capacity. */
size_t m_size{0};
/** The size of m_buffer, expressed as a multiple of the size of T. */
size_t m_capacity{0};
/** Returns the number of populated objects between m_offset and the end of the buffer. */
size_t FirstPart() const noexcept { return std::min(m_capacity - m_offset, m_size); }
void Reallocate(size_t capacity)
{
Assume(capacity >= m_size);
Assume((m_offset == 0 && m_capacity == 0) || m_offset < m_capacity);
// Allocate new buffer.
T* new_buffer = capacity ? std::allocator<T>().allocate(capacity) : nullptr;
if (capacity) {
if constexpr (std::is_trivially_copyable_v<T>) {
// When T is trivially copyable, just copy the data over from old to new buffer.
size_t first_part = FirstPart();
if (first_part != 0) {
std::memcpy(new_buffer, m_buffer + m_offset, first_part * sizeof(T));
}
if (first_part != m_size) {
std::memcpy(new_buffer + first_part, m_buffer, (m_size - first_part) * sizeof(T));
}
} else {
// Otherwise move-construct in place in the new buffer, and destroy old buffer objects.
size_t old_pos = m_offset;
for (size_t new_pos = 0; new_pos < m_size; ++new_pos) {
std::construct_at(new_buffer + new_pos, std::move(*(m_buffer + old_pos)));
std::destroy_at(m_buffer + old_pos);
++old_pos;
if (old_pos == m_capacity) old_pos = 0;
}
}
}
// Deallocate old buffer and update housekeeping.
std::allocator<T>().deallocate(m_buffer, m_capacity);
m_buffer = new_buffer;
m_offset = 0;
m_capacity = capacity;
Assume((m_offset == 0 && m_capacity == 0) || m_offset < m_capacity);
}
/** What index in the buffer does logical entry number pos have? */
size_t BufferIndex(size_t pos) const noexcept
{
Assume(pos < m_capacity);
// The expression below is used instead of the more obvious (pos + m_offset >= m_capacity),
// because the addition there could in theory overflow with very large deques.
if (pos >= m_capacity - m_offset) {
return (m_offset + pos) - m_capacity;
} else {
return m_offset + pos;
}
}
/** Specialization of resize() that can only shrink. Separate so that clear() can call it
* without requiring a default T constructor. */
void ResizeDown(size_t size) noexcept
{
Assume(size <= m_size);
if constexpr (std::is_trivially_destructible_v<T>) {
// If T is trivially destructible, we do not need to do anything but update the
// housekeeping record. Default constructor or zero-filling will be used when
// the space is reused.
m_size = size;
} else {
// If not, we need to invoke the destructor for every element separately.
while (m_size > size) {
std::destroy_at(m_buffer + BufferIndex(m_size - 1));
--m_size;
}
}
}
public:
VecDeque() noexcept = default;
/** Resize the deque to be exactly size size (adding default-constructed elements if needed). */
void resize(size_t size)
{
if (size < m_size) {
// Delegate to ResizeDown when shrinking.
ResizeDown(size);
} else if (size > m_size) {
// When growing, first see if we need to allocate more space.
if (size > m_capacity) Reallocate(size);
while (m_size < size) {
std::construct_at(m_buffer + BufferIndex(m_size));
++m_size;
}
}
}
/** Resize the deque to be size 0. The capacity will remain unchanged. */
void clear() noexcept { ResizeDown(0); }
/** Destroy a deque. */
~VecDeque()
{
clear();
Reallocate(0);
}
/** Copy-assign a deque. */
VecDeque& operator=(const VecDeque& other)
{
if (&other == this) [[unlikely]] return *this;
clear();
Reallocate(other.m_size);
if constexpr (std::is_trivially_copyable_v<T>) {
size_t first_part = other.FirstPart();
Assume(first_part > 0 || m_size == 0);
if (first_part != 0) {
std::memcpy(m_buffer, other.m_buffer + other.m_offset, first_part * sizeof(T));
}
if (first_part != other.m_size) {
std::memcpy(m_buffer + first_part, other.m_buffer, (other.m_size - first_part) * sizeof(T));
}
m_size = other.m_size;
} else {
while (m_size < other.m_size) {
std::construct_at(m_buffer + BufferIndex(m_size), other[m_size]);
++m_size;
}
}
return *this;
}
/** Swap two deques. */
void swap(VecDeque& other) noexcept
{
std::swap(m_buffer, other.m_buffer);
std::swap(m_offset, other.m_offset);
std::swap(m_size, other.m_size);
std::swap(m_capacity, other.m_capacity);
}
/** Non-member version of swap. */
friend void swap(VecDeque& a, VecDeque& b) noexcept { a.swap(b); }
/** Move-assign a deque. */
VecDeque& operator=(VecDeque&& other) noexcept
{
swap(other);
return *this;
}
/** Copy-construct a deque. */
VecDeque(const VecDeque& other) { *this = other; }
/** Move-construct a deque. */
VecDeque(VecDeque&& other) noexcept { swap(other); }
/** Equality comparison between two deques (only compares size+contents, not capacity). */
bool friend operator==(const VecDeque& a, const VecDeque& b)
{
if (a.m_size != b.m_size) return false;
for (size_t i = 0; i < a.m_size; ++i) {
if (a[i] != b[i]) return false;
}
return true;
}
/** Comparison between two deques, implementing lexicographic ordering on the contents. */
std::strong_ordering friend operator<=>(const VecDeque& a, const VecDeque& b)
{
size_t pos_a{0}, pos_b{0};
while (pos_a < a.m_size && pos_b < b.m_size) {
auto cmp = a[pos_a++] <=> b[pos_b++];
if (cmp != 0) return cmp;
}
return a.m_size <=> b.m_size;
}
/** Increase the capacity to capacity. Capacity will not shrink. */
void reserve(size_t capacity)
{
if (capacity > m_capacity) Reallocate(capacity);
}
/** Make the capacity equal to the size. The contents does not change. */
void shrink_to_fit()
{
if (m_capacity > m_size) Reallocate(m_size);
}
/** Construct a new element at the end of the deque. */
template<typename... Args>
void emplace_back(Args&&... args)
{
if (m_size == m_capacity) Reallocate((m_size + 1) * 2);
std::construct_at(m_buffer + BufferIndex(m_size), std::forward<Args>(args)...);
++m_size;
}
/** Move-construct a new element at the end of the deque. */
void push_back(T&& elem) { emplace_back(std::move(elem)); }
/** Copy-construct a new element at the end of the deque. */
void push_back(const T& elem) { emplace_back(elem); }
/** Construct a new element at the beginning of the deque. */
template<typename... Args>
void emplace_front(Args&&... args)
{
if (m_size == m_capacity) Reallocate((m_size + 1) * 2);
std::construct_at(m_buffer + BufferIndex(m_capacity - 1), std::forward<Args>(args)...);
if (m_offset == 0) m_offset = m_capacity;
--m_offset;
++m_size;
}
/** Copy-construct a new element at the beginning of the deque. */
void push_front(const T& elem) { emplace_front(elem); }
/** Move-construct a new element at the beginning of the deque. */
void push_front(T&& elem) { emplace_front(std::move(elem)); }
/** Remove the first element of the deque. Requires !empty(). */
void pop_front()
{
Assume(m_size);
std::destroy_at(m_buffer + m_offset);
--m_size;
++m_offset;
if (m_offset == m_capacity) m_offset = 0;
}
/** Remove the last element of the deque. Requires !empty(). */
void pop_back()
{
Assume(m_size);
std::destroy_at(m_buffer + BufferIndex(m_size - 1));
--m_size;
}
/** Get a mutable reference to the first element of the deque. Requires !empty(). */
T& front() noexcept
{
Assume(m_size);
return m_buffer[m_offset];
}
/** Get a const reference to the first element of the deque. Requires !empty(). */
const T& front() const noexcept
{
Assume(m_size);
return m_buffer[m_offset];
}
/** Get a mutable reference to the last element of the deque. Requires !empty(). */
T& back() noexcept
{
Assume(m_size);
return m_buffer[BufferIndex(m_size - 1)];
}
/** Get a const reference to the last element of the deque. Requires !empty(). */
const T& back() const noexcept
{
Assume(m_size);
return m_buffer[BufferIndex(m_size - 1)];
}
/** Get a mutable reference to the element in the deque at the given index. Requires idx < size(). */
T& operator[](size_t idx) noexcept
{
Assume(idx < m_size);
return m_buffer[BufferIndex(idx)];
}
/** Get a const reference to the element in the deque at the given index. Requires idx < size(). */
const T& operator[](size_t idx) const noexcept
{
Assume(idx < m_size);
return m_buffer[BufferIndex(idx)];
}
/** Test whether the contents of this deque is empty. */
bool empty() const noexcept { return m_size == 0; }
/** Get the number of elements in this deque. */
size_t size() const noexcept { return m_size; }
/** Get the capacity of this deque (maximum size it can have without reallocating). */
size_t capacity() const noexcept { return m_capacity; }
};
#endif // BITCOIN_UTIL_VECDEQUE_H