/bitcoin/src/prevector.h

Source
// Copyright (c) 2015-present 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_PREVECTOR_H
#define BITCOIN_PREVECTOR_H

#include <algorithm>
#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <iterator>
#include <new>
#include <type_traits>
#include <utility>

/** Implements a drop-in replacement for std::vector<T> which stores up to N
 *  elements directly (without heap allocation). The types Size and Diff are
 *  used to store element counts, and can be any unsigned + signed type.
 *
 *  Storage layout is either:
 *  - Direct allocation:
 *    - Size _size: the number of used elements (between 0 and N)
 *    - T direct[N]: an array of N elements of type T
 *      (only the first _size are initialized).
 *  - Indirect allocation:
 *    - Size _size: the number of used elements plus N + 1
 *    - Size capacity: the number of allocated elements
 *    - T* indirect: a pointer to an array of capacity elements of type T
 *      (only the first _size are initialized).
 *
 *  The data type T must be movable by memmove/realloc(). Once we switch to C++,
 *  move constructors can be used instead.
 */
template<unsigned int N, typename T, typename Size = uint32_t, typename Diff = int32_t>
class prevector {
    static_assert(std::is_trivially_copyable_v<T>);

public:
    static constexpr unsigned int STATIC_SIZE{N};

    typedef Size size_type;
    typedef Diff difference_type;
    typedef T value_type;
    typedef value_type& reference;
    typedef const value_type& const_reference;
    typedef value_type* pointer;
    typedef const value_type* const_pointer;

    class iterator {
        T* ptr{};
    public:
        typedef Diff difference_type;
        typedef T* pointer;
        typedef T& reference;
        using element_type = T;
        using iterator_category = std::contiguous_iterator_tag;
        iterator() = default;
        iterator(T* ptr_) : ptr(ptr_) {}
        T& operator*() const { return *ptr; }
        T* operator->() const { return ptr; }
        T& operator[](size_type pos) const { return ptr[pos]; }
        iterator& operator++() { ptr++; return *this; }
        iterator& operator--() { ptr--; return *this; }
        iterator operator++(int) { iterator copy(*this); ++(*this); return copy; }
        iterator operator--(int) { iterator copy(*this); --(*this); return copy; }
        difference_type friend operator-(iterator a, iterator b) { return (&(*a) - &(*b)); }
        iterator operator+(size_type n) const { return iterator(ptr + n); }
        iterator friend operator+(size_type n, iterator x) { return x + n; }
        iterator& operator+=(size_type n) { ptr += n; return *this; }
        iterator operator-(size_type n) const { return iterator(ptr - n); }
        iterator& operator-=(size_type n) { ptr -= n; return *this; }
        bool operator==(iterator x) const { return ptr == x.ptr; }
        auto operator<=>(iterator x) const { return ptr <=> x.ptr; }
    };

    class const_iterator {
        const T* ptr{};
    public:
        typedef Diff difference_type;
        typedef const T* pointer;
        typedef const T& reference;
        using element_type = const T;
        using iterator_category = std::contiguous_iterator_tag;
        const_iterator() = default;
        const_iterator(const T* ptr_) : ptr(ptr_) {}
        const_iterator(iterator x) : ptr(&(*x)) {}
        const T& operator*() const { return *ptr; }
        const T* operator->() const { return ptr; }
        const T& operator[](size_type pos) const { return ptr[pos]; }
        const_iterator& operator++() { ptr++; return *this; }
        const_iterator& operator--() { ptr--; return *this; }
        const_iterator operator++(int) { const_iterator copy(*this); ++(*this); return copy; }
        const_iterator operator--(int) { const_iterator copy(*this); --(*this); return copy; }
        difference_type friend operator-(const_iterator a, const_iterator b) { return (&(*a) - &(*b)); }
        const_iterator operator+(size_type n) const { return const_iterator(ptr + n); }
        const_iterator friend operator+(size_type n, const_iterator x) { return x + n; }
        const_iterator& operator+=(size_type n) { ptr += n; return *this; }
        const_iterator operator-(size_type n) const { return const_iterator(ptr - n); }
        const_iterator& operator-=(size_type n) { ptr -= n; return *this; }
        bool operator==(const_iterator x) const { return ptr == x.ptr; }
        auto operator<=>(const_iterator x) const { return ptr <=> x.ptr; }
    };

private:
#pragma pack(push, 1)
    union direct_or_indirect {
        char direct[sizeof(T) * N];
        struct {
            char* indirect;
            size_type capacity;
        } indirect_contents;
    };
#pragma pack(pop)
    alignas(char*) direct_or_indirect _union = {};
    size_type _size = 0;

    static_assert(alignof(char*) % alignof(size_type) == 0 && sizeof(char*) % alignof(size_type) == 0, "size_type cannot have more restrictive alignment requirement than pointer");
    static_assert(alignof(char*) % alignof(T) == 0, "value_type T cannot have more restrictive alignment requirement than pointer");

    T* direct_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.direct) + pos; }
    const T* direct_ptr(difference_type pos) const { return reinterpret_cast<const T*>(_union.direct) + pos; }
    T* indirect_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.indirect_contents.indirect) + pos; }
    const T* indirect_ptr(difference_type pos) const { return reinterpret_cast<const T*>(_union.indirect_contents.indirect) + pos; }
    bool is_direct() const { return _size <= N; }

    void change_capacity(size_type new_capacity) {
        if (new_capacity <= N) {
            if (!is_direct()) {
                T* indirect = indirect_ptr(0);
                T* src = indirect;
                T* dst = direct_ptr(0);
                memcpy(dst, src, size() * sizeof(T));
                free(indirect);
                _size -= N + 1;
            }
        } else {
            if (!is_direct()) {
                /* FIXME: Because malloc/realloc here won't call new_handler if allocation fails, assert
                    success. These should instead use an allocator or new/delete so that handlers
                    are called as necessary, but performance would be slightly degraded by doing so. */
                _union.indirect_contents.indirect = static_cast<char*>(realloc(_union.indirect_contents.indirect, ((size_t)sizeof(T)) * new_capacity));
                assert(_union.indirect_contents.indirect);
                _union.indirect_contents.capacity = new_capacity;
            } else {
                char* new_indirect = static_cast<char*>(malloc(((size_t)sizeof(T)) * new_capacity));
                assert(new_indirect);
                T* src = direct_ptr(0);
                T* dst = reinterpret_cast<T*>(new_indirect);
                memcpy(dst, src, size() * sizeof(T));
                _union.indirect_contents.indirect = new_indirect;
                _union.indirect_contents.capacity = new_capacity;
                _size += N + 1;
            }
        }
    }

    T* item_ptr(difference_type pos) { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); }
    const T* item_ptr(difference_type pos) const { return is_direct() ? direct_ptr(pos) : indirect_ptr(pos); }

    void fill(T* dst, ptrdiff_t count, const T& value = T{}) {
        std::fill_n(dst, count, value);
    }

    template <std::input_iterator InputIterator>
    void fill(T* dst, InputIterator first, InputIterator last) {
        while (first != last) {
            new(static_cast<void*>(dst)) T(*first);
            ++dst;
            ++first;
        }
    }

public:
    void assign(size_type n, const T& val) {
        clear();
        if (capacity() < n) {
            change_capacity(n);
        }
        _size += n;
        fill(item_ptr(0), n, val);
    }

    template <std::input_iterator InputIterator>
    void assign(InputIterator first, InputIterator last) {
        size_type n = last - first;
        clear();
        if (capacity() < n) {
            change_capacity(n);
        }
        _size += n;
        fill(item_ptr(0), first, last);
    }

    prevector() = default;

    explicit prevector(size_type n) {
        resize(n);
    }

    explicit prevector(size_type n, const T& val) {
        change_capacity(n);
        _size += n;
        fill(item_ptr(0), n, val);
    }

    template <std::input_iterator InputIterator>
    prevector(InputIterator first, InputIterator last) {
        size_type n = last - first;
        change_capacity(n);
        _size += n;
        fill(item_ptr(0), first, last);
    }

    prevector(const prevector<N, T, Size, Diff>& other) {
        size_type n = other.size();
        change_capacity(n);
        _size += n;
        fill(item_ptr(0), other.begin(),  other.end());
    }

    prevector(prevector<N, T, Size, Diff>&& other) noexcept
        : _union(std::move(other._union)), _size(other._size)
    {
        other._size = 0;
    }

    prevector& operator=(const prevector<N, T, Size, Diff>& other) {
        if (&other == this) {
            return *this;
        }
        assign(other.begin(), other.end());
        return *this;
    }

    prevector& operator=(prevector<N, T, Size, Diff>&& other) noexcept {
        if (!is_direct()) {
            free(_union.indirect_contents.indirect);
        }
        _union = std::move(other._union);
        _size = other._size;
        other._size = 0;
        return *this;
    }

    size_type size() const {
        return is_direct() ? _size : _size - N - 1;
    }

    bool empty() const {
        return size() == 0;
    }

    iterator begin() { return iterator(item_ptr(0)); }
    const_iterator begin() const { return const_iterator(item_ptr(0)); }
    iterator end() { return iterator(item_ptr(size())); }
    const_iterator end() const { return const_iterator(item_ptr(size())); }

    size_t capacity() const {
        if (is_direct()) {
            return N;
        } else {
            return _union.indirect_contents.capacity;
        }
    }

    T& operator[](size_type pos) {
        return *item_ptr(pos);
    }

    const T& operator[](size_type pos) const {
        return *item_ptr(pos);
    }

    void resize(size_type new_size) {
        size_type cur_size = size();
        if (cur_size == new_size) {
            return;
        }
        if (cur_size > new_size) {
            erase(item_ptr(new_size), end());
            return;
        }
        if (new_size > capacity()) {
            change_capacity(new_size);
        }
        ptrdiff_t increase = new_size - cur_size;
        fill(item_ptr(cur_size), increase);
        _size += increase;
    }

    void reserve(size_type new_capacity) {
        if (new_capacity > capacity()) {
            change_capacity(new_capacity);
        }
    }

    void shrink_to_fit() {
        change_capacity(size());
    }

    void clear() {
        resize(0);
    }

    iterator insert(iterator pos, const T& value) {
        size_type p = pos - begin();
        size_type new_size = size() + 1;
        if (capacity() < new_size) {
            change_capacity(new_size + (new_size >> 1));
        }
        T* ptr = item_ptr(p);
        T* dst = ptr + 1;
        memmove(dst, ptr, (size() - p) * sizeof(T));
        _size++;
        new(static_cast<void*>(ptr)) T(value);
        return iterator(ptr);
    }

    void insert(iterator pos, size_type count, const T& value) {
        size_type p = pos - begin();
        size_type new_size = size() + count;
        if (capacity() < new_size) {
            change_capacity(new_size + (new_size >> 1));
        }
        T* ptr = item_ptr(p);
        T* dst = ptr + count;
        memmove(dst, ptr, (size() - p) * sizeof(T));
        _size += count;
        fill(item_ptr(p), count, value);
    }

    template <std::input_iterator InputIterator>
    void insert(iterator pos, InputIterator first, InputIterator last) {
        size_type p = pos - begin();
        difference_type count = last - first;
        size_type new_size = size() + count;
        if (capacity() < new_size) {
            change_capacity(new_size + (new_size >> 1));
        }
        T* ptr = item_ptr(p);
        T* dst = ptr + count;
        memmove(dst, ptr, (size() - p) * sizeof(T));
        _size += count;
        fill(ptr, first, last);
    }

    inline void resize_uninitialized(size_type new_size) {
        // resize_uninitialized changes the size of the prevector but does not initialize it.
        // If size < new_size, the added elements must be initialized explicitly.
        if (capacity() < new_size) {
            change_capacity(new_size);
            _size += new_size - size();
            return;
        }
        if (new_size < size()) {
            erase(item_ptr(new_size), end());
        } else {
            _size += new_size - size();
        }
    }

    iterator erase(iterator pos) {
        return erase(pos, pos + 1);
    }

    iterator erase(iterator first, iterator last) {
        // Erase is not allowed to the change the object's capacity. That means
        // that when starting with an indirectly allocated prevector with
        // size and capacity > N, the result may be a still indirectly allocated
        // prevector with size <= N and capacity > N. A shrink_to_fit() call is
        // necessary to switch to the (more efficient) directly allocated
        // representation (with capacity N and size <= N).
        iterator p = first;
        char* endp = (char*)&(*end());
        _size -= last - p;
        memmove(&(*first), &(*last), endp - ((char*)(&(*last))));
        return first;
    }

    template<typename... Args>
    void emplace_back(Args&&... args) {
        size_type new_size = size() + 1;
        if (capacity() < new_size) {
            change_capacity(new_size + (new_size >> 1));
        }
        new(item_ptr(size())) T(std::forward<Args>(args)...);
        _size++;
    }

    void push_back(const T& value) {
        emplace_back(value);
    }

    void pop_back() {
        erase(end() - 1, end());
    }

    T& front() {
        return *item_ptr(0);
    }

    const T& front() const {
        return *item_ptr(0);
    }

    T& back() {
        return *item_ptr(size() - 1);
    }

    const T& back() const {
        return *item_ptr(size() - 1);
    }

    void swap(prevector<N, T, Size, Diff>& other) noexcept
    {
        std::swap(_union, other._union);
        std::swap(_size, other._size);
    }

    ~prevector() {
        if (!is_direct()) {
            free(_union.indirect_contents.indirect);
            _union.indirect_contents.indirect = nullptr;
        }
    }

    constexpr bool operator==(const prevector& other) const {
        return std::ranges::equal(*this, other);
    }

    bool operator<(const prevector<N, T, Size, Diff>& other) const {
        if (size() < other.size()) {
            return true;
        }
        if (size() > other.size()) {
            return false;
        }
        const_iterator b1 = begin();
        const_iterator b2 = other.begin();
        const_iterator e1 = end();
        while (b1 != e1) {
            if ((*b1) < (*b2)) {
                return true;
            }
            if ((*b2) < (*b1)) {
                return false;
            }
            ++b1;
            ++b2;
        }
        return false;
    }

    size_t allocated_memory() const {
        if (is_direct()) {
            return 0;
        } else {
            return ((size_t)(sizeof(T))) * _union.indirect_contents.capacity;
        }
    }

    value_type* data() {
        return item_ptr(0);
    }

    const value_type* data() const {
        return item_ptr(0);
    }
};

#endif // BITCOIN_PREVECTOR_H

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Created: 2026-07-14 18:13