vector_with_small_size_optimization.h

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/* +------------------------------------------------------------------------+
   |                     Mobile Robot Programming Toolkit (MRPT)            |
   |                          https://www.mrpt.org/                         |
   |                                                                        |
   | Copyright (c) 2005-2019, Individual contributors, see AUTHORS file     |
   | See: https://www.mrpt.org/Authors - All rights reserved.               |
   | Released under BSD License. See: https://www.mrpt.org/License          |
   +------------------------------------------------------------------------+ */
#pragma once

#include <mrpt/core/aligned_allocator.h>  // aligned_allocator_cpp11
#include <array>
#include <cstddef>  // size_t
#include <type_traits>  // conditional_t, ...
#include <vector>

namespace mrpt::containers
{
struct UnspecializedBool
{
    operator const bool&() const { return b; }
    operator bool&() { return b; }

    bool b;
};

/** Container that transparently and dynamically switches between a std::array
 * and std::vector. Used to avoid heap allocations with small vectors.
 *
 * \note In `#include <mrpt/containers/vector_with_small_size_optimization.h>`
 * \ingroup mrpt_containers_grp
 */
template <typename VAL, size_t small_size, size_t alignment = 16>
class vector_with_small_size_optimization
{
   private:
    using T =
        std::conditional_t<std::is_same_v<VAL, bool>, UnspecializedBool, VAL>;
    using ALLOC = mrpt::aligned_allocator_cpp11<T>;
    using vector =
        vector_with_small_size_optimization<VAL, small_size, alignment>;
    using large_vec = std::vector<T, ALLOC>;
    using small_array = std::array<T, small_size>;

    large_vec m_v;
    alignas(alignment) small_array m_a;
    bool m_is_small = true;
    size_t m_size = 0;

   public:
    using value_type = T;
    using reference = T&;
    using const_reference = const T&;
    using difference_type = typename large_vec::difference_type;
    using size_type = typename large_vec::size_type;
    template <typename TYPE, typename POINTER, typename REFERENCE>
    class iteratorImpl
    {
        using STORAGE = std::conditional_t<
            std::is_same_v<POINTER, bool*>, UnspecializedBool*,
            std::conditional_t<
                std::is_same_v<POINTER, const bool*>, const UnspecializedBool*,
                POINTER>>;
        using self = iteratorImpl<TYPE, POINTER, REFERENCE>;

       public:
        using value_type = TYPE;
        using reference = REFERENCE;
        using pointer = POINTER;
        using iterator_category = std::random_access_iterator_tag;
        using difference_type = typename large_vec::difference_type;
        iteratorImpl() = default;
        iteratorImpl(STORAGE ptr) : m_ptr(ptr) {}
        self operator++()
        {
            self i = *this;
            m_ptr++;
            return i;
        }
        self operator--()
        {
            self i = *this;
            m_ptr--;
            return i;
        }
        self operator++(int)
        {
            m_ptr++;
            return *this;
        }
        self operator--(int)
        {
            m_ptr--;
            return *this;
        }
        self operator+(difference_type n)
        {
            self i = *this;
            i.m_ptr += n;
            return i;
        }
        self operator-(difference_type n)
        {
            self i = *this;
            i.m_ptr -= n;
            return i;
        }
        self operator+=(difference_type n)
        {
            m_ptr += n;
            return *this;
        }
        self operator-=(difference_type n)
        {
            m_ptr -= n;
            return *this;
        }
        difference_type operator-(const self& o) const
        {
            return m_ptr - o.m_ptr;
        }
        REFERENCE operator*() { return *m_ptr; }
        const REFERENCE operator*() const { return *m_ptr; }
        POINTER operator->() { return m_ptr; }
        const POINTER operator->() const { return m_ptr; }
        bool operator==(const self& o) { return m_ptr == o.m_ptr; }
        bool operator!=(const self& o) { return m_ptr != o.m_ptr; }

       private:
        STORAGE m_ptr{nullptr};
    };

    using iterator = iteratorImpl<VAL, VAL*, VAL&>;
    using const_iterator = iteratorImpl<VAL, const VAL*, const VAL&>;

    void resize(size_type n)
    {
        if (m_size)
        {
            if (m_is_small && n > small_size)
            {
                m_v.assign(m_a.begin(), m_a.begin() + m_size);
            }
            else if (!m_is_small && n <= small_size)
            {
                std::copy(m_v.begin(), m_v.begin() + n, m_a.begin());
            }
        }
        m_size = n;
        m_is_small = (n <= small_size);
        if (!m_is_small)
        {
            m_v.resize(m_size);
        }
    }

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

    reference operator[](size_type n) { return m_is_small ? m_a[n] : m_v[n]; }

    const_reference operator[](size_type n) const
    {
        return m_is_small ? m_a[n] : m_v[n];
    }

    const_reference back() const
    {
        return m_is_small ? m_a[m_size - 1] : m_v.back();
    }
    reference back() { return m_is_small ? m_a[m_size - 1] : m_v.back(); }

    const_reference front() const
    {
        return m_is_small ? m_a.front() : m_v.front();
    }
    reference front() { return m_is_small ? m_a.front() : m_v.front(); }

    void swap(vector& x)
    {
        if (m_is_small & x.m_is_small)
        {
            m_a.swap(x.m_a);
        }
        else if (!m_is_small && !x.m_is_small)
        {
            m_v.swap(x.m_v);
        }
        else if (!m_is_small && x.m_is_small)
        {
            std::copy(x.m_a.begin(), x.m_a.begin() + x.m_size, m_a.begin());
            x.m_v.swap(m_v);
        }
        else
        {
            m_v.swap(x.m_v);
            std::copy(m_a.begin(), m_a.begin() + m_size, x.m_a.begin());
        }
        std::swap(m_size, x.m_size);
        std::swap(m_is_small, x.m_is_small);
    }

    iterator begin() noexcept { return m_is_small ? m_a.data() : m_v.data(); }
    const_iterator begin() const noexcept
    {
        return m_is_small ? m_a.data() : m_v.data();
    }

    iterator end() noexcept
    {
        return m_is_small ? m_a.data() + m_size : m_v.data() + m_size;
    }
    const_iterator end() const noexcept
    {
        return m_is_small ? m_a.data() + m_size : m_v.data() + m_size;
    }
};

}  // namespace mrpt::containers