StackHeapVector.h

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/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
 
#ifndef META_OCEAN_BASE_STACK_HEAP_VECTOR_H
#define META_OCEAN_BASE_STACK_HEAP_VECTOR_H
 
#include "ocean/base/Base.h"
#include "ocean/base/StaticBuffer.h"
 
namespace Ocean
{
 
/**
 * Vector like data structure combining stack and heap memory.
 * This class implements a vector-like data structure which stores the first `tStackCapacity` elements on the stack, and any additional elements on the heap.<br>
 * This approach can optimize performance and memory usage when the number of elements is often within the `tStackCapacity` but can occasionally exceed it.
 * @tparam T Data type of the elements that will be stored
 * @tparam tStackCapacity Number of elements that can be stored on the stack, with range [1, infinity)
 * @ingroup base
 */
template <typename T, size_t tStackCapacity>
class StackHeapVector
{
    static_assert(tStackCapacity >= 1, "Invalid stack capacity!");
 
    public:
 
        /**
         * Definition of an iterator allowing to iterate through the vector.
         * The iterator allows to modified the element to which the iterator is pointing.
         */
        class Iterator
        {
            friend class StackHeapVector;
 
            public:
 
                /**
                 * (Pre-) increments this iterator.
                 * @return Reference to the incremented iterator
                 */
                Iterator& operator++();
 
                /**
                 * (Post-) increments this iterator.
                 * @return The current (not yet) incremented iterator.
                 */
                Iterator operator++(int);
 
                /**
                 * De-references the iterator and provides access to the underlying element in the vector.
                 * @return The vector element to which this iterator points
                 */
                T& operator*();
 
                /**
                 * Compares two iterators and returns whether both iterators point to the same vector element.
                 * @return True, if so
                 */
                bool operator==(const Iterator& iterator) const;
 
                /**
                 * Compares two iterators and returns whether both iterators do not point to the same vector element.
                 * @return True, if so
                 */
                bool operator!=(const Iterator& iterator) const;
 
            protected:
 
                /**
                 * Creates a new iterator pointing to a specified elements in the vector.
                 * @param vector The vector owning this iterator
                 * @param index The index of the element to which the iterator points
                 */
                Iterator(StackHeapVector& vector, const size_t index);
 
            protected:
 
                /// The vector owning this iterator.
                StackHeapVector& vector_;
 
                /// The index of the element within the vector to which the iterator points.
                size_t index_ = size_t(-1);
        };
 
        /**
         * Definition of an iterator allowing to iterate through the vector.
         * The iterator does ont allow to modified the element to which the iterator is pointing.
         */
        class ConstIterator
        {
            friend class StackHeapVector;
 
            public:
 
                /**
                 * (Pre-) increments this iterator.
                 * @return Reference to the incremented iterator
                 */
                ConstIterator& operator++();
 
                /**
                 * (Post-) increments this iterator.
                 * @return The current (not yet) incremented iterator.
                 */
                ConstIterator operator++(int);
 
                /**
                 * De-references the iterator and provides access to the underlying element in the vector.
                 * @return The vector element to which this iterator points
                 */
                const T& operator*() const;
 
                /**
                 * Compares two iterators and returns whether both iterators point to the same vector element.
                 * @return True, if so
                 */
                bool operator==(const ConstIterator& iterator) const;
 
                /**
                 * Compares two iterators and returns whether both iterators do not point to the same vector element.
                 * @return True, if so
                 */
                bool operator!=(const ConstIterator& iterator) const;
 
            protected:
 
                /**
                 * Creates a new iterator pointing to a specified elements in the vector.
                 * @param vector The vector owning this iterator
                 * @param index The index of the element to which the iterator points
                 */
                ConstIterator(const StackHeapVector& vector, const size_t index);
 
            protected:
 
                /// The vector owning this iterator.
                const StackHeapVector& vector_;
 
                /// The index of the element within the vector to which the iterator points.
                size_t index_ = size_t(-1);
        };
 
    public:
 
        /**
         * Creates a new vector object.
         */
        StackHeapVector();
 
        /**
         * Creates a new vector object.
         * @param size The number of elements to be created
         * @param element The value that will be created in the first 'number' elements of this vector
         */
        StackHeapVector(const size_t size, const T& element);
 
        /**
         * Pushes a new element to the end of this vector.
         * @param element The new element to be pushed
         */
        void pushBack(T&& element);
 
        /**
         * Pushes a new element to the end of this vector.
         * @param element The new element to be pushed
         */
        void pushBack(const T& element);
 
        /**
         * Emplaces a new element to the end of this vector.
         * @param args The arguments of the new element
         * @tparam TArgs The data types of the arguments
         */
        template <typename... TArgs>
        T& emplaceBack(TArgs&&... args);
 
        /**
         * Removed the last elements from the vector.
         * The vector must not be empty.
         */
        void popBack();
 
        /**
         * Resizes the vector.
         * @param size The new size of the vector
         */
        void resize(const size_t size);
 
        /**
         * Replaces the content of the vector with 'size' copies of the provided element.
         * @param size The new size of the vector, with range [0, infinity)
         * @param element The element which will be copied into all places of the resized vector
         */
        void assign(const size_t size, const T& element);
 
        /**
         * Returns the number of elements of this vector.
         * @return The vector's number of elements, with range [0, infinity)
         */
        inline size_t size() const;
 
        /**
         * Returns the overall capacity of this vector (including the capacity on the stack and on the heap).
         * @return The vector's capacity, with range [size(), infinity)
         */
        inline size_t capacity() const;
 
        /**
         * Returns whether this vector is empty.
         * @return True, if so
         */
        inline bool isEmpty() const;
 
        /**
         * Clears this vector.
         * All stack elements will be overwritten with default values.
         */
        void clear();
 
        /**
         * Sets the capacity of this vector to a specified number of elements.
         * In case the specified capacity is smaller than the current capacity or is smaller than the number of elements in this vector, nothing happens.
         * @param capacity The capacity to set, with range [0, infinity)
         */
        void setCapacity(const size_t capacity);
 
        /**
         * Returns the first element of this vector.
         * Ensure that the vector is not empty before calling this function.
         * @return The vector's first element
         */
        const T& front() const;
 
        /**
         * Returns the first element of this vector.
         * Ensure that the vector is not empty before calling this function.
         * @return The vector's first element
         */
        T& front();
 
        /**
         * Returns the last element of this vector.
         * Ensure that the vector is not empty before calling this function.
         * @return The vector's last element
         */
        const T& back() const;
 
        /**
         * Returns the last element of this vector.
         * Ensure that the vector is not empty before calling this function.
         * @return The vector's last element
         */
        T& back();
 
        /**
         * Returns an iterator to the first element in this vector.
         * @return The vector's iterator to the first element
         */
        Iterator begin();
 
        /**
         * Returns an iterator to the element following the last element in this vector.
         * @return The vector's iterator to the element following the last element
         */
        Iterator end();
 
        /**
         * Returns a const iterator to the first element in this vector.
         * @return The vector's iterator to the first element
         */
        ConstIterator begin() const;
 
        /**
         * Returns an iterator to the element following the last element in this vector.
         * @return The vector's iterator to the element following the last element
         */
        ConstIterator end() const;
 
        /**
         * Elements access operator.
         * @param index The index of the element to access, with range [0, size() - 1]
         * @return The element with specified index
         */
        inline const T& operator[](const size_t index) const;
 
        /**
         * Elements access operator.
         * @param index The index of the element to access, with range [0, size() - 1]
         * @return The element with specified index
         */
        inline T& operator[](const size_t index);
 
    protected:
 
        /// The elements located on the stack.
        T stackElements_[tStackCapacity];
 
        /// The remaining elements located on the heap.
        std::vector<T> heapElements_;
 
        /// The number of elements in this vector.
        size_t size_ = 0;
};
 
template <typename T, size_t tStackCapacity>
StackHeapVector<T, tStackCapacity>::Iterator::Iterator(StackHeapVector& vector, const size_t index) :
    vector_(vector),
    index_(index)
{
    ocean_assert(index <= vector_.size_);
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::Iterator& StackHeapVector<T, tStackCapacity>::Iterator::operator++()
{
    ++index_;
 
    return *this;
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::Iterator StackHeapVector<T, tStackCapacity>::Iterator::operator++(int)
{
    Iterator iterator(*this);
 
    ++index_;
 
    return iterator;
}
 
template <typename T, size_t tStackCapacity>
T& StackHeapVector<T, tStackCapacity>::Iterator::operator*()
{
    return vector_[index_];
}
 
template <typename T, size_t tStackCapacity>
bool StackHeapVector<T, tStackCapacity>::Iterator::operator==(const Iterator& iterator) const
{
    ocean_assert(&vector_ == &iterator.vector_);
 
    return &vector_ == &iterator.vector_ && index_ == iterator.index_;
}
 
template <typename T, size_t tStackCapacity>
bool StackHeapVector<T, tStackCapacity>::Iterator::operator!=(const Iterator& iterator) const
{
    return !(*this == iterator);
}
 
template <typename T, size_t tStackCapacity>
StackHeapVector<T, tStackCapacity>::ConstIterator::ConstIterator(const StackHeapVector& vector, const size_t index) :
    vector_(vector),
    index_(index)
{
    ocean_assert(index <= vector_.size_);
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::ConstIterator& StackHeapVector<T, tStackCapacity>::ConstIterator::operator++()
{
    ++index_;
 
    return *this;
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::ConstIterator StackHeapVector<T, tStackCapacity>::ConstIterator::operator++(int)
{
    Iterator iterator(*this);
 
    ++index_;
 
    return iterator;
}
 
template <typename T, size_t tStackCapacity>
const T& StackHeapVector<T, tStackCapacity>::ConstIterator::operator*() const
{
    return vector_[index_];
}
 
template <typename T, size_t tStackCapacity>
bool StackHeapVector<T, tStackCapacity>::ConstIterator::operator==(const ConstIterator& iterator) const
{
    ocean_assert(&vector_ == &iterator.vector_);
 
    return &vector_ == &iterator.vector_ && index_ == iterator.index_;
}
 
template <typename T, size_t tStackCapacity>
bool StackHeapVector<T, tStackCapacity>::ConstIterator::operator!=(const ConstIterator& iterator) const
{
    return !(*this == iterator);
}
 
template <typename T, size_t tStackCapacity>
StackHeapVector<T, tStackCapacity>::StackHeapVector()
{
    // nothing to do here
}
 
template <typename T, size_t tStackCapacity>
StackHeapVector<T, tStackCapacity>::StackHeapVector(const size_t size, const T& element)
{
    setCapacity(size);
 
    for (size_t n = 0; n < size; ++n)
    {
        pushBack(element);
    }
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::pushBack(T&& element)
{
    if (size_ < tStackCapacity)
    {
        stackElements_[size_] = std::move(element);
    }
    else
    {
        heapElements_.emplace_back(std::move(element));
    }
 
    ++size_;
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::pushBack(const T& element)
{
    if (size_ < tStackCapacity)
    {
        stackElements_[size_] = element;
    }
    else
    {
        heapElements_.push_back(element);
    }
 
    ++size_;
}
 
template <typename T, size_t tStackCapacity>
template <typename... TArgs>
T& StackHeapVector<T, tStackCapacity>::emplaceBack(TArgs&&... args)
{
    const size_t index = size_;
 
    ++size_;
 
    if (index < tStackCapacity)
    {
        stackElements_[index] = T(std::forward<TArgs>(args)...);
 
        return stackElements_[index];
    }
    else
    {
        return heapElements_.emplace_back(std::forward<TArgs>(args)...);
    }
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::popBack()
{
    ocean_assert(size_ >= 1);
 
    --size_;
 
    if (size_ >= tStackCapacity)
    {
        heapElements_.pop_back();
    }
    else
    {
        stackElements_[size_] = T();
    }
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::resize(const size_t size)
{
    if (size == size_)
    {
        return;
    }
 
    if (size < size_)
    {
        // we have to remove elements
 
        for (size_t n = size; n < tStackCapacity; ++n) // in case size >= tStackCapacity, nothing happens
        {
            stackElements_[n] = T();
        }
 
        if (size_ > tStackCapacity)
        {
            if (size < tStackCapacity)
            {
                heapElements_.clear();
            }
            else
            {
                heapElements_.resize(size - tStackCapacity);
            }
        }
    }
    else
    {
        ocean_assert(size > size_);
 
        if (size > tStackCapacity)
        {
            heapElements_.resize(size - tStackCapacity);
        }
    }
 
    size_ = size;
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::assign(const size_t size, const T& element)
{
    for (size_t n = 0; n < std::min(size, tStackCapacity); ++n) // we assign as many elements in the stack
    {
        stackElements_[n] = element;
    }
 
    for (size_t n = size; n < std::min(tStackCapacity, size_); ++n) // if necessary (if the new size is smaller than the previous size), we overwrite stack elements with default values
    {
        stackElements_[n] = T();
    }
 
    if (size < tStackCapacity)
    {
        heapElements_.clear();
    }
    else
    {
        const size_t newHeapSize = size - tStackCapacity;
 
        heapElements_.assign(newHeapSize, element);
    }
 
    size_ = size;
}
 
template <typename T, size_t tStackCapacity>
inline size_t StackHeapVector<T, tStackCapacity>::size() const
{
    ocean_assert(size_ <= tStackCapacity || size_ == tStackCapacity + heapElements_.size());
 
    return size_;
}
 
template <typename T, size_t tStackCapacity>
inline size_t StackHeapVector<T, tStackCapacity>::capacity() const
{
    return tStackCapacity + heapElements_.capacity();
}
 
template <typename T, size_t tStackCapacity>
inline bool StackHeapVector<T, tStackCapacity>::isEmpty() const
{
    return size() == 0;
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::clear()
{
    for (size_t nStack = 0; nStack < std::min(size_, tStackCapacity); ++nStack)
    {
        stackElements_[nStack] = T();
    }
 
    heapElements_.clear();
 
    size_ = 0;
}
 
template <typename T, size_t tStackCapacity>
void StackHeapVector<T, tStackCapacity>::setCapacity(const size_t capacity)
{
    if (capacity > size_)
    {
        if (capacity > tStackCapacity)
        {
            heapElements_.reserve(capacity - tStackCapacity);
        }
    }
}
 
template <typename T, size_t tStackCapacity>
const T& StackHeapVector<T, tStackCapacity>::front() const
{
    ocean_assert(!isEmpty());
 
    return stackElements_[0];
}
 
template <typename T, size_t tStackCapacity>
T& StackHeapVector<T, tStackCapacity>::front()
{
    ocean_assert(!isEmpty());
 
    return stackElements_[0];
}
 
template <typename T, size_t tStackCapacity>
const T& StackHeapVector<T, tStackCapacity>::back() const
{
    ocean_assert(!isEmpty());
 
    if (size_ <= tStackCapacity)
    {
        return stackElements_[size_ - 1];
    }
    else
    {
        return heapElements_.back();
    }
}
 
template <typename T, size_t tStackCapacity>
T& StackHeapVector<T, tStackCapacity>::back()
{
    ocean_assert(!isEmpty());
 
    if (size_ <= tStackCapacity)
    {
        return stackElements_[size_ - 1];
    }
    else
    {
        return heapElements_.back();
    }
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::Iterator StackHeapVector<T, tStackCapacity>::begin()
{
    return Iterator(*this, 0);
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::Iterator StackHeapVector<T, tStackCapacity>::end()
{
    return Iterator(*this, size_);
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::ConstIterator StackHeapVector<T, tStackCapacity>::begin() const
{
    return ConstIterator(*this, 0);
}
 
template <typename T, size_t tStackCapacity>
typename StackHeapVector<T, tStackCapacity>::ConstIterator StackHeapVector<T, tStackCapacity>::end() const
{
    return ConstIterator(*this, size_);
}
 
template <typename T, size_t tStackCapacity>
inline const T& StackHeapVector<T, tStackCapacity>::operator[](const size_t index) const
{
    ocean_assert(index < size());
 
    if (index < tStackCapacity)
    {
        return stackElements_[index];
    }
 
    return heapElements_[index - tStackCapacity];
}
 
template <typename T, size_t tStackCapacity>
inline T& StackHeapVector<T, tStackCapacity>::operator[](const size_t index)
{
    ocean_assert(index < size());
 
    if (index < tStackCapacity)
    {
        return stackElements_[index];
    }
 
    return heapElements_[index - tStackCapacity];
}
 
}
 
#endif // META_OCEAN_BASE_STACK_HEAP_VECTOR_H