gedeng/Vector.h

#pragma once
#include <algorithm>
#include <cassert>
#include <iostream>

template <class T>
class Vector {
  public:
    // Constructor
    // If the needed capacity is known, it can be passed; other wise, a default
    // capacity is reserved.
    Vector(unsigned int capacity = 10)
        : capacity(capacity), element_count(0),
          data((T *)::operator new(capacity * sizeof(T))) {
    }

    // Copy Constructor
    Vector(const Vector &other)
        : capacity(other.capacity), element_count(other.element_count),
          data((T *)::operator new(capacity * sizeof(T))) {
        // `std::copy` is used because it is more flexible than `std::memcpy`,
        // and the compiler will replace it with `memcpy` anyway if appropriate,
        // so there is no performance loss.
        std::copy(other.data, other.data + element_count, data);
    }

    // Move Constructor using the copy-and-swap-idiom
    Vector(Vector &&other) : data(nullptr) {
        swap(*this, other);
    }

    // Copy Assignment Operator using the copy-and-swap-idiom
    // Since this takes a value rather than a const reference due to
    // copy-and-swap, this is also the Move Assignment Operator.
    // This works because we pass-by-value, causing the copy constructor (which
    // does the actual copying) to be called, and then we swap the resulting
    // temporary into `this`.
    Vector &operator=(Vector other) {
        swap(*this, other);

        return *this;
    }

    // Destructor
    ~Vector() {
        delete[] data;
    }

    // Bracket Operator for accessing elements
    T &operator[](unsigned int position) const {
        return at(position);
    }

    // Equals Operator: Returns true if the number of elements is identical and
    // each of these elements are equal
    bool operator==(const Vector &other) {
        if (size() != other.size()) return false;

        for (unsigned int i = 0; i < size(); i++) {
            if (at(i) != other[i]) {
                return false;
            }
        }

        return true;
    }

    // Swap function for the copy-and-swap idiom.
    // See also:
    // https://stackoverflow.com/questions/3279543/what-is-the-copy-and-swap-idiom
    // The `friend` keyword is used so it is found through ADL.
    friend void swap(Vector &first, Vector &second) {
        // Enable ADL (good practice)
        using std::swap;

        swap(first.capacity, second.capacity);
        swap(first.element_count, second.element_count);
        swap(first.data, second.data);
    }

    void push_back(const T &element) {
        if (new_size_exceeds_capacity(size() + 1)) {
            reallocate(get_capacity_for_size(size() + 1));
        }

        // Use placement new to directly use pre-allocated memory
        new (data + element_count) T(element);
        element_count++;
    }

    void erase(unsigned int position) {
        assert(position < element_count);

        // Call the destructor on the given element
        data[position].~T();

        // Copy the other elements forwards
        std::copy_backward(data + position + 1, data + element_count,
                           data + element_count - 1);

        element_count--;

        if (is_using_excessive_memory()) {
            reallocate(get_capacity_for_size(size()));
        }
    }

    // Returns the number of elements in the vector, regardless of the actually
    // reserved memory.
    unsigned int size() const {
        return element_count;
    }

    // Returns the number of elements which could be in the vector using the
    // currently allocated memory, regardless of how many of these slots are
    // currently actually used.
    unsigned int length() const {
        return capacity;
    }

    // Returns a reference to the element at the given position
    T &at(unsigned int position) const {
        assert(position < element_count);
        return data[position];
    }

    // Expand the capacity of the vector by at least the given addition
    void reserve(unsigned int addition) {
        capacity += addition;
        reallocate(capacity);
    }

    // Resize the size of the vector to the given new_size
    // If this decreases the size, some elements are deleted
    // If this increases the size, some default-constructed elements are added
    void resize(unsigned int new_size) {
        int difference = new_size - size();

        if (difference > 0) {
            if (new_size_exceeds_capacity(new_size)) {
                reallocate(new_size);
            }

            // Add additional default-constructed items
            for (int i = 0; i < difference; i++) {
                data[element_count + i] = T();
            }
        } else if (difference < 0) {
            // Call the destructor on all excess items
            for (int i = -1; i > difference; i--) {
                data[element_count + i].~T();
            }

            if (is_using_excessive_memory()) {
                reallocate(get_capacity_for_size(new_size));
            }
        }

        element_count += difference;
    }

  private:
    unsigned int capacity;
    unsigned int element_count;
    T *data;

    bool is_using_excessive_memory() {
        return 10 + element_count * 2 < capacity;
    }

    bool new_size_exceeds_capacity(unsigned int new_size) {
        return new_size > capacity;
    }

    // Allocate enough memory to hold at least new_size entries.
    // Note that this function doesn't validate its input for optimization
    // reasons!
    void reallocate(unsigned int new_size) {
        T *new_data = (T *)::operator new(new_size * sizeof(T));
        std::copy(data, data + element_count, new_data);
        delete[] data;

        data = new_data;
    }

    // Return a sensible capacity for the given size (including some
    // padding).
    unsigned int get_capacity_for_size(unsigned int size) {
        return size * 2 + 5;
    }
};