Yet another hashmap :)

dictionary.cpp

template<typename Key, typename Value>
class Dictionary
{
public:
	
	using size_t = std::size_t;
	
	using pre_hash_t = std::function<size_t(const Key&)>;
	
	static const size_t minimum_capacity = 20;
	
	Dictionary(size_t capacity = minimum_capacity,
			   const pre_hash_t& pre_hash = std::hash<Key>())
	: _nodes(new Node*[capacity])
	, _size(0)
	, _capacity(capacity/_load_factor)
	, _threshold(capacity)
	, _pre_hash(pre_hash)
	{
		std::fill(_nodes, _nodes + _capacity, nullptr);
	}
	
	Dictionary(std::initializer_list<std::pair<Key, Value>> list,
				size_t capacity = minimum_capacity,
				const pre_hash_t& pre_hash = std::hash<Key>())
	: Dictionary(std::max(list.size(), capacity), pre_hash)
	{
		for (const auto& item : list)
		{
			insert(item.first, item.second);
		}
	}
	
	Dictionary(const Dictionary& other)
	: _nodes(new Node*[other._capacity])
	, _size(other._size)
	, _capacity(other._capacity)
	, _threshold(other._threshold)
	, _pre_hash(other._pre_hash)
	{
		for (size_t i = 0; i < _capacity; ++i)
		{
			_nodes[i] = nullptr;
			
			for (auto other_node = other._nodes[i];
				 other_node;
				 other_node = other_node->next)
			{
				auto node = new Node(other_node->key,
									 other_node->value,
									 _nodes[i]);
				
				_nodes[i] = node;
			}
		}
	}
	
	Dictionary(Dictionary&& other) noexcept
	: Dictionary()
	{
		swap(other);
	}
	
	void swap(Dictionary& other) noexcept
	{
		// Enable ADL
		using std::swap;
		
		swap(_nodes, other._nodes);
		
		swap(_size, other._size);
		
		swap(_capacity, other._capacity);
		
		swap(_threshold, other._threshold);
		
		swap(_pre_hash, other._pre_hash);
	}
	
	friend void swap(Dictionary& first, Dictionary& second) noexcept
	{
		first.swap(second);
	}
	
	~Dictionary()
	{
		_clear();
	}
	
	
	void insert(const Key& key, const Value& value)
	{
		auto index = _hash(key);
		
		for (auto node = _nodes[index]; node; node = node->next)
		{
			if (node->key == key)
			{
				node->value = value;
				
				return;
			}
		}
		
		auto node = new Node(key, value, _nodes[index]);
		
		_nodes[index] = node;
		
		if (++_size == _threshold) _resize(_capacity * 2);
	}
	
	
	void erase(const Key& key)
	{
		auto index = _hash(key);
		
		Node* previous = nullptr;
		
		for (auto node = _nodes[index]; node; node = node->next)
		{
			if (node->key == key)
			{
				if (previous) previous->next = node->next;
				
				delete node;
				
				if (--_size == _threshold/4) _resize(_capacity * 2);
			}
		}
		
		throw std::invalid_argument("No such key!");
	}
	
	void clear()
	{
		_clear();
		
		_size = 0;
		
		_resize(minimum_capacity);
	}
	
	
	Value& get(const Key& key)
	{
		auto found = _get(key);
		
		if (! found) throw std::invalid_argument("No such key!");
		
		return found;
	}
	
	const Value& get(const Key& key) const
	{
		auto found = _get(key);
		
		if (! found) throw std::invalid_argument("No such key!");
		
		return found;
	}
	
	
	bool contains(const Key& key) const
	{
		return static_cast<bool>(_get(key));
	}
	
	
	Value& operator[](const Key& key)
	{
		auto index = _hash(key);
		
		for (auto node = _nodes[index]; node; node = node->next)
		{
			if (node->key == key)
			{
				return node->value;
			}
		}
		
		auto node = new Node(key);
		
		node->next = _nodes[index];
		
		_nodes[index] = node;
		
		return node->value;
	}
	
	
	size_t size() const
	{
		return _size;
	}
	
	bool is_empty() const
	{
		return _size == 0;
	}
	
	pre_hash_t pre_hash()
	{
		return _pre_hash;
	}
	
	
	void rehash()
	{
		auto old = _nodes;
		
		_nodes = new Node*[_capacity];
		
		_rehash(old, _capacity);
		
		delete [] old;
	}
	
	void resize(size_t new_size)
	{
		_resize(new_size);
	}
	
private:
	
	static const size_t _load_factor = 4;

	struct Node
	{
		Node(const Key& key_ = Key(),
			 const Value& value_ = Value(),
			 Node* next_ = nullptr)
		: key(key_)
		, value(value_)
		, next(next_)
		{ }
		
		Key key;
		
		Value value;
		
		Node* next;
	};
	
	void _clear()
	{
		for (size_t i = 0; i < _capacity; ++i)
		{
			for (auto node = _nodes[i]; node; )
			{
				auto next = node->next;
				
				delete node;
				
				node = next;
			}
		}
	}
	
	Node* _get(const Key& key) const
	{
		auto index = _hash(key);
		
		for (auto node = _nodes[index]; node; node = node->next)
		{
			if (node->key == key)
			{
				return node;
			}
		}
		
		return nullptr;
	}
	
	size_t _hash(const Key& key) const
	{
		return _pre_hash(key) % _capacity;
	}
	
	
	void _resize(size_t new_capacity)
	{
		if (new_capacity < minimum_capacity) return;
		
		size_t old_capacity =  new_capacity;
		
		_capacity = new_capacity;
		
		_threshold = _capacity * _load_factor;
		
		auto old = _nodes;
		
		_nodes = new Node*[_capacity];
		
		_rehash(old, old_capacity);
		
		delete [] old;
	}
	
	void _rehash(Node** old, size_t old_capacity)
	{
		std::fill(_nodes, _nodes + _capacity, nullptr);
		
		for (size_t i = 0; i < old_capacity; ++i)
		{
			for (auto node = old[i]; node; )
			{
				auto next = node->next;
				
				auto index = _hash(node->key);
				
				node->next = _nodes[index];
				
				_nodes[index] = node;
				
				node = next;
			}
		}
	}
	
	
	pre_hash_t _pre_hash;
	
	Node** _nodes;
	
	size_t _size;
	
	size_t _capacity;
	
	size_t _threshold;
};