A hash-set implementation with iterator support.

HashSet.cpp

template<typename T>
class HashSet
{
private:
	
	struct Node
	{
		Node(const T& t = T(),
			 Node* following = nullptr,
			 Node* prev = nullptr)
		: item(t)
		, next(following)
		, previous(prev)
		{ }
		
		T item;
		
		Node* next;
		Node* previous;
	};
	
public:
	
	using size_t = std::size_t;
	
	using hash_function_t = std::function<size_t(const T&)>;
	
	static const size_t minimum_capacity = 16;
	
	class iterator
	{
	public:
		
		iterator(size_t index = 0,
				 Node* node = nullptr,
				 Node** nodes = nullptr)
		: _index(index)
		, _current(node)
		, _nodes(nodes)
		{ }
		
		iterator(const iterator& other)
		: _index(other._index)
		, _current(other._current)
		, _nodes(other._nodes)
		{ }
		
		iterator(iterator&& other)
		: iterator()
		{
			swap(other);
		}
		
		iterator& operator=(iterator other)
		{
			swap(other);
			
			return *this;
		}
		
		void swap(iterator& other) noexcept
		{
			using std::swap;
			
			swap(_index, other._index);
			
			swap(_current, other._current);
			
			swap(_nodes, other._nodes);
		}
		
		friend void swap(iterator& first, iterator& second) noexcept
		{
			first.swap(second);
		}

		const T& operator*() const
		{
			return _current->item;
		}
		
		const T* operator->() const
		{
			return &_current->item;
		}
		
		iterator& operator++()
		{
			if (_current && _current->next)
			{
				_current = _current->next;
			}
			
			else
			{
				do _current = _nodes[++_index];
				
				while (! _current);
			}
			
			return *this;
		}
		
		iterator& operator++(int)
		{
			auto previous = *this;
			
			++*this;
			
			return previous;
		}
		
		iterator& operator--()
		{
			if (_current->previous) _current = _current->previous;
			
			else
			{
				while (! _nodes[_index]) --_index;
				
				while (_current->next) _current = _current->next;
			}
			
			return *this;
		}
		
		iterator& operator--(int)
		{
			auto following = *this;
			
			--*this;
			
			return following;
			
		}
		
		bool operator==(const iterator& other) const
		{
			return _current == other._current;
		}
		
		bool operator!=(const iterator& other) const
		{
			return _current != other._current;
		}
		
	private:
		
		size_t _index;
		
		Node* _current;
		
		Node** _nodes;
	};
	
	using const_iterator = iterator;
	
	HashSet(const hash_function_t& hash_function = std::hash<T>(),
			size_t capacity = minimum_capacity)
	: _nodes(new Node*[capacity / _bin_size + 1])
	, _hash_function(hash_function)
	, _threshold(capacity)
	, _capacity(_threshold/_bin_size)
	, _size(0)
	{
		for (size_t i = 0; i < _capacity; ++i)
		{
			_nodes[i] = nullptr;
		}
		
		_nodes[_capacity] = new Node;
	}
	
	HashSet(const std::initializer_list<T>& items,
			const hash_function_t& hash_function = std::hash<T>(),
			size_t capacity = minimum_capacity)
	: _hash_function(hash_function)
	, _size(0)
	{
		_threshold = std::max(items.size(), capacity);
		
		_capacity = _threshold / _bin_size;
		
		_nodes = new Node*[_capacity + 1];
		
		for (size_t i = 0, end = _capacity; i < end; ++i)
		{
			_nodes[i] = nullptr;
		}
		
		for (const auto& i : items) insert(i);
		
		_nodes[_capacity] = new Node;
	}
	
	HashSet(const HashSet& other)
	: _nodes(new Node*[other._capacity])
	, _hash_function(other._hash_function)
	, _capacity(other._capacity)
	, _threshold(other._threshold)
	{
		for (size_t i = 0; i < _capacity; ++i)
		{
			_nodes[i] = nullptr;
			
			for (Node* node = other._nodes[i]; node; node = node->next)
			{
				insert(node->item);
			}
		}
		
		_nodes[_capacity] = new Node;
	}
	
	HashSet(HashSet&& other) noexcept
	: HashSet()
	{
		swap(other);
	}
	
	HashSet& operator=(HashSet other)
	{
		swap(other);
	}
	
	void swap(HashSet& other) noexcept
	{
		// Enable ADL
		using std::swap;
		
		swap(_nodes, other._nodes);
		
		swap(_hash_function, other._hash_function);
		
		swap(_capacity, other._capacity);
		
		swap(_size, other._size);
		
		swap(_threshold, other._threshold);
	}
	
	friend void swap(HashSet& first, HashSet& second) noexcept
	{
		first.swap(second);
	}
	
	~HashSet()
	{
		clear();
	}
	
	
	iterator begin() const
	{
		if (is_empty()) return end();
		
		return {0, _nodes[0], _nodes};
	}
	
	iterator end() const
	{
		return {_capacity, _nodes[_capacity], _nodes};
	}
	
	
	void insert(const T& item)
	{
		size_t index = _hash(item);
		
		Node* node = _nodes[index];
		
		while (node)
		{
			if (node->item == item) return;
			
			node = node->next;
		}
		
		node = new Node(item, _nodes[index]);
		
		if (_nodes[index]) _nodes[index]->previous = node;
		
		_nodes[index] = node;
		
		if (++_size == _threshold) _resize();
	}
	
	template<typename Itr>
	void insert(Itr begin, Itr end)
	{
		for ( ; begin != end; ++begin)
		{
			insert(*begin);
		}
	}
	
	bool contains(const T& item) const
	{
		size_t index = _hash(item);
		
		for (Node* node = _nodes[index]; node; node = node->next)
		{
			if (node->item == item) return true;
		}
		
		return false;

	}
	
	iterator find(const T& item) const
	{
		size_t index = _hash(item);
		
		for (Node* node = _nodes[index]; node; node = node->next)
		{
			if (node->item == item)
			{
				return {index, node, _nodes};
			}
		}
		
		return end();
	}
	
	size_t count(const T& item) const
	{
		auto itr = find(item);
		
		return itr != end();
	}
	
	void erase(const T& item)
	{
		size_t index = _hash(item);
		
		for (Node* node = _nodes[index]; node; node = node->next)
		{
			if (node->item == item)
			{
				if (node->previous) node->previous->next = node->next;
				
				else _nodes[index] = node->next;
				
				delete node;
				
				if (--_size == _threshold/4) _resize();
				
				return;
			}
		}
		
		throw std::invalid_argument("No such item!");
	}
	
	template<typename Itr>
	void erase(Itr begin, Itr end)
	{
		for ( ; begin != end; ++begin)
		{
			erase(*begin);
		}
	}
	
	void clear()
	{
		for (size_t i = 0; i < _capacity; ++i)
		{
			for (Node* node = _nodes[i], *next; node; node = next)
			{
				next = node->next;
				
				delete node;
			}
			
			_nodes[i] = nullptr;
		}
		
		_size = 0;
	}
	
	
	size_t size() const
	{
		return _size;
	}
	
	bool is_empty() const
	{
		return _size == 0;
	}
	
	const hash_function_t& hash_function() const
	{
		return _hash_function;
	}
	
private:
	
	static const size_t _bin_size = 4;

	size_t _hash(const T& item) const
	{
		return _hash_function(item) % _capacity;
	}
	
	void _resize()
	{
		size_t new_size = _size * 2;
		
		if (new_size < minimum_capacity) return;
		
		size_t old_capacity = _capacity;
		
		_capacity = new_size / _bin_size;
		
		_threshold = new_size;
		
		Node** old = _nodes;
		
		_nodes = new Node*[_capacity + 1];
		
		// for the end node
		_nodes[_capacity] = old[old_capacity];
		
		for (size_t i = 0; i < _capacity; ++i)
		{
			_nodes[i] = nullptr;
		}
		
		_rehash(old, old_capacity);
		
		delete [] old;
	}
	
	void _rehash(Node** old, size_t old_capacity)
	{
		for (size_t i = 0; i < old_capacity; ++i)
		{
			for (Node* node = old[i], *next; node; node = next)
			{
				size_t index = _hash(node->item);
				
				next = node->next;
				
				node->next = _nodes[index];
				
				if (_nodes[index]) _nodes[index]->previous = node;
				
				_nodes[index] = node;
			}
			
			old[i] = nullptr;
		}
	}
	
	
	Node** _nodes;

	
	size_t _threshold;
	
	size_t _capacity;
	
	size_t _size;

	
	hash_function_t _hash_function;
	
};