local function _remove(list, m)

	if m.prev then

		m.prev.next = m.next;

	end

	if m.next then

		m.next.prev = m.prev;

	end

	if list._tail == m then

		list._tail = m.prev;

	end

	if list._head == m then

		list._head = m.next;

	end

	list._count = list._count - 1;

end

local function _insert(list, m)

	if list._head then

		list._head.prev = m;

	end

	m.prev, m.next = nil, list._head;

	list._head = m;

	if not list._tail then

		list._tail = m;

	end

	list._count = list._count + 1;

end

local cache_methods = {};

local cache_mt = { __index = cache_methods };

function cache_methods:set(k, v)

	local m = self._data[k];

	if m then

		-- Key already exists

		if v ~= nil then

			-- Bump to head of list

			_remove(self, m);

			_insert(self, m);

			m.value = v;

		else

			-- Remove from list

			_remove(self, m);

			self._data[k] = nil;

		end

		return true;

	end

	-- New key

	if v == nil then

		return true;

	end

	-- Check whether we need to remove oldest k/v

	if self._count == self.size then

		local tail = self._tail;

		local on_evict, evicted_key, evicted_value = self._on_evict, tail.key, tail.value;

		if on_evict ~= nil and (on_evict == false or on_evict(evicted_key, evicted_value) == false) then

			-- Cache is full, and we're not allowed to evict

			return false;

		end

		_remove(self, tail);

		self._data[evicted_key] = nil;

	end

	m = { key = k, value = v, prev = nil, next = nil };

	self._data[k] = m;

	_insert(self, m);

	return true;

end

function cache_methods:get(k)

	local m = self._data[k];

	if m then

		return m.value;

	end

	return nil;

end

function cache_methods:items()

	local m = self._head;

	return function ()

		if not m then

			return;

		end

		local k, v = m.key, m.value;

		m = m.next;

		return k, v;

	end

end

function cache_methods:count()

	return self._count;

end

function cache_methods:head()

	local head = self._head;

	if not head then return nil, nil; end

	return head.key, head.value;

end

function cache_methods:tail()

	local tail = self._tail;

	if not tail then return nil, nil; end

	return tail.key, tail.value;

end

local function new(size, on_evict)

	size = assert(tonumber(size), "cache size must be a number");

	size = math.floor(size);

	assert(size > 0, "cache size must be greater than zero");

	local data = {};

	return setmetatable({ _data = data, _count = 0, size = size, _head = nil, _tail = nil, _on_evict = on_evict }, cache_mt);

end

return {

	new = new;

}