Yet another Lua Set class

set.lua

local SET = {}

if gmod then debug.getregistry().Set = SET end

function SET:Add(item)
	self._data[item] = true
end

function SET:Remove(item)
	self._data[item] = nil
end

function SET:Contains(item)
	return self._data[item] == true
end

function SET:Iter()
	return next, self._data
end

function SET:Count()
	local len = 0

	for _ in self:Iter() do
		len = len + 1
	end

	return len
end

SET.__len = SET.Count

function SET:Unioned(other)
	local out = Set()

	for item in other:Iter() do
		out:Add(item)
	end

	for item in self:Iter() do
		out:Add(item)
	end

	return out
end

function SET:Subtracted(other)
	local out = Set()

	for item in self:Iter() do
		if not other:Contains(item) then
			out:Add(item)
		end
	end

	return out
end

function SET:Intersected(other)
	local out = Set()

	for item in self:Iter() do
		if other:Contains(item) then
			out:Add(item)
		end
	end

	return out
end

-- syntactic sugar for checking membership
function SET:__index(k)
	return SET[k] or self._data[k]
end

function SET:__tostring()
	return string.format("set (%d values)", self:Count())
end

function Set(...)
	local self = setmetatable({ _data = {} }, SET)

	for _, item in pairs({ ... }) do
		self:Add(item)
	end

	return self
end