class.lua

-- This convenience function is copied from http://lua-users.org/wiki/ObjectOrientationTutorial.
function class(...)
	-- "cls" is the new class
	local cls, bases = {}, {...}
	-- copy base class contents into the new class
	for i, base in ipairs(bases) do
		for k, v in pairs(base) do
			cls[k] = v
		end
	end
	-- set the class's __index, and start filling an "is_a" table that contains this class and all of its bases
	-- so you can do an "instance of" check using my_instance.is_a[MyClass]
	cls.__index, cls.is_a = cls, {[cls] = true}
	for i, base in ipairs(bases) do
		for c in pairs(base.is_a) do
			cls.is_a[c] = true
		end
		cls.is_a[base] = true
	end
	-- the class's __call metamethod
	setmetatable(
		cls,
		{
			__call = function(c, ...)
				local instance = setmetatable({}, c)
				-- run the init method if it's there
				local init = instance._init
				if init then
					init(instance, ...)
				end
				return instance
			end
		}
	)
	-- return the new class table, that's ready to fill with methods
	return cls
end