batteries/class.lua

--[[
	barebones oop basics

	call the class object to construct a new instance

	classes are used as metatables directly so that
	metamethods "just work" - except for index, which is
	used to hook up instance methods

	classes do use a prototype chain for inheritance, but
	also copy their interfaces (including superclass)

		we copy interfaces in classes rather than relying on
		a prototype chain, so that pairs on the class gets
		all the methods when implemented as an interface

		class properties are not copied and should likely
		be accessed through the concrete class object so
		that everything refers to the same object

	arguments (all optional):
		name (string):
			the name to use for type()
		extends (class):
			superclass for basic inheritance
		implements (ordered table of classes):
			mixins/interfaces
		default_tostring (boolean):
			whether or not to provide a default tostring function

]]

--generate unique increasing class ids
local class_id_gen = 0
local function next_class_id()
	class_id_gen = class_id_gen + 1
	return class_id_gen
end

--implement an interface into c
local function implement(c, interface)
	c.__is[interface] = true
	for k, v in pairs(interface) do
		if c[k] == nil and type(v) == "function" then
			c[k] = v
		end
	end
end

--build a new class
local function class(config)
	local class_id = next_class_id()

	config = config or {}
	local extends = config.extends
	local implements = config.implements
	local name = config.name or ("unnamed class %d)"):format(class_id)

	local c = {}

	--unique generated id per-class
	c.__id = class_id

	--the class name
	c.__name = name

	--prototype
	c.__index = c

	--return the name of the class
	function c:type()
		return name
	end

	if config.default_tostring then
		function c:__tostring()
			return name
		end
	end

	--class metatable to set up constructor call
	setmetatable(c, {
		__call = function(self, ...)
			local instance = setmetatable({}, self)
			instance:new(...)
			return instance
		end,
		__index = extends,
	})

	--checking class membership for probably-too-dynamic code
	--returns true for both extended classes and implemented interfaces
	--(implemented with a hashset for fast lookups)
	c.__is = {}
	c.__is[c] = true
	function c:is(t)
		return self.__is[t] == true
	end

	--get the inherited class for super calls if/as needed
	--allows overrides that still refer to superclass behaviour
	c.__super = extends
	--nop by default
	function c:super() end

	if c.__super then
		--perform a super construction for an instance
		function c:super(...)
			c.__super.new(self, ...)
		end
		--implement superclass interface
		implement(c, c.__super)
	end


	--implement all the passed interfaces/mixins
	--in order provided
	if implements then
		for _, interface in ipairs(implements) do
			implement(c, interface)
		end
	end

	--default constructor, just proxy to the super constructor
	--override it and use to set up the properties of the instance
	--but don't forget to call the super constructor!
	function c:new(...)
		self:super(...)
	end

	--done
	return c
end

return class