HC/class.lua

--[[
Copyright (c) 2010-2011 Matthias Richter

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local function __NULL__() end

-- class "inheritance" by copying functions
local function inherit(class, interface, ...)
	if not interface or type(interface) ~= "table" then return end

	-- __index and construct are not overwritten as for them class[name] is defined
	for name, func in pairs(interface) do
		if not class[name] and type(func) == "function" then
			class[name] = func
		end
	end
	for super in pairs(interface.__is_a) do
		class.__is_a[super] = true
	end

	inherit(class, ...)
end

-- class builder
local function new(args)
	local super = {}
	local name = '<unnamed class>'
	local constructor = args or __NULL__
	if type(args) == "table" then
		-- nasty hack to check if args.inherits is a table of classes or a class or nil
		super = (args.inherits or {}).__is_a and {args.inherits} or args.inherits or {}
		name = args.name or name
		constructor = args[1] or __NULL__
	end
	assert(type(constructor) == "function", 'constructor has to be nil or a function')

	-- build class
	local class = {}
	class.__index = class
	class.__tostring = function() return ("<instance of %s>"):format(tostring(class)) end
	class.construct, class.Construct = constructor or __NULL__, constructor or __NULL__
	class.Construct = class.construct
	class.inherit, class.Inherit = inherit, inherit
	class.__is_a = {[class] = true}
	class.is_a = function(self, other) return not not self.__is_a[other] end

	-- intercept assignment in global environment to infer the class name
	if not (args and args.name) then
		local env, env_meta, interceptor = getfenv(0), getmetatable(getfenv(0)), {}
		function interceptor:__newindex(key, value)
			if value == class then
				local name = tostring(key)
				getmetatable(class).__tostring = function() return name end
			end
			-- restore old metatable and insert value
			setmetatable(env, env_meta)
			if env.global then env.global(key) end -- handle 'strict' module
			env[key] = value
		end
		setmetatable(env, interceptor)
	end

	-- inherit superclasses (see above)
	inherit(class, unpack(super))

	-- syntactic sugar
	local meta = {
		__call = function(self, ...)
			local obj = {}
			self.construct(obj, ...)
			return setmetatable(obj, self)
		end,
		__tostring = function() return name end
	}
	return setmetatable(class, meta)
end

-- the module
return setmetatable({new = new, inherit = inherit},
	{__call = function(_,...) return new(...) end})