light_world.lua/examples/vendor/hump/class.lua

--[[
Copyright (c) 2010-2013 Matthias Richter

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local function include_helper(to, from, seen)
	if from == nil then
		return to
	elseif type(from) ~= 'table' then
		return from
	elseif seen[from] then
		return seen[from]
	end

	seen[from] = to
	for k,v in pairs(from) do
		k = include_helper({}, k, seen) -- keys might also be tables
		if not to[k] then
			to[k] = include_helper({}, v, seen)
		end
	end
	return to
end

-- deeply copies `other' into `class'. keys in `other' that are already
-- defined in `class' are omitted
local function include(class, other)
	return include_helper(class, other, {})
end

-- returns a deep copy of `other'
local function clone(other)
	return setmetatable(include({}, other), getmetatable(other))
end

local function new(class)
	-- mixins
	local inc = class.__includes or {}
	if getmetatable(inc) then inc = {inc} end

	for _, other in ipairs(inc) do
		include(class, other)
	end

	-- class implementation
	class.__index = class
	class.init    = class.init    or class[1] or function() end
	class.include = class.include or include
	class.clone   = class.clone   or clone

	-- constructor call
	return setmetatable(class, {__call = function(c, ...)
		local o = setmetatable({}, c)
		o:init(...)
		return o
	end})
end

-- interface for cross class-system compatibility (see https://github.com/bartbes/Class-Commons).
if class_commons ~= false and not common then
	common = {}
	function common.class(name, prototype, parent)
		return new{__includes = {prototype, parent}}
	end
	function common.instance(class, ...)
		return class(...)
	end
end


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