classlib.lua

-- classlib.lua 2.04.04
--
-- Changes from 2.03:
--
-- 1.	Included patches and ideas from Peter Schaefer
-- 		(peter.schaefer@gmail.com) considerably improving the efficiency of
-- 		build(), __init() and other parts of the code.
--
-- 2.	The former remove_ambiguous() function was removed, improving the
-- 		efficiency of both build() (instance creation) and mt:__call() (class
-- 		creation). Ambiguities are now processed using the ambiguous_keys
-- 		tables introduced by Peter.
--
-- 3.	Removed inheritance of class properties, which was inconsistent with
-- 		the way instance properties are handled (see pages 4-5 of the manual).
-- 		Mostly harmless, but confusing. Now only methods are inherited.
--

-- PRIVATE

--[[
	Define unique value for identifying ambiguous base objects and inherited
	attributes. Ambiguous values are normally removed from classes and objects,
	but if keep_ambiguous == true they are left there and the ambiguous value
	is made to behave in a way useful for debugging.
]]

local ambiguous

if keep_ambiguous then

	ambiguous = { _type = 'ambiguous' }

	local function invalid(operation)
		return function()
			error('Invalid ' .. operation .. ' on ambiguous')
		end
	end

	-- Make ambiguous complain about everything except tostring()
	local ambiguous_mt =
	{
		__add		= invalid('addition'),
		__sub		= invalid('substraction'),
		__mul		= invalid('multiplication'),
		__div		= invalid('division'),
		__mod		= invalid('modulus operation'),
		__pow		= invalid('exponentiation'),
		__unm		= invalid('unary minus'),
		__concat	= invalid('concatenation'),
		__len		= invalid('length operation'),
		__eq		= invalid('equality comparison'),
		__lt		= invalid('less than'),
		__le		= invalid('less or equal'),
		__index		= invalid('indexing'),
		__newindex	= invalid('new indexing'),
		__call		= invalid('call'),
		__tostring	= function() return 'ambiguous' end,
		__tonumber	= invalid('conversion to number')
	}
	setmetatable(ambiguous, ambiguous_mt)

end


--[[
	Reserved attribute names.
]]

local reserved =
{
	__index			= true,
	__newindex		= true,
	__type			= true,
	__class			= true,
	__bases			= true,
	__inherited		= true,
	__from			= true,
	__shared		= true,
	__user_init		= true,
	__name			= true,
	__initialized	= true
}

--[[
	Some special user-set attributes are renamed.
]]

local rename =
{
	__init	= '__user_init',
	__set	= '__user_set',
	__get	= '__user_get'
}

--[[
	The metatable of all classes, containing:

	To be used by the classes:
 	__call()		for creating instances
 	__init() 		default constructor
 	is_a()			for checking object and class types
	implements()	for checking interface support

	For internal use:
	__newindex()	for controlling class population
]]

local class_mt = {}
class_mt.__index = class_mt

--[[
	This controls class population.
	Here 'self' is a class being populated by inheritance or by the user.
]]

function class_mt:__newindex(name, value)

	-- Rename special user-set attributes
	if rename[name] then name = rename[name] end

	-- __user_get() needs an __index() handler
	if name == '__user_get' then
		self.__index = value and function(obj, k)
			local v = self[k]
			if v == nil and not reserved[k] then v = value(obj, k) end
			return v
		end or self

	-- __user_set() needs a __newindex() handler
	elseif name == '__user_set' then
		self.__newindex = value and function(obj, k, v)
			if reserved[k] or not value(obj, k, v) then rawset(obj, k, v) end
		end or nil

	end

	-- Assign the attribute
	rawset(self, name, value)
end

--[[
	This function creates an object of a certain class and calls itself
	recursively to create one child object for each base class. Base objects
	of unnamed base classes are accessed by using the base class as an index
	into the object, base objects of named base classes are accessed as fields
	of the object with the names of their respective base classes.
	Classes derived in shared mode will create only a single base object.
	Unambiguous grandchildren are inherited by the parent if they do not
	collide with direct children.
]]

local function build(class, shared_objs, shared)

	-- If shared, look in the repository of shared objects
	-- and return any previous instance of this class.
	if shared then
		local prev_instance = shared_objs[class]
		if prev_instance then return prev_instance end
	end

	-- Create new object
	local obj = { __type = 'object' }

	-- Build child objects if there are base classes
	local nbases = #class.__bases
	if nbases > 0 then

		-- Repository for storing inherited base objects
		local inherited = {}

		-- List of ambiguous keys
		local ambiguous_keys = {}

		-- Build child objects for each base class
		for i = 1, nbases do
			local base = class.__bases[i]
			local child = build(base, shared_objs, class.__shared[base])
			obj[base.__name] = child

			-- Get inherited grandchildren from this child
			for c, grandchild in pairs(child) do

				-- We can only accept one inherited grandchild of each class,
				-- otherwise this is an ambiguous reference
				if not ambiguous_keys[c] then
					if not inherited[c] then inherited[c] = grandchild
					elseif inherited[c] ~= grandchild then
						inherited[c] = ambiguous
						table.insert(ambiguous_keys, c)
					end
				end
			end
		end

		-- Accept inherited grandchildren if they don't collide with
		-- direct children
		for k, v in pairs(inherited) do
			if not obj[k] then obj[k] = v end
		end

	end

	-- Object is ready
	setmetatable(obj, class)

	-- If shared, add it to the repository of shared objects
	if shared then shared_objs[class] = obj end

	return obj

end

--[[
	The __call() operator creates an instance of the class and initializes it.
]]

function class_mt:__call(...)
	local obj = build(self, {}, false)
	obj:__init(...)
	return obj
end

--[[
	The implements() method checks that an object or class supports the
	interface of a target class. This means it can be passed as an argument to
	any function that expects the target class. We consider only functions
	and callable objects to be part of the interface of a class.
]]

function class_mt:implements(class)

	-- Auxiliary function to determine if something is callable
	local function is_callable(v)
		if v == ambiguous then return false end
		if type(v) == 'function' then return true end
		local mt = getmetatable(v)
		return mt and type(mt.__call) == 'function'
	end

	-- Check we have all the target's callables (except reserved names)
	for k, v in pairs(class) do
		if not reserved[k] and is_callable(v) and not is_callable(self[k]) then
			return false
		end
	end
	return true
end

--[[
	The is_a() method checks the type of an object or class starting from
	its class and following the derivation chain upwards looking for
	the target class. If the target class is found, it checks that its
	interface is supported (this may fail in multiple inheritance because
	of ambiguities).
]]

function class_mt:is_a(class)

	-- If our class is the target class this is trivially true
	if self.__class == class then return true end

	-- Auxiliary function to determine if a target class is one of a list of
	-- classes or one of their bases
	local function find(target, classlist)
		for i = 1, #classlist do
			local class = classlist[i]
			if class == target or find(target, class.__bases) then
				return true
			end
		end
		return false
	end

	-- Check that we derive from the target
	if not find(class, self.__bases) then return false end

	-- Check that we implement the target's interface.
	return self:implements(class)
end

--[[
	Factory-supplied constructor, calls the user-supplied constructor if any,
	then calls the constructors of the bases to initialize those that were
	not initialized before. Objects are initialized exactly once.
]]

function class_mt:__init(...)
	if self.__initialized then return end
	if self.__user_init then self:__user_init(...) end
	for i = 1, #self.__bases do
		local base = self.__bases[i]
		self[base.__name]:__init(...)
	end
	self.__initialized = true
end


-- PUBLIC

--[[
	Utility type and interface checking functions
]]

function typeof(value)
	local t = type(value)
	return t =='table' and value.__type or t
end

function classof(value)
	local t = type(value)
	return t == 'table' and value.__class or nil
end

function classname(value)
	if not classof(value) then return nil end
	local name = value.__name
	return type(name) == 'string' and name or nil
end

function implements(value, class)
	return classof(value) and value:implements(class) or false
end

function is_a(value, class)
	return classof(value) and value:is_a(class) or false
end

--[[
	Use a table to control class creation and naming.
]]

class = {}
local mt = {}
setmetatable(class, mt)

--[[
	Create a named or unnamed class by calling class([name, ] ...).
	Arguments are an optional string to set the class name and the classes or
	shared classes to be derived from.
]]

function mt:__call(...)

	local arg = {...}

	-- Create a new class
	local c =
	{
		__type = 'class',
		__bases = {},
		__shared = {}
	}
	c.__class = c
	c.__index = c

	-- A first string argument sets the name of the class.
	if type(arg[1]) == 'string' then
		c.__name = arg[1]
		table.remove(arg, 1)
	else
		c.__name = c
	end

	-- Repository of inherited attributes
	local inherited = {}
	local from = {}

	-- List of ambiguous keys
	local ambiguous_keys = {}

	-- Inherit from the base classes
	for i = 1, #arg do
		local base = arg[i]

		-- Get the base and whether it is inherited in shared mode
		local basetype = typeof(base)
		local shared = basetype == 'share'
		assert(basetype == 'class' or shared,
				'Base ' .. i .. ' is not a class or shared class')
		if shared then base = base.__class end

		-- Just in case, check this base is not repeated
		assert(c.__shared[base] == nil, 'Base ' .. i .. ' is duplicated')

		-- Accept it
		c.__bases[i] = base
		c.__shared[base] = shared

		-- Get methods that could be inherited from this base
		for k, v in pairs(base) do

			-- Skip reserved and ambiguous methods
			if type(v) == 'function' and not reserved[k] and
				not ambiguous_keys[k] then

				-- Where does this method come from?
				local new_from

				-- Check if the method was inherited by the base
				local base_inherited = base.__inherited[k]
				if base_inherited then

					-- If it has been redefined, cancel this inheritance
					if base_inherited ~= v then		-- (1)
						base.__inherited[k] = nil
						base.__from[k] = nil

					-- It is still inherited, get it from the original
					else
						new_from = base.__from[k]
					end
				end

				-- If it is not inherited by the base, it originates there
				new_from = new_from or { class = base, shared = shared }

				-- Accept a first-time inheritance
				local current_from = from[k]
				if not current_from then
					from[k] = new_from
					local origin = new_from.class

					-- We assume this is an instance method (called with
					-- self as first argument) and wrap it so that it will
					-- receive the correct base object as self. For class
					-- functions this code is unusable.
					inherited[k] = function(self, ...)
						return origin[k](self[origin.__name], ...)
					end

				-- Methods inherited more than once are ambiguous unless
				-- they originate in the same shared class.
				elseif current_from.class ~= new_from.class or
						not current_from.shared or not new_from.shared then
					inherited[k] = ambiguous
					table.insert(ambiguous_keys, k)
					from[k] = nil
				end
			end
		end
	end

	-- Set the metatable now, it monitors attribute setting and does some
	-- special processing for some of them.
	setmetatable(c, class_mt)

	-- Set inherited attributes in the class, they may be redefined afterwards
	for k, v in pairs(inherited) do c[k] = v end	-- checked at (1)
	c.__inherited = inherited
	c.__from = from

	return c
end

--[[
	Create a named class and assign it to a global variable of the same name.
	Example: class.A(...) is equivalent to (global) A = class('A', ...).
]]

function mt:__index(name)
	return function(...)
		local c = class(name, ...)
		getfenv()[name] = c
		return c
	end
end

--[[
	Wrap a class for shared derivation.
]]

function shared(class)
	assert(typeof(class) == 'class', 'Argument is not a class')
	return { __type = 'share', __class = class }
end