luajit ffi reference counting

refcount.lua

local ffi = require('ffi')

ffi.cdef[[
typedef struct Other Other;
typedef struct Thing Thing;

struct Other {
	Thing *thing;
	int index;
};

struct Thing {
	Other *others[2];
	int ref;
};

void *malloc(size_t);
void free (void *);
]]

local C = ffi.C

local Thing = {}
Thing.__index = Thing

local function release(self)
	self.ref = self.ref - 1
	print("release", self)
	if self.ref == 0 then
		print("adios")
		=C.free(self)
	end
end

function Thing:new(a, b)
	local self = ffi.gc(ffi.cast("Thing *", C.malloc(ffi.sizeof("Thing"))), release)
	self.others[0] = a
	self.others[1] = b
	self.ref = 3
	a.index = 0
	b.index = 1
	a.thing = self
	b.thing = self
	return self
end

function Thing:__tostring()
	return string.format("Thing: others=[0x%08x,0x%08x], ref=%d",
		tonumber(ffi.cast("uintptr_t", self.others[0])),
		tonumber(ffi.cast("uintptr_t", self.others[1])),
		self.ref)
end

ffi.metatype("Thing", Thing)


local Other = {}
Other.__index = Other

function Other:new()
	return self.alloc(nil)
end

function Other:__gc()
	self.thing.others[self.index] = nil
	release(self.thing)
end

Other.alloc = ffi.metatype("Other", Other)


local a = Other:new()
local b = Other:new()
local t = Thing:new(a,b)
print(t)

t = nil
collectgarbage("collect")

print(a.thing)
print(b.thing)

a = nil
collectgarbage("collect")

print(b.thing)

b = nil
collectgarbage("collect")