"Message buffer too small for table" and recursive tables

Questions
1

I have a bunch of sprites that form ad hoc groups during gameplay, and I need each sprite to have a reference to the group. I implemented the group as a table in the level, but when I went to send this table to the sprites, I get the “Message buffer too small for table” error.

I should think a recursive table is too big, if it’s trying to send the whole table!

I suppose it could just be that it’s trying to send everything up to the recursive points, but regardless, I would much prefer if I could send the table by reference, instead of trying to send the whole thing. Is there a way I could send just a reference to the group instead of the entire group? This group is going to be changing during play, and I only want to update the group once, not the group copy in every sprite.

Well, I suppose I might be able to rethink some of the structure of what needs to know what if I can’t; but this does raise another concern as well: objects in tables are references, right? When I have an object linking to a sprite that also has the group object in a table, which in turn holds a table of sprite objects, which have the group object… We are only talking about one group object and one sprite object here, right? I read somewhere that lua variables are always references, but this table size issue (being more than a single reference to 1 variable) has me a bit concerned.

So I guess that’s two questions: passing a reference to the table, and making sure I’m not making a giant (non)recursive blob even without worrying about messaging.

2

Yes Lua tables are passed by reference. But when you post a message the table data get serialised into a binary format and at that point each reference to the table will be serialised and take up precious bytes in the message buffer.

3

It is often the case that the developer uses a shared Lua module to store game state in.
Perhaps this would help in your situation too.

4

Yes, like Mathias wrote, store the state in a Lua module, keyed by some kind of group id, and only send the group id in the messages.

Here’s an example with a “grouper” module that tracks groups and objects belonging to groups.

-- grouper.lua
local M = {}

local groups = {}

local count = 0

-- create a group
-- @return id of the created group
function M.create()
	local group_id = hash("group"..count)
	local group = {
		id = group_id,
		objects = {},
	}
	groups[group_id] = group
	return group_id
end

-- add an object to a group
function M.add(group_id, object)
	local group = groups[group_id]
	assert(group)
	group.objects[#group.objects + 1] = object
end

function M.remove(group_id, object)
	local group = groups[group_id]
	assert(group)
	local objects = group.objects
	for i=1,#objects do
		if objects[i] == object then
			table.remove(objects, i)
			return true
		end
	end
	return false
end

function M.get(group_id)
	local group = groups[group_id]
	assert(group)
	return group.objects
end

return M

Here’s an example of how it can be used:

-- foo.script

local grouper = require("grouper")

local function leave_group(self)
	if self.group_id then
		grouper.remove(self.group_id, go.get_id())
	end
end

local function join_group(self, group_id)
	self.group_id = message.group_id
	grouper.add(message.group_id, go.get_id())
end

function init(self)
	-- create a group and add this game object to the group
	self.group_id = grouper.create()
	grouper.add(go.get_id())

	-- some kind of movement direction
	self.direction = vmath.vector3()

	-- raycast 100 pixels in front of the object
	-- if we hit something we bring it to our group
	local raycast_groups = { hash("objects") }
	timer.delay(0.5, true, function()
		local from = go.get_position()
		local to = from + self.direction * 100
		local hits = physics.raycast(from, to, raycast_groups)
		if hits then
			for i=1,#hits do
				-- send a message to the object we hit that it should join our group
				msg.post(hits[i].id, "join_group", { group_id = self.group_id })
			end
		end
	end)
end

function final(self)
	-- remove the object from the group it belongs to
	leave_group(self)
end

function update(self, dt)
	-- apply some kind of steering behaviour and movement logic
end

function on_message(self, message_id, message, sender)
	if message_id == hash("join_group") then
		leave_group(self)
		join_group(self, message.group_id)
	end
end
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