--- Children system
-- Allow to build a hierarchy between entities.
-- Children are stored directly in the .children entity table: they are added when their parent is added, and removed when it is removed from the world.

return {
	name = "children",
	filter = true,
	default = {
		parent = nil, -- reference to parent entity, if any
		-- ... list of children to add when the entity is added to the world
		children = {}, -- [children]=true,children... map+list of children currently in the entity children (don't set this yourself)
	},
	onAdd = :(c, e)
		if c.parent then -- add to parent
			let parentcc = c.parent.children.children
			table.insert(parentcc, e)
			parentcc[e] = true
		end
		for _, o in ipairs(c) do -- add children
			if not o.children then o.children = {} end
			o.children.parent = e
			@world:add(o)
		end
	end,
	onRemove = :(c, e)
		for i=#c.children, 1, -1 do -- remove children
			@world:remove(c.children[i])
		end
		if c.parent then -- remove from parent
			let parentcc = c.parent.children.children
			for i=#parentcc, 1, -1 do
				if parentcc[i] == e then
					table.remove(parentcc, i)
					break
				end
			end
			parentcc[e] = nil
		end
	end
}