intercept.lua

local intercept = {}

function intercept.before(func, callback)
	return function(...)
		callback(...)
		return func(...)
	end
end

function intercept.after(func, callback)
	return function(...)
		local result = func(...)
		callback(..., result)
		return result
	end
end

function intercept.around(func, callback)
	return function(...) return callback(func, ...) end
end

return intercept

main.lua

local userClass = require("userclass")
require("usercoordination") --installs the coordination aspect

local user1 = userClass.create()
local user2 = userClass.create()
local user3 = userClass.create()
local user4 = userClass.create()
local user5 = userClass.create()
local user6 = userClass.create()
local user7 = userClass.create()
local user8 = userClass.create()
local user9 = userClass.create()

print("user1", user1:findSlot())
print("user2", user2:findSlot())
print("user3", user3:findSlot())
print("user4", user4:findSlot())
print("user5", user5:findSlot())
print("user6", user6:findSlot())
print("user7", user7:findSlot())
print("user8", user8:findSlot())
print("user9", user9:findSlot())

userclass.lua

local userClass = {}

function userClass:findSlot()
	while true do
		local slot = math.random(0, 10)
		if self:canUseSlot(slot) then
			self:reserveSlot(slot)
			return slot
		end
	end
end

function userClass:canUseSlot(slot)
	return not self.slots[slot]
end

function userClass:reserveSlot(slot)
	self.slots[slot] = true
end

function userClass.create()
	local instance = { slots = {} }
	setmetatable(instance, { __index = userClass })
	return instance
end

return userClass

usercoordination.lua

local userClass = require("userclass")
local intercept = require("intercept")

--Create a weak table that allows the users to be garbage collected
local users = setmetatable({}, { __mode = "kv" })
local counter = 0
userClass.create = intercept.after(userClass.create, function(_, inst)
	--Save the users in the table
	counter = counter + 1
	users[inst] = counter
	print("registered user #" .. counter)
end)

userClass.canUseSlot = intercept.around(userClass.canUseSlot, function(func, user, slot)
	--Check with all existing users
	local result = true
	print("Organized reservation for slot #" .. slot)
	for existingUser, index in pairs(users) do
		print("checking with user #" .. index)
		if not func(existingUser, slot) then
			print("Collision with user schedule, returning false")
			return false
		end
	end
	return true
end)