Faster? id allocation without recursion

id_alloc_fast.lua

local MaxConcurrent = 512 -- Max concurrent active streams
local TopID = 0 -- Current top-most id.

local IsSequential = true -- Boost for initial creation when no IDs have been dropped.

local OccupiedCount = 0
local Occupied = {}

local function register(id)
    Occupied[id] = true
    OccupiedCount = OccupiedCount + 1
    TopID = id
    return id
end

local function allocID()
	if OccupiedCount >= MaxConcurrent then return end
	
	if IsSequential then
	    return register(TopID+1)
    end

	local left_until_loop = MaxConcurrent-TopID
	
	for i = 1, left_until_loop do
	    local i2 = TopID + i
        if not Occupied[i2] then
            return register(i2)
        end
	end
	
	for i = 1, TopID do
        if not Occupied[i] then
            return register(i)
        end
	end
end

local function dropID(id)
	if Occupied[id] then
		Occupied[id] = nil
		OccupiedCount = OccupiedCount - 1
		IsSequential = false
		return true
	end
	return false
end