Queue and Stack Classes

Queues and Stacks are data storage that allow incremental additions and subtractions. For easy and quick implementation, these classes can be added to your scripts.

The Queue class uses the colon to access functions. Data is public in this class. In addition to the normal Queue functions, a PushFirst() is included to add to the front of the queue for priority usage. See the Code Example below for specific usage.

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--A Simple Queue Class
Queue = {}

function Queue:New()
  list = { first = 1, last = 0 }
  self.__index = self
  setmetatable(list, self)
  return list
end

function Queue:Push(value)
  self.last = self.last + 1 
  self[self.last] = value
end

function Queue:PushFirst(value)
  self.first = self.first - 1 
  self[self.first] = value
end

function Queue:Pop()
  if self.first > self.last then 
    error("Queue is empty!") 
  end
  local value=self[self.first]
  self[self.first] = nil
  self.first = self.first + 1
  return value
end

function Queue:IndexOf(value)
  for i=self.first,self.last do
    if self[i] == value then
      return i
    end
  end
  return nil
end

function Queue:Clear()
  for i=self.first,self.last do
    self[i] = nil
  end
  self.first = 1
  self.last = 0
end

function Queue:IsEmpty()
  return self.first > self.last
end
Accessor Description

:New()

Returns a new Queue object

:Push( value )

Add a value to the end of the Queue

:Pop()

Return a value removed from the front of the queue

:PushFirst( value )

Add a value at the head of the Queue

:IndexOf( value )

Return the index within the Queue of the value, or nil.

:Clear()

Remove all data from the Queue

:IsEmpty()

Returns false if the Queue contains data, true otherwise

.first

Index of the first data in the Queue (oldest)

.last

Index of the last data in the Queue (newest)

An alternative is a Stack data structure. This code can be used to add a Stack class to Lua code:

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-- A Simple Stack Class
Stack = {}

function Stack:New()
  list = {top = 0}
  self.__index = self
  setmetatable(list, self)
  return list
end

function Stack:Push(value)
  self.top = self.top + 1 
  self[self.top] = value
end

function Stack:Pop()
  if self.top < 1 then 
    error("Stack is empty!") 
  end
  local value = self[self.top]
  self[self.top] = nil
  self.top = self.top - 1
  return value
end

function Stack:IndexOf(value)
  for i=1,self.top do
    if self[i] == value then
      return i
    end
  end
  return nil
end

function Stack:Clear()
  for i=1,self.top do
    self[i] = nil
  end
  self.top = 0
end

function Stack:IsEmpty()
  return self.top < 1
end

Similar to the Queue functionality, the Stack implementation does a Last On, First Off method for pushing and popping data.

Accessor Description

:New()

Returns a new Stack object

:Push( value )

Add a value to the top of the Stack

:Pop()

Return a value removed from the top of the Stack

:IndexOf( value )

Return the index within the Stack of the value, or nil

:Clear()

Remove all data from the Stack

:IsEmpty()

Returns false if the Stack contains data, true otherwise

.top

Index of the top item of the Stack

Using these classes in a control script allows sequential data handling with single function calls.

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--Example Code
myQ = Queue:New()
myStack = Stack:New()

Controls["Add"].EventHandler = function()
  local value = math.random(10)
  myQ:Push(value)
  myStack:Push(value)
end

Controls["Remove"].EventHandler = function()
  if not myQ:IsEmpty() then
    Controls["QueueOutputText"].String = myQ:Pop()
    Controls["StackOutputText"].String = myStack:Pop()
  else
    Controls["QueueOutputText"].String = "Queue is empty"
    Controls["StackOutputText"].String = "Stack is empty"
  end
end

This control script uses two buttons, “Add” and “Remove”, and two text fields, “QueueOutputText” and “StackOutputText”, to store and retrieve data following the queue and stack paradigms.

Binding this class into the TCP example, the queue can be used to serialize TCP communication.

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--A Simple Queue Class
Queue = {}

function Queue:New()
  list = { first = 1, last = 0 }
  self.__index = self
  setmetatable(list, self)
  return list
end

function Queue:Push(value)
  self.last = self.last + 1 
  self[self.last] = value
end

function Queue:PushFirst(value)
  self.first = self.first - 1 
  self[self.first] = value
end

function Queue:Pop()
  if self.first > self.last then 
    error("Queue is empty!") 
  end
  local value = self[self.first]
  self[self.first] = nil
  self.first = self.first + 1
  return value
end

function Queue:IndexOf(value)
  for i = self.first,self.last do
    if self[i] == value then
      return i
    end
  end
  return nil
end

function Queue:Clear()
  for i = self.first,self.last do
    self[i] = nil
  end
  self.first = 1
  self.last = 0
end

function Queue:IsEmpty()
  return self.first > self.last
end


-- Connection Variables
IPAddress = "192.168.1.1"
Port = 10001
UserName = "user"
Password = "password"
EOL = "\r"                      -- End of line character as defined in device's API
LoginPrompt = "login"           -- Match user prompt string as defined in the API
PasswordPrompt = "Password"     -- Match password prompt as defined in the API
LoginMatch = "Login successful" -- Match sucessful login string as defined in the API
ResponseText = ""               -- Hold the last message received from the remote device

-- Timers
PollTimer = Timer.New() -- Timer for polling commands
PollTime = 3            -- Time between polls in seconds 

-- Sockets
TCP = TcpSocket.New()     -- Create new TcpSocket object
TCP.ReadTimeout = 5       -- Set the timeout to 5 seconds
TCP.WriteTimeout = 5      -- Set the timeout to 5 seconds
TCP.ReconnectTimeout = 5  -- Set the wait time bfore reconnecting
CommandQueue = Queue:New()
CommandInFlight = false

-- Variables
LoggedIn = false  -- Flag for when a successful login is made


-- Functions
-- Helper Functions
-- Returns true if TCP Socket is connected
function IsConnected()  
  return TCP.IsConnected
end

-- On connect functionality
function Connected()  
  --Start ending any command hanging in the queue on connect
  if not CommandInFlight and not CommandQueue:IsEmpty() then
    CommandInFlight = true
    TCP:Write( CommandQueue:Pop() ) 
  end
  PollTimer:Start(PollTime)
end

-- On disconnect functionality
function Disconnected()  
  PollTimer:Stop()
  CommandInFlight = false
  LoggedIn = false
end

-- TCP socket callbacks
-- Called when TCP socket is connected
TCP.Connected = function()  
  print("TCP Connected")
  Connected()
end

-- Called when TCP socket is reconnecting
TCP.Reconnect = function()  
  print("Socket reconnecting...")
  Disconnected()
end

-- Called when TCP socket is closed
TCP.Closed = function()
  print("Socket closed")
  Disconnected()
end

-- Called when TCP socket has an error
TCP.Error = function()  
  print("Socket error")
  Disconnected()
end

-- Called when TCP socket times out
TCP.Timeout = function()
  print("Socket timeout")
  Disconnected()
end

-- Called when TCP socket receives data
TCP.Data = function()
  print("Data received")
  ParseResponse()  -- Call ParseResponse when the TCP socket has data
end

-- Use the TCPSocket object to send a string (cmd)
function Send(cmd)
  if TCP.IsConnected then   -- If the TCP socket is connected write the command and EOL to the socket
    local command = cmd .. EOL
    print("Sending: " .. command)
    CommandQueue:Push(command)
    if not CommandInFlight then
      CommandInFlight = true
      TCP:Write( CommandQueue:Pop() ) 
    end
  else
    print("Error - Disconnected; unable to send " .. cmd .. EOL)
  end
end

-- If the Username or Password prompt is matched then the Username or Password is sent
function Authenticate(rx)  
  if rx:match(LoginPrompt) then 
    print("Found \"" .. LoginPrompt .. "\" Sending UserName: " .. UserName.String) 
    Send(UserName.String)
  elseif rx:match(PasswordPrompt) then 
    print("Found \"" .. PasswordPrompt .. "\" Sending Password: " .. Password.String) 
    Send(Password.String)
  end
end

--Parsers
function ParseResponse()
  -- Read the data in the buffer
  local rx = TCP:Read(TCP.BufferLength)
  --Send the next command if the queue is not empty
  if CommandQueue:IsEmpty() then
    CommandInFlight = false
  else
    TCP:Write( CommandQueue:Pop() ) 
  end
  if not LoggedIn then
    if rx:find(LoginMatch) then  -- If sucessful login string matched set LoggedIn high and call Connected()
      print("Logged in")
      LoggedIn = true
      Connected()
    elseif rx:find(LoginPrompt) or rx:find(PasswordPrompt) then  -- If sucessful Password or Username string matched set LoggedIn low and call Authenticate()
      LoggedIn = false
      Authenticate(rx)
    else
      print("Waiting for login prompt...")
    end
  else 
    print("Rx: " .. rx)  -- Print recieved data
    ResponseData = rx  -- Show the received data through the user interface

    -- Add data handling routines here

  end
end

-- Event Handlers
PollTimer.EventHandler = function PollDevice()  -- Call PollDevice when the PollTimer EventHandler is called
  Send("PING")
end

-- Run at start
Connect()

The Queue is used to build a list of commands in the Send() function. A flag is set when a command is in flight, and in case of a response, the next command is sent. Connect() and Disconnect() are also bound to start and stop sends from the Queue to handle those states cleanly.

  • Indexing uses Lua numbers, which will encounter a rollover error at 2^52 consecutive pushes to the object.

  • While the Stack and Queue objects data list can be indexed directly, it is not advisable because it may break stack/queue functionality.