Finite State Machine

wiki

This was originally asked for on the mailing list:

 > From: romeo kalebic <romeo.kalebic@k...>
 >
 > I'm learning lua with small fsm machine built in C and all actions are
 > built in lua.
 > FSM is described with: old_state, event, new_state and action.
 >
 >  state1, event1, state2, action1
 >  state1, event2, state3, action2
 >  etc..
 >
 > Now, I like to describe fsm in lua, maybe in lua table or ?.

One easy way to do this in Lua would be to create a table in exactly the form above:

fsm = FSM{ 
  {state1, event1, state2, action1},
  {state1, event2, state3, action2},
  ...
}

where FSM is a function to be described soon.

Then, to run the machine, do something like this in a loop:

local a = fsm[state][event]
a.action()
state = a.new

plus some error checking if needed (eg, if a is nil, then the transition (state, event) is invalid). Also, you might want to do a.action(state, event, a.new)} if the action can use this info.

The function FSM takes the simple syntax above and creates tables for (state, event) pairs with fields (action, new):

function FSM(t)
  local a = {}
  for _,v in ipairs(t) do
    local old, event, new, action = v[1], v[2], v[3], v[4]
    if a[old] == nil then a[old] = {} end
    a[old][event] = {new = new, action = action}
  end
  return a
end

Note that this scheme works for states and events of any type: number, string, functions, tables, anything. Such is the power of associate arrays.

Have fun. --LuizHenriqueDeFigueiredo

running version:

function action1()
 print("action1")
end

function action2()
 print("action2")
end


function FSM(t)
  local a = {}
  for _,v in ipairs(t) do
    local old, event, new, action = v[1], v[2], v[3], v[4]
    if a[old] == nil then a[old] = {} end
    a[old][event] = {new = new, action = action}
  end
  return a
end

fsm = FSM{
  {"state1", "event1", "state2", action1},
  {"state1", "event2", "state3", action2},
}

local a = fsm["state1"]["event1"]
a.action()
state = a.new

Note: above code is Lua 5.0 and 5.1 compatible.