A simple reusable state machine written in Lua.

StateMachine.lua

StateMachine = {}
StateMachine.__index = StateMachine
function StateMachine:Create()
    local this =
    {
        mEmpty =
        {
            HandleInput = function() end,
            Update = function() end,
            Enter = function() end,
            Exit = function() end
        },
        mCurrent = nil,
        mStates = {}
    }
    this.mCurrent = this.mEmpty
    setmetatable(this, self)
    return this
end

function StateMachine:Change(stateName, enterParams)
    assert(self.mStates[stateName]) -- state must exist!
    self.mCurrent:Exit()
    self.mCurrent = self.mStates[stateName]
    self.mCurrent:Enter(enterParams)
end

function StateMachine:Update(dt)
    self.mCurrent:Update(dt)
end

function StateMachine:Add(id, state)
    self.mStates[id] = state
end

function StateMachine:Remove(id)

    if self.mCurrent == self.mStates[id] then
        self.mCurrent = self.mEmpty
    end

    self.mStates[id] = nil
end

function StateMachine:Clear()
    self.mStates = {}
    self.mCurrent = self.mEmpty
end