state_migration.lua

-- state migration prototype inspired by circa's demo video and circa's
-- notes/stateful_code

---- handles state migration --------------------------------------------------

local saved_states = {}
local index = 1

function prepare_for_reload()
  index = 1
end

function state(value)
  -- to be more resilient to code changes this key would have to be determined
  -- based on an analysis of the control flow, using the lua debug library.
  local key = index
  index = index + 1

  if saved_states[key] == nil then
    saved_states[key] = value
  end

  return {
    set = function (new_value) saved_states[key] = new_value end,
    get = function () return saved_states[key] end,
  }
end

---- test game ----------------------------------------------------------------

function make_the_game(timer_init)
  local timer = state(timer_init)
  print('game initialized, timer is ' .. timer.get())
  return {
    update = function ()
      timer.set(timer.get() + 1)
      print('game updated, timer is ' .. timer.get())
    end
  }
end

---- simulate load and re-load ------------------------------------------------

-- initialize the game for the first time
print('initial load')
game = make_the_game(5)
game.update()
game.update()
game.update()

-- simulate 're-loading' the game after a code change
prepare_for_reload()
print('second load')
game = make_the_game(5)
game.update()

-- the output is:
--
-- initial load
-- game initialized, timer is 5
-- game updated, timer is 6
-- game updated, timer is 7
-- game updated, timer is 8
-- second load
-- game initialized, timer is 8
-- game updated, timer is 9
-- 
-- MAGIC! :D