local Frames = {}
local CurrentFrame = 0

local Versions = {}
local CurrentVersion = 0


function Canvas:addFrame()
   local key = tostring(math.random()):sub(3)
   local img = image(Canvas.width, Canvas.height) -- Create new empty image
   CurrentFrame = CurrentFrame + 1

   saveImage("Documents:Pixelboy_"..key, img)

   table.insert(Frames, CurrentFrame, key)

   Version:saveState()
end

function Canvas:deleteFrame(frame)
   frame = frame or CurrentFrame

   if frame <= #Frames then
       table.remove(Frames, CurrentFrame, frame)
       CurrentFrame = math.max(CurrentFrame - 1, 1)

       if #Frames == 0 then
           CurrentFrame = 0
           Canvas:addFrame()
       else
           Version:saveState()
       end
   end
end


-- Delete all data from previous project session
function Version:clearAll()
	local list = assetList("Documents")
	for id, imgKey in ipairs(list) do
       if imgKey:find("Pixelboy_") then
           saveImage("Documents:"..imgKey, nil)
       end
	end
end

function Version:saveState()
   -- Lua memory storage is restricted to ~4MB = 4096Kb on iPad. If Versions stack rises to high, lower it (this also cleans "Documents")
   while #Versions > 1 and collectgarbage("count") > 4096 do
       local whitelist = table.concat(Versions[2].Frames, " ")
       for imgKey, _ in pairs(Versions[1].Frames) do
           if not whitelist:find(imgKey) then
               saveImage("Documents:Pixelboy_"..imgKey, nil)
           end
       end
       table.remove(Versions, 1)
       collectgarbage()
   end

   -- Undid some Versions until CurrentVersion -> Delete all redundant Versions from stack until #Versions
   if CurrentVersion then
       local whitelist = table.concat(Versions[CurrentVersion].Frames, " ")
       for id = CurrentVersion + 1, #Versions do
           for imgKey, pos in pairs(Versions[id].Frames) do
               if pos > 1 and not whitelist:find(imgKey) then
                   saveImage("Documents:Pixelboy_"..imgKey, nil)
               end
           end
           Versions[id] = nil
       end
       CurrentVersion = nil
   end

   -- Add new Version to the stack
   local frames = {}
   for pos, imgKey in ipairs(Frames) do
       frames[imgKey] = pos
   end
   table.insert(Versions, {CurrentFrame = CurrentFrame, Frames = frames})
end

function Version:restoreState()
   if CurrentVersion then
       local action = Versions[CurrentVersion]
       CurrentFrame = action.CurrentFrame
       Frames = {}

       for imgKey, pos in pairs(action.Frames) do
           Frames[pos] = imgKey
       end

       Canvas.image = readImage("Documents:Pixelboy_"..Frames[CurrentFrame])
   end
end

function Version:undo()
   if #Versions > 0 then
       CurrentVersion = math.max((CurrentVersion or #Versions) - 1, 1)
       Version:restoreState()
   end
end

function Version:redo()
   if CurrentVersion then
       CurrentVersion = math.min(CurrentVersion + 1, #Versions)
       Version:restoreState()

       if CurrentVersion == #Versions then CurrentVersion = nil end
   end
end