local function getUDBT(keyPrefix)
  return setmetatable({}, {
     __index = function(luatable, key)
       return userdata.get(keyPrefix .. "." .. key)
     end,
     __newindex = function(luatable, key, value)
       if (value == nil) then
         userdata.remove(keyPrefix .. "." .. key)
       else
         userdata.set(keyPrefix .. "." .. key, value)
       end       
     end,
  })
end

local function getUDBTStack(keyPrefix)
  local udbtStack = getUDBT(keyPrefix)
  local len = udbtStack.len
  if (type(len) ~= "number" or len < 0) then
    udbtStack.len = 0
  end
  return udbtStack
end

local function UDBTStack_length(udbtStack)
  return udbtStack.len
end

local function UDBTStack_top(udbtStack)
  local len = udbtStack.len
  if (len > 0) then
    return udbtStack[len]
  else
    return nil
  end
end

local function UDBTStack_push(udbtStack, element)
  local newLen = udbtStack.len + 1
  udbtStack.len = newLen
  udbtStack[newLen] = element
end

local function UDBTStack_pop(udbtStack)
  local len = udbtStack.len
  if (len > 0) then
    local value = udbtStack[len]
    udbtStack[len] = nil
    udbtStack.len = len - 1
    return value
  else
    return nil
  end
end

local function UDBTStack_clear(udbtStack)
  local len = udbtStack.len
  udbtStack.len = 0
  for i = 1, len do
    udbtStack[i] = nil
  end
end

local onFrameEndListenerName = "Genesis Lemmings TAS lag frames tracker"
local function OnAfterFrameUpdate()
  -- Base64 210DE077-C471-44D5-A5FF-ECAC5FE52E2F
  local trackingData = getUDBT("d+ANIXHE1USl/+ysX+UuLw")
  -- Base64 c4f782da-27aa-4f27-bcd2-d0e1212d74a7
  local lagFrameTimes = getUDBTStack("2oL3xKonJ0+80tDhIS10pw")
  -- Base64 8bf16b4e-6062-4279-a78f-ffe6d8296f96
  local lagFrameTimesBeyondCurrentFrame = getUDBTStack("Tmvxi2JgeUKnj//m2Clvlg")

  local currentCounterValue = memory.readbyte(0xF54D)
  local currentF9E2Value = memory.readbyte(0xF9E2)
  local currentFrameCounter = emu.framecount()

  if (currentCounterValue ~= 0) then
    if (trackingData.stoptracking == nil) then
      trackingData.stoptracking = true
    elseif ((not trackingData.stoptracking) and 
            (currentFrameCounter == trackingData.previousFrameCounter + 1)) then

      local maybeNextLagFrame = UDBTStack_top(lagFrameTimesBeyondCurrentFrame)
      if ((trackingData.previousF54DCounter == currentCounterValue) and 
          (currentF9E2Value ~= 0)) then
        trackingData.lagFramesCount = trackingData.lagFramesCount + 1
        -- see comment below on why we ignore the first one
        if (trackingData.lagFramesCount > 1) then
          if (currentFrameCounter == maybeNextLagFrame) then
            UDBTStack_pop(lagFrameTimesBeyondCurrentFrame)
          end
          console.writeline("lag at frame #" .. currentFrameCounter)
          UDBTStack_push(lagFrameTimes, currentFrameCounter)
        end
      else
        if (currentFrameCounter == maybeNextLagFrame) then
          UDBTStack_clear(lagFrameTimesBeyondCurrentFrame)
        end
      end

      if ((trackingData.previousF9E2Value ~= 0) and (currentF9E2Value == 0)) then
        console.writeline()
        console.writeline("***** end of level observed on frame " .. 
                          currentFrameCounter .. " *****");
        console.writeline(UDBTStack_length(lagFrameTimes) .. 
                          " lag frames (frame counter values listed below):")
        local lagFramesListingString = ""
        for i = 1, UDBTStack_length(lagFrameTimes) do
          lagFramesListingString = lagFramesListingString .. "    " .. lagFrameTimes[i] .. "\r\n"
        end
        console.write(lagFramesListingString)
        console.writeline("*****")
        console.writeline()
      end
    elseif ((not trackingData.stoptracking) and 
            (currentFrameCounter == trackingData.previousFrameCounter - 1)) then
      if ((trackingData.previousF54DCounter == currentCounterValue) and 
          (trackingData.previousF9E2Value ~= 0)) then
        trackingData.lagFramesCount = trackingData.lagFramesCount - 1
        if (trackingData.lagFramesCount > 0) then
          UDBTStack_push(lagFrameTimesBeyondCurrentFrame, UDBTStack_pop(lagFrameTimes))
        end
      end    
    else
      trackingData.stoptracking = true
    end

    -- effectiveLagFramesCount: when the level starts we observe the following
    -- per-frame changes in 0xF54D: 0->1, 1->1, then the regular cadence of 1,2,3,1,2,3
    -- Therefore, the straightforward calculation of lagFramesCount always reports 
    -- one extra frame from that phenomenon. This appears unavoidable, so we compensate 
    -- for that initial 1->1 in the output below by not counting it.

    local effectiveLagFramesCount = trackingData.lagFramesCount
    if (trackingData.lagFramesCount > 0) then 
      effectiveLagFramesCount = trackingData.lagFramesCount - 1
    end

    local offsetFromBottomOfWindow = 5
    if (trackingData.stoptracking) then
      gui.text(0, offsetFromBottomOfWindow, 
               "lag frames: [aborted tracking, most recent value] " .. effectiveLagFramesCount, 
               nil, nil, "bottomleft")
    else
      gui.text(0, offsetFromBottomOfWindow, 
               "lag frames: " .. effectiveLagFramesCount, 
               nil, nil, "bottomleft")
    end

    trackingData.previousF54DCounter = currentCounterValue
    trackingData.previousF9E2Value = currentF9E2Value
  else
    trackingData.stoptracking = false
    trackingData.lagFramesCount = 0
    trackingData.previousF54DCounter = 0
    trackingData.previousF9E2Value = 0
    UDBTStack_clear(lagFrameTimes)
    UDBTStack_clear(lagFrameTimesBeyondCurrentFrame)
  end

  trackingData.previousFrameCounter = currentFrameCounter
end

console.clear()
event.unregisterbyname(onFrameEndListenerName)
event.onframestart(OnAfterFrameUpdate, onFrameEndListenerName)