local onFrameEndListenerName = "Genesis Lemmings TAS lag frames tracker";

local previousFrameCounter = -1
local previousF54DCounter = 0
local lagFramesCount = 0
local stoptracking = false

local function OnAfterFrameUpdate()
  local counterAddr = 0xF54D
  local currentCounterValue = memory.readbyte(counterAddr)
  local currentFrameCounter = emu.framecount()

  if (currentCounterValue ~= 0) then
    if ((not stoptracking) and (currentFrameCounter == previousFrameCounter + 1)) then
      if (previousF54DCounter == currentCounterValue) then
        lagFramesCount = lagFramesCount + 1 
      end    
    elseif ((not stoptracking) and (currentFrameCounter == previousFrameCounter - 1)) then
      if (previousF54DCounter == currentCounterValue) then
        lagFramesCount = lagFramesCount - 1 
      end    
    else
      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 = lagFramesCount
    if (lagFramesCount > 0) then 
      effectiveLagFramesCount = lagFramesCount - 1
    end

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

    previousF54DCounter = currentCounterValue
  else
    stoptracking = false
    lagFramesCount = 0
    previousF54DCounter = 0
  end

  previousFrameCounter = currentFrameCounter
end

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