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

local previousFrameCounter = -1
local previousF54DCounter = 0
local previousF9E2Value = 0
local lagFramesCount = 0
local stoptracking = false
local lagFrameTimes = {}

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

  if (currentCounterValue ~= 0) then
    if ((not stoptracking) and (currentFrameCounter == previousFrameCounter + 1)) then
      if ((previousF54DCounter == currentCounterValue) and (currentF9E2Value ~= 0)) then
        lagFramesCount = lagFramesCount + 1
        -- see comment below on why we ignore the first one
        if (lagFramesCount > 1) then
          console.writeline("lag at frame #" .. currentFrameCounter)
          table.insert(lagFrameTimes, currentFrameCounter)
        end
      end

      if ((previousF9E2Value ~= 0) and (currentF9E2Value == 0)) then
        console.writeline()
        console.writeline("***** end of level observed on frame " .. currentFrameCounter .. " *****");
        console.writeline(#lagFrameTimes .. " lag frames (frame counter values listed below):")
        for idx,frameCounterValue in ipairs(lagFrameTimes) do
          console.writeline("    " .. frameCounterValue);
        end
        console.writeline("*****");
        console.writeline();
      end
    elseif ((not stoptracking) and (currentFrameCounter == previousFrameCounter - 1)) then
      if ((previousF54DCounter == currentCounterValue) and (previousF9E2Value ~= 0)) then
        lagFramesCount = lagFramesCount - 1
        if (lagFramesCount > 0) then
          table.remove(lagFrameTimes)
        end
      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: [aborted tracking, most recent value] " .. effectiveLagFramesCount, nil, nil, "bottomleft")
    else
      gui.text(0, offsetFromBottomOfWindow, "lag frames: " .. effectiveLagFramesCount, nil, nil, "bottomleft")
    end

    previousF54DCounter = currentCounterValue
    previousF9E2Value = currentF9E2Value
  else
    stoptracking = false
    lagFramesCount = 0
    previousF54DCounter = 0
    previousF9E2Value = 0
    lagFrameTimes = {}
  end

  previousFrameCounter = currentFrameCounter
end

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