PICO-8 Timeline module with Beat sync & drift correction

timeline.lua

function init_timeline(state)
	tl_scenes={}
	tl_state={
		time=0,duration=0,
		seek_time=0,frame=0,
		loop=true,
		music_bpm=120.5,
		music_sig=4,
		music_idx=nil,
		fixsync=true,
		sync_time=0
	}
	for k,v in pairs(state) do
		tl_state[k]=v
	end
	return tl_state
end
function set_music(idx,spd)
	tl_state.music_idx=idx
	music_spd(spd)
end
function music_spd(spd,bars)
	tl_state.music_bpm=(7229.50819/(bars or 4))/(spd or 15)
end
function music_beat(time)
	return (tl_state.music_bpm/60)*time
end
function music_bar(time)
	return (tl_state.music_bpm/60)*time/tl_state.music_sig
end
function bars_to_seconds(bars)
  return (bars * tl_state.music_sig * 60) / tl_state.music_bpm
end
function add_scene(sstart,sstop,init,update,draw,unload,start,stop)
	add(tl_scenes,{
		start=start or sstart and bars_to_seconds(sstart) or 0,
		stop=stop or sstop and bars_to_seconds(sstop) or 0,
		show=false,time=-1,
		init=init,update=update,draw=draw,unload=unload
	})
	for scn in all(tl_scenes) do
		tl_state.duration=max(tl_state.duration,scn.stop)
	end
end

function get_time()
	return tl_state.seek_time+tl_state.sync_time+t()
end

function local_time(scn)
	return (tl_state.time-scn.start)/(scn.stop-scn.start)
end

function seek_time(bars)
	local seek_seconds=bars_to_seconds(bars)
	tl_state.seek_time+=seek_seconds
	local music_pos=music_bar(get_time())%music_bar(tl_state.duration)
	if music_pos>63 then
		music_pos=63
	end
	music(music_pos)
end

function update_timeline()
	local s=tl_state
	s.time=get_time()
	s.frame+=1
	-- bpm detection
	s.music_bar_last=s.music_bar or 0
	-- sync correction
	s.music_bar=music_bar(s.time)
	if s.fixsync then
		s.music_bar=stat(54)+stat(56)/256
		s.sync_time+=bars_to_seconds(
			s.music_bar-music_bar(s.time)
		)
		s.time=get_time()
	end
	s.music_bar_active=flr(s.music_bar)!=flr(s.music_bar_last)
	s.music_beat_last=s.music_beat or 0
	s.music_beat=music_beat(s.time)
	s.music_beat_active=flr(s.music_beat)!=flr(s.music_beat_last)

	for scn in all(tl_scenes) do
		if not scn.show and (s.time>=scn.start)
			and s.time<scn.stop then
			scn.show=true
			if scn.init then scn.init(s) end
		elseif scn.show and (s.time>=scn.stop or s.time<scn.start) then
			scn.show=false
			if scn.unload then scn.unload(s) end
		end
		if scn.show and scn.update then
			s.local_time=local_time(scn)
			scn.update(s)
		end
	end
	if s.time>s.duration then
		if s.loop then
			extcmd('reset')
		else
			extcmd('shutdown')
		end
	end
	if s.debug then
		if(btnp(⬅️)) seek_time(-1)
		if(btnp(➡️)) seek_time(1)
	end
end

function draw_timeline(s)
	local s=tl_state
	for scn in all(tl_scenes) do
		if scn.show and scn.draw then
			scn.draw(local_time(scn),s)
		end
	end
	if (btnp(❎) and btnp(🅾️)) then
		s.debug=not s.debug
	end
	if s.debug then
		line(0,127,127,127,1)
		line(0,127,flr(s.time/s.duration*127),127,5)
		for si,scn in ipairs(tl_scenes) do
			line(
				flr(scn.start/s.duration*127),126-si,
				flr(scn.stop/s.duration*127),126-si,si)
		end
		print('bar '..flr(s.music_bar)..':'..(
			{'-','\\','|','/'})[flr(s.music_beat%4)+1]..'='..s.music_beat,
			0,0,2)
		print('trk '..(s.seek_time),0,6)
		print('snc '..(s.sync_time),0,12)
	end
end