Simple beat sync mechanism for PICO-8

beat_sync.lua

sync = (function()
  local speed = 16 -- can be hardcoded for this project
  local prev_elapsed = 0 -- ticks elapsed in current note

  function execute(f) f() end

  local callbacks = {
    [32] = {}, [16] = {}, [8] = {}, [4] = {}, [2] = {}, [1] = {}
  }

  return {
    on = function(note_length, f)
      add(callbacks[note_length], f)
    end,

    off = function(note_length, f)
      del(callbacks[note_length], f)
    end,

    update = function()
      tick = stat(26)
      elapsed = tick % speed
      if elapsed < prev_elapsed then
        -- new beat has begun
        beats = flr(tick / speed)
        foreach(callbacks[32], execute)
        if (beats % 2 == 0) foreach(callbacks[16], execute)
        if (beats % 4 == 0) foreach(callbacks[8], execute)
        if (beats % 8 == 0) foreach(callbacks[4], execute)
        if (beats % 16 == 0) foreach(callbacks[2], execute)
        if (beats % 32 == 0) foreach(callbacks[1], execute)
      end
      prev_elapsed = elapsed
    end
  }
end)()