function pause_toggle()
  original_volume_gain = original_volume_gain or mp.get_property_native("volume-gain")
  local paused = mp.get_property_native("pause")

  if timer then
    mp.commandv("print-text", "kill fade-in or fade-out is in progress")
    niler:kill()
    if timer then -- in case it was just nil-ed
      timer:kill()
      timer = nil
    end
    niler = nil
  end

  local step = 10
  local timeout = 0.05
  local total_period = step * timeout
  local multiplier = 2.0 -- value between 0.25 and 2.0
  local gain = original_volume_gain

  if not paused then
    local to_step = 1
    timer = mp.add_periodic_timer(timeout, function()
      mp.set_property_native("volume-gain", gain - to_step * multiplier)
      if to_step == step then
        mp.set_property_native("pause", true)
        -- restore original gain while paused
        mp.set_property_native("volume-gain", gain)
        paused = true
        timer:kill()
        return
      end
      to_step = to_step + 1
    end)
  else
    timer = mp.add_periodic_timer(timeout, function()
      mp.set_property_native("volume-gain", gain - step * multiplier)
      if paused then
        mp.set_property_native("pause", false)
        paused = false
      end
      if step <= 0 then
        timer:kill()
        return
      end
      step = step - 1
    end)
  end
  niler = mp.add_timeout(total_period + 2 * timeout, function()
    timer = nil
    original_volume_gain = nil
  end)
end


mp.add_forced_key_binding("SPACE", "key_space", pause_toggle)