sub-skip.lua

-- feel free to modify and/or redistribute as long as you give credit to the original creator; © 2022 Ben Kerman

local cfg = {
	default_state = false,
	seek_mode_default = false,
	min_skip_interval = 3,
	speed_skip_speed = 2.5,
	lead_in = 0,
	lead_out = 1,
	speed_skip_speed_delta = 0.1,
	min_skip_interval_delta = 0.25
}
require("mp.options").read_options(cfg, nil, function(changes)
	if changes.default_state then toggle_script() end
	if changes.seek_mode_default then switch_mode() end
	if changes.min_skip_interval then set_min_interval(cfg.min_skip_interval) end
	if changes.speed_skip_speed then set_speed_skip_speed(cfg.speed_skip_speed) end
end)

local active = cfg.default_state
local seek_skip = cfg.seek_mode_default
local skipping = false
local sped_up = false
local last_sub_end, next_sub_start

function calc_next_delay()
	-- hide subtitles, otherwise sub could briefly flash on screen when stepping
	local was_visible = mp.get_property_bool("sub-visibility")
	if was_visible then
		mp.set_property_bool("sub-visibility", false)
	end

	local initial_delay = mp.get_property_number("sub-delay")

	-- get time to next line by adjusting subtitle delay
	-- so that the next line starts at the current time
	mp.commandv("sub-step", "1")
	local new_delay = mp.get_property_number("sub-delay")
	mp.set_property_number("sub-delay", initial_delay)

	if was_visible then
		mp.set_property_bool("sub-visibility", true)
	end

	-- if the delay didn't change, the next line hasn't been demuxed yet
	-- (or there are no more lines)
	-- else calculate difference between previous delay and shifted delay
	if new_delay == initial_delay then return nil
	else return -(new_delay - initial_delay) end
end

-- SEEK SKIP --
-- Seek skip generally works by directly skipping to the calculated start
-- of the next line. If it is not available, the script repeatedly seeks
-- to the end of the demuxer cache until a line is found.

function end_seek_skip(next_sub_begin)
	mp.set_property_number("time-pos", next_sub_begin - cfg.lead_out)
	end_skip()
end

-- create timer that determines if the next line has been found and reacts accordingly
local seek_skip_timer
seek_skip_timer = mp.add_periodic_timer(128, function()
	if mp.get_property_bool("seeking") == false then
		seek_skip_timer:stop()
		local time_pos = mp.get_property_number("time-pos")
		local next_delay = calc_next_delay()
		if next_delay == nil then
			-- if no line found, seek to end of demuxer cache (potentially unstable!)
			local cache_duration = mp.get_property_number("demuxer-cache-duration")
			local seek_time = cache_duration or 1
			mp.set_property_number("time-pos", time_pos + seek_time)
			seek_skip_timer:resume()
		else
			-- if line found, finish seek skip
			seek_skip_timer:kill()
			end_seek_skip(time_pos + next_delay)
			mp.set_property_bool("pause", false)
		end
	end
end)
-- make sure that the timer never fires while the script is being loaded
seek_skip_timer:kill()
seek_skip_timer.timeout = 0.05

function start_seek_skip()
	mp.unobserve_property(handle_tick)
	local next_delay = calc_next_delay()
	if next_delay ~= nil then
		-- if next line is visible seek immediately
		end_seek_skip(mp.get_property_number("time-pos") + next_delay)
	else
		mp.set_property_bool("pause", true)
		seek_skip_timer:resume()
	end
end

-- SPEED SKIP (mostly) --
-- Speed skip works by simply speeding up playback between the end of the last line
-- (+lead in) until the start of the next one (-lead out).
-- If the start of the next line is not known, playback is sped up and it's checked
-- on each tick (i.e. change of time-pos) if the next line has been loaded.

local initial_speed = mp.get_property_number("speed")
local initial_video_sync = mp.get_property("video-sync")
function handle_tick(_, time_pos)
	-- time_pos might be nil after the file changes
	if time_pos == nil then return end

	if not sped_up and time_pos > last_sub_end + cfg.lead_in then
		if seek_skip then start_seek_skip()
		else
			initial_speed = mp.get_property_number("speed")
			-- Setting video-sync to desync and then audio prevents audio issues
			-- after resetting speed back to its initial value.
			initial_video_sync = mp.get_property("video-sync")
			mp.set_property("video-sync", "desync")
			mp.set_property_number("speed", cfg.speed_skip_speed)
			sped_up = true
		end
	elseif sped_up and next_sub_start == nil then
		-- next_sub_start == nil means that no next line has been
		-- found during blind skip
		local next_delay = calc_next_delay()
		if next_delay ~= nil then
			next_sub_start = time_pos + next_delay
		end
	elseif sped_up and time_pos > next_sub_start - cfg.lead_out then
		end_skip()
	end
end

-- INITIALIZATION/SHARED FUNCTIONALITY --

function start_skip()
	skipping = true
	mp.observe_property("time-pos", "number", handle_tick)
end

function end_skip()
	mp.unobserve_property(handle_tick)
	skipping = false
	sped_up = false
	mp.set_property_number("speed", initial_speed)
	mp.set_property("video-sync", "audio")
	mp.set_property("video-sync", initial_video_sync)
	last_sub_end, next_sub_start = nil
end

function handle_sub_change(_, sub_end)
	if not sub_end and not skipping then
		local time_pos = mp.get_property_number("time-pos")
		local next_delay = calc_next_delay()

		-- if time-pos is nil the file probably just started playing
		if not time_pos then
			last_sub_end = -cfg.lead_in
		else
			last_sub_end = time_pos
		end
		if next_delay ~= nil then
			if next_delay < cfg.min_skip_interval then return
			else next_sub_start = time_pos + next_delay end
		end
		start_skip()
	elseif skipping and sub_end then end_skip() end
end

function activate()
	mp.observe_property("sub-end", "number", handle_sub_change)
	active = true
end

function deactivate()
	seek_skip_timer:kill()
	end_skip()
	mp.unobserve_property(handle_sub_change)
	active = false
end

if active then activate() end

-- CONFIG --

function toggle_script()
	if active then
		deactivate()
		mp.osd_message("Non-subtitle skip disabled")
	else
		activate()
		mp.osd_message("Non-subtitle skip enabled")
	end
end

mp.add_key_binding("Ctrl+n", "toggle", toggle_script)

function switch_mode()
	seek_skip = not seek_skip
	mp.osd_message("Seek skip " .. (seek_skip and "enabled" or "disabled"))
end

mp.add_key_binding("Ctrl+Alt+n", "switch-mode", switch_mode)

function set_speed_skip_speed(new_value)
	cfg.speed_skip_speed = new_value
	if skipping then mp.set_property_number("speed", new_value) end
	mp.osd_message("Skip speed: " .. new_value)
end

mp.add_key_binding("Ctrl+Alt+[", "decrease-speed", function()
	set_speed_skip_speed(cfg.speed_skip_speed - cfg.speed_skip_speed_delta)
end, {repeatable = true})

mp.add_key_binding("Ctrl+Alt+]", "increase-speed", function()
	set_speed_skip_speed(cfg.speed_skip_speed + cfg.speed_skip_speed_delta)
end, {repeatable = true})

function set_min_interval(new_value)
	cfg.min_skip_interval = new_value
	mp.osd_message("Minimum interval: " .. new_value)
end

mp.add_key_binding("Ctrl+Alt+-", "decrease-interval", function()
	set_min_interval(cfg.min_skip_interval - cfg.min_skip_interval_delta)
end, {repeatable = true})

mp.add_key_binding("Ctrl+Alt++", "increase-interval", function()
	set_min_interval(cfg.min_skip_interval + cfg.min_skip_interval_delta)
end, {repeatable = true})