0x.RetimeTransforms.lua

local tr = aegisub.gettext

script_name = tr't-retime'
script_description = tr'Set start/end timestamps while preserving transform timing'
script_author = 'The0x539'
script_version = '1'

local function splice(str, a, b, replacement)
	return str:sub(1, a - 1) .. replacement .. str:sub(b + 1)
end

local function parse_transform(contents)
	-- accel just gets lumped in with tags because we can do that
	local a, b, t1, t2, tags = contents:find('^%((%-?%d+),(%-?%d+),(.+)%)$')
	if a ~= nil then
		return tonumber(t1), tonumber(t2), tags
	end

	local a, b, tags = contents:find('^%((.+)%)$')
	if a ~= nil then
		return nil, nil, tags
	end

	error('failed to parse transform args: ' .. contents)
end

local function parse_move(contents)
	local a, b, coords, t1, t2 = contents:find('^%((%-?[%d.]+,%-?[%d.]+,%-?[%d.]+,%-?[%d.]+),(%-?%d+),(%-?%d+)%)$')
	if a ~= nil then
		return coords, tonumber(t1), tonumber(t2)
	end

	local a, b, coords, t1, t2 = contents:find('^%((%-?[%d.]+,%-?[%d.]+,%-?[%d.]+,%-?[%d.]+)%)$')
	if a ~= nil then
		return coords, nil, nil
	end

	error('failed to parse move args: ' .. contents)
end

local function parse_fade(contents)
	local a, b, alphas, t1, t2, t3, t4 = contents:find('^%((%d+,%d+,%d+),(%-?%d+),(%-?%d+),(%-?%d+),(%-?%d+)%)$')
	if a ~= nil then
		return alphas, t1, t2, t3, t4
	end
	
	error('failed to parse fade args: ' .. contents)
end

local function process(line, delta_start, delta_end)
	local block_index = 1
	while true do
		local block_a, block_b = line.text:find('{.-}', block_index)
		if block_a == nil or block_b == nil then
			break
		end
		block_index = block_b + 1
		
		local block = line.text:sub(block_a, block_b)

		local index = 1
		while true do
			local a, b, contents = block:find('\\t(%b())', index)
			if a == nil or b == nil then
				break
			end
			index = b + 1
			
			local t1, t2, tags = parse_transform(contents)
			t1 = (t1 or 0) - delta_start
			t2 = (t2 or line.duration) - delta_start

			local new_tag = ('\\t(%d,%d,%s)'):format(t1, t2, tags)
			line.text = splice(line.text, block_a - 1 + a, block_a - 1 + b, new_tag)
		end

		local a, b, contents = block:find('\\move(%b())')
		if contents ~= nil then
			local coords, t1, t2 = parse_move(contents)
			t1 = (t1 or 0) - delta_start
			t2 = (t2 or line.duration) - delta_start

			local new_tag = ('\\move(%s,%d,%d)'):format(coords, t1, t2)
			line.text = splice(line.text, block_a - 1 + a, block_a - 1 + b, new_tag)
		end

		local a, b, contents = block:find('\\fade(%b())', index)
		if contents ~= nil then
			local alphas, t1, t2, t3, t4 = parse_fade(contents)
			t1 = tonumber(t1) - delta_start
			t2 = tonumber(t2) - delta_start
			t3 = tonumber(t3) - delta_start
			t4 = tonumber(t4) - delta_start
			local new_tag = ('\\fade(%s,%d,%d,%d,%d)'):format(alphas, t1, t2, t3, t4)
			line.text = splice(line.text, block_a - 1 + a, block_a - 1 + b, new_tag)
		end
	end

	line.start_time = line.start_time + delta_start
	line.end_time = line.end_time + delta_end
end

function process_all(subs, sel, changing_start)
	local video_position = aegisub.ms_from_frame(aegisub.project_properties().video_position)
	video_position = video_position - (video_position % 10)

	for _, li in ipairs(sel) do
		local line = subs[li]

		local delta_start, delta_end = 0, 0
		if changing_start then
			delta_start = video_position - line.start_time
		else
			delta_end = video_position - line.end_time
		end
		process(line, delta_start, delta_end)

		subs[li] = line
	end
end

local function set_start(subs, sel, i)
	process_all(subs, sel, true)
	aegisub.set_undo_point('Snap start to video, preserving transform timing')
end

local function set_end(subs, sel, i)
	process_all(subs, sel, false)
	aegisub.set_undo_point('Snap end to video, preserving transform timing')
end

aegisub.register_macro('t-retime/Set Start', script_description, set_start)
aegisub.register_macro('t-retime/Set End', script_description, set_end)