Module:LCCN

require('strict')
local p = {}

local function blank_to_nil(s)
	if s and #s > 0 then return s end
end

local function pad(padlen, ...)
	padlen = tonumber(padlen) or 0
	if 1 > select('#', ...) then
		return ("0"):rep(padlen)
	end
	local padsrc = tostring((...))
	local srclen = mw.ustring.len(padsrc)
	if 1 > padlen or 1 > srclen then
		return ""
	end
	return mw.ustring.sub(mw.ustring.rep(padsrc, math.ceil(padlen / srclen)), 1, padlen)
end

local function padleft(str, ...)
	str = tostring(str)
	if 1 > select('#', ...) then
		return str
	end
	local strlen = mw.ustring.len(str)
	if 1 > strlen then
		return pad((...), select(2, ...))
	end
	return pad((tonumber((...)) or 0) - strlen, select(2, ...)) .. str
end

local function padright(str, ...)
	str = tostring(str)
	if 1 > select('#', ...) then
		return str
	end
	local strlen = mw.ustring.len(str)
	if 1 > strlen then
		return pad((...), select(2, ...))
	end
	return str .. pad((tonumber((...)) or 0) - strlen , select(2, ...))
end

local function expr(...)
	return mw.ext.ParserFunctions.expr( ... )
end

function p.main(frame)
	local args = frame:getParent().args
	local id = blank_to_nil(args[1] or args.id)
	if not id then
		error("Template:LCCN: You must provide an ID.")
	end
	local title = blank_to_nil(args[2] or args.title or args.name)

	local letter_width = (id:match("^%s*[0-9][0-9]") and id:len() < 10) and 0 or 2
	local left_part = id:sub(1, letter_width + 2)
	local right_part = expr(padright(id, letter_width + 8, '.00000'):sub(letter_width + 3, letter_width + 8))
	local url = "https://www.loc.gov/item/" .. left_part .. padleft(right_part, 6, 0)
		
	if args.long == "yes" then
		return ("Catalog record for [%s %s] at the United States [[LCCN (identifier)|Library of Congress]]"):format(
			url, title or mw.title.getCurrentTitle().text)
	end
	
	return ("[[LCCN (identifier)|LCCN]]&nbsp;[%s %s-%s]%s"):format(
		url, left_part, right_part, title and ('&nbsp;&ndash;&nbsp;' .. title) or '')
end

return p
Module:LCCN

Usage
