--[[

   For a given 3- or 4-digit year or decade, find the most specific portal

   which actually exists.

   Takes one parameter, which must be either a year (e.g. "1879", "1123")

   or a decade (e.g. "1940s", "790s").

   If a portal is found, return its name without the namespace prefix

   (e.g. for "Portal:1980s" return "1980s"); otherwise return an empty string.

   If the parameter is missing, empty, or does not fit the required format,

   an empty string is returned"

]]



local p = {}



-- This table of existing decade portals is the first check of whether a portal

-- exists for a given decade.

-- If the decade is listed in this table, then a system call is made to verify its existence

-- This approach has two advantages:

-- 1/ It reduces server load by reducing the number of expensive system calls

-- 2/ It avoids creating backlinks to non-existing portals, because a each .exists check

--    generates a backlink

local existingDecadePortals = {

	"1920s" = true,

	"1950s" = true,

	"1960s" = true,

	"1980s" = true,

	"1990s" = true,

	"2010s" = true

}



-- check for the existence of a portal with the given name

-- if it exists, returns the name

-- otherwise returns nil

function doesPortalExist(portalName)

	local portalPage = mw.title.new( portalName, "Portal" )

	if (portalPage.exists) then

		return portalName

	end

	return nil

end





-- check for the existence of a portal with the name of that year

-- if it exists, returns the year

-- otherwise calls decadeCheck, and returns that result

-- otherwise returns nil

function checkYear(yearParam)

--[[

the year portals have all been deleted, so comment out this section

	if doesPortalExist(yearParam) then

		return yearParam

	end

]]--

-- myDecade = the year, with the last digit stripped

	--            e.g. "1694" → "1690"

	--            Note that a decade is written as usul=ally written "YYY0s"

	local myDecade = mw.ustring.gsub(yearParam, "%d$", "")

	return checkDecade(myDecade)

end



-- check for the existence of a portal with the name of that decade

-- if it exists, returns the year

-- otherwise calls decadeCheck, and returns that result

-- otherwise returns nil

function checkDecade(decadeParam)

	local mydecade = decadeParam .. "0s"

	if (existingDecadePortalsmydecade == true) then

		if doesPortalExist(mydecade) then

			return mydecade

		end

	end

	-- We don't have a portal for the decade, so now try the century.

--[[

the century portals have all been deleted, so comment out this section



	local myCenturyString = mw.ustring.gsub(decadeParam, "%d$", "")

	local myCenturyNum = tonumber(myCenturyString)

	local myCenturyNum = tonumber(myCenturyString)

	-- increment by one, because we have now conveted e.g. "1870s" to "18"

	-- but that's the 19th century

	myCenturyNum = myCenturyNum + 1

	-- the century portals have all been deleted, so disable the centeury checking

	-- return checkCentury(tostring(myCenturyNum))

]]--	return ""

end



-- check for the existence of a portal with the name of that century

-- if it exists, returns the century

-- otherwise returns an empty string

function checkCentury(centuryParam)

	local myCenturyString = ordinal_numbers(centuryParam) .. " century"

	if doesPortalExist(myCenturyString) then

		return myCenturyString

	end

	return ""

end





-- converts a string number to an string ordinal

-- e.g. 21 → 21st

--      17 → 17th

-- code copied from https://stackoverflow.com/questions/20694133/how-to-to-add-th-or-rd-to-the-date (license:CC BY-SA 3.0 )

function ordinal_numbers(n)

  local ordinal, digit = {"st", "nd", "rd"}, string.sub(n, -1)

  if tonumber(digit) > 0 and tonumber(digit) <= 3 and string.sub(n,-2) ~= 11 and string.sub(n,-2) ~= 12 and string.sub(n,-2) ~= 13 then

    return n .. ordinaltonumber(digit)]

  else

    return n .. "th"

  end

end



function trim(s)

   return s:match "^%s*(.-)%s*$"

end



function p.findydcportal(frame)

	-- Expects one parameter

	-- {{{1}}} = a 3- or 4-digit year or deacde

	--    e.g. 1916

	--         1504

	--         1630s

	--         920s

	local arg1 = frame.args1

	if arg1 == nil then

		return ""

	end

	arg1 = trim(arg1) -- strip leading and trailing spaces

	if (mw.ustring.match(arg1, "^%d%d%d%d?$")) then

		-- it's a 3- or 4-digit-year

		return checkYear(arg1)

	elseif (mw.ustring.match(arg1, "^%d%d%d?0s$")) then

		-- it's a 3- or 4-digit decade

		-- so strip the trailing "0s"

		local decadeArg = mw.ustring.gsub(arg1, "0s$", "")

		return checkDecade(decadeArg)

	end

	-- If we get here, then arg1 was neither a year nor a decade

	-- This is going to be a helper template, and diagnostics woud be intrusive

	-- So just return an empty string

	return ""

end



return p