-- This module provides functions for getting the target of a redirect page.
-- da https://en.wikipedia.org/w/index.php?title=Module:Redirect&oldid=773537701
local p = {}

-- Gets a mw.title object, using pcall to avoid generating script errors if we
-- are over the expensive function count limit (among other possible causes).
local function getTitle(...)
	local success, titleObj = pcall(mw.title.new, ...)
	if success then
		return titleObj
	else
		return nil
	end
end

-- Gets the name of a page that a redirect leads to, or nil if it isn't a
-- redirect.
function p.getTargetFromText(text)
	return string.match(
		text,
		"^%s*#[Rr][Ee][Dd][Ii][Rr][Ee][Cc][Tt]%s*:?%s*%[%[([^%[%]|]-)%]%]"
	) or string.match(
		text,
		"^%s*#[Rr][Ee][Dd][Ii][Rr][Ee][Cc][Tt]%s*:?%s*%[%[([^%[%]|]-)|[^%[%]]-%]%]"
	)
end

-- Gets the target of a redirect. If the page specified is not a redirect,
-- returns nil.
function p.getTarget(page, fulltext)
	-- Get the title object. Both page names and title objects are allowed
	-- as input.
	local titleObj
	if type(page) == 'string' or type(page) == 'number' then
		titleObj = getTitle(page)
	elseif type(page) == 'table' and type(page.getContent) == 'function' then
		titleObj = page
	else
		error(string.format(
			"bad argument #1 to 'getTarget'"
				.. " (string, number, or title object expected, got %s)",
			type(page)
		), 2)
	end
	if not titleObj or not titleObj.isRedirect then
		return nil
	end
	
	-- Find the target by using string matching on the page content.
	local target = p.getTargetFromText(titleObj:getContent() or "")
	if target then
		local targetTitle = getTitle(target)
		if targetTitle then
			if fulltext then
				return targetTitle.fullText
			else
				return targetTitle.prefixedText
			end
		else
			return nil
		end
	else
		-- The page is a redirect, but matching failed. This indicates a bug in
		-- the redirect matching pattern, so throw an error.
		error(string.format(
			'could not parse redirect on page "%s"',
			fulltext and titleObj.fullText or titleObj.prefixedText
		))
	end
end

--[[
-- Given a single page name determines what page it redirects to and returns the
-- target page name, or the passed page name when not a redirect. The passed
-- page name can be given as plain text or as a page link.
-- 
-- Returns page name as plain text, or when the bracket parameter is given, as a
-- page link. Returns an error message when page does not exist or the redirect
-- target cannot be determined for some reason.
--]]
function p.luaMain(rname, bracket, fulltext)
	if type(rname) ~= "string" or not rname:find("%S") then
		return nil
	end
	bracket = bracket and "[[%s]]" or "%s"
	rname = rname:match("%[%[(.+)%]%]") or rname
	local target = p.getTarget(rname, fulltext)
	local ret = target or rname
	ret = getTitle(ret)
	if ret then
		if fulltext then
			ret = ret.fullText
		else
			ret = ret.prefixedText
		end
		return bracket:format(ret)
	else
		return nil
	end
end

-- Provides access to the luaMain function from wikitext.
function p.main(frame)
	local args = require('Module:Arguments').getArgs(frame, {frameOnly = true})
	return p.luaMain(args[1], args.bracket, args.fulltext) or ''
end

-- Returns true if the specified page is a redirect, and false otherwise.
function p.luaIsRedirect(page)
	local titleObj = getTitle(page)
	if not titleObj then
		return false
	end
	if titleObj.isRedirect then
		return true
	else
		return false
	end
end

-- Provides access to the luaIsRedirect function from wikitext, returning 'yes'
-- if the specified page is a redirect, and the blank string otherwise.
function p.isRedirect(frame)
	local args = require('Module:Arguments').getArgs(frame, {frameOnly = true})
	if p.luaIsRedirect(args[1]) then
		return 'yes'
	else
		return ''
	end
end

return p