local p = {}

function p.main(frame)

local mRedirect = require("Module:Redirect")
local function noredir(page)
  local link = mw.title.new(page):fullUrl("redirect=no")
  return "<span class=\"plainlinks\">["..link.." "..page.."]</span>"
end
local function process(page)
  return mw.title.new(page).prefixedText
end
local function exists(page)
  return mw.title.new(page).exists
end

local args = require("Module:Arguments").getArgs(frame,{removeBlanks=false})
local thisPage = tostring(mw.title.getCurrentTitle())
local otherPage = args[1] or ""

--Demo parameters, for demonstrating behavior with certain redirect 
--targets and avoiding categorization (do not use in articles)
local thisDemoTarget = args.thistarget
local otherDemoTarget = args.othertarget
local noError = args.noerror
local demo = args.demo or noError or thisDemoTarget or otherDemoTarget

--"Process" pages to remove section links, standardize capitalization, etc.
otherPage = otherPage=="" and "" or process(otherPage)
thisDemoTarget = thisDemoTarget=="" and "" or thisDemoTarget and process(thisDemoTarget)
otherDemoTarget = otherDemoTarget=="" and "" or otherDemoTarget and process(otherDemoTarget)

--Determine redirect targets (getTarget returns nil if page is not a redirect)
local thisTarget = thisDemoTarget or mRedirect.getTarget(thisPage)
local otherTarget = otherDemoTarget or mRedirect.getTarget(otherPage)
--For double redirects
local thisDoubleTarget = thisTarget and mRedirect.getTarget(thisTarget)
local otherDoubleTarget = otherTarget and mRedirect.getTarget(otherTarget)
-- Allow setting demo parameters to empty string to demonstrate a non-redirect
thisTarget = thisDemoTarget~="" and thisTarget
otherTarget = otherDemoTarget~="" and otherTarget

--Errors
local err
if not thisTarget and exists(thisPage) then --Check existence of thisPage to avoid errors in preview mode
  err = "This page is not a redirect."
elseif otherPage=="" then
  err = "No other page was specified."
elseif not exists(otherPage) then
  err = "[["..otherPage.."]] does not exist."
elseif thisTarget==thisPage or (not exists(thisTarget) and exists(thisPage)) then
  err = "This is a broken redirect (it redirects to itself or to a non-existing page)."
elseif otherTarget and (otherTarget==otherPage or not exists(otherTarget)) then
  err = noredir(otherPage).." is a broken redirect (it redirects to itself or to a non-existing page)."
elseif otherPage==thisPage then
  err = "The current page was passed as parameter."
elseif not otherTarget and thisTarget==otherPage then
  err = "This page already redirects to [["..otherPage.."]]. Please remove this template."
elseif otherPage==thisTarget and otherTarget==thisPage then
  err = "This is a circular redirect. Please change the target of both this redirect and "..noredir(otherPage).." to the correct article."
elseif thisDoubleTarget and otherDoubleTarget and thisDoubleTarget==otherDoubleTarget then
  err = "Both this page and "..noredir(otherPage).." are double redirects. Please change the redirect target of both to "
        ..(mRedirect.luaIsRedirect(thisDoubleTarget) and "the correct article." or "[["..thisDoubleTarget.."]].")
elseif otherTarget and (thisTarget==otherPage or thisDoubleTarget==otherTarget) then
  err = "This is a [[Wikipedia:Double redirects|double redirect]]."
        .." Please change the redirect target to [["..otherTarget.."]]."
elseif otherTarget and thisDoubleTarget then
  err = "This is a [[Wikipedia:Double redirects|double redirect]] to [["..thisDoubleTarget.."]], while "
        ..noredir(otherPage).." redirects to [["..otherTarget.."]]."
elseif thisDoubleTarget then
  err = "This is a [[Wikipedia:Double redirects|double redirect]] to [["..thisDoubleTarget.."]]."
elseif otherTarget==thisPage then
  err = noredir(otherPage).." is a [[Wikipedia:Double redirects|double redirect]]."
        .." Please change its target to [["..thisTarget.."]]."
elseif otherDoubleTarget then
  err = noredir(otherPage).." is a [[Wikipedia:Double redirects|double redirect]]"
        .." to [["..otherDoubleTarget.."]] via [["..otherTarget.."]]."
elseif otherTarget and thisTarget~=otherTarget then
  err = "This page and "..noredir(otherPage).." redirect to different articles."
end

--Return either redirect template or error
local embed = args.embed
local from, info, cat
if not err or noError then
  if otherTarget then
    from = "an alternative title for "..noredir(otherPage)..", another redirect to the same title"
    info = "Because [[Wikipedia:Double redirects|double redirects]] are disallowed,"
           .." both pages currently point to [["..otherTarget.."]].\n"
           .."**In case "..noredir(otherPage).." is expanded into a separate"
           .." article, this redirect should be changed to point to that article instead."
    cat = demo and "" or "Avoided double redirects"
  else
    from = "an alternative title for [["..otherPage.."]], a former redirect to the same title"
    info = "\n**Since [["..otherPage.."]] is now a separate article, please update"
           .." this redirect's target to [["..otherPage.."]] and remove this template."
    cat = demo and "" or "Avoided double redirects to be updated"
  end
  return frame:expandTemplate({title="Redirect template",
                               args={from=from,info=info,["main category"]=cat,
                                     name=(embed=="yes" and "Avoided double redirect" or nil))))
else
  return "<span class=\"error\">Error in [[Module:R avoided double redirect]]: "..err.."</span>"
         ..(demo and "" or "[[Category:Avoided double redirects/error]]")
end

end

return p