    {-# LANGUAGE CPP #-}
    {-
    Copyright (C) 2009 John MacFarlane <jgm@berkeley.edu>,
    Anton van Straaten <anton@appsolutions.com>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- Functions for content conversion.
    -}

    module Network.Gitit.ContentTransformer
      (
      -- * ContentTransformer runners
        runPageTransformer
      , runFileTransformer
      -- * Gitit responders
      , showRawPage
      , showFileAsText
      , showPage
      , exportPage
      , showHighlightedSource
      , showFile
      , preview
      , applyPreCommitPlugins
      -- * Cache support for transformers
      , cacheHtml
      , cachedHtml
      -- * Content retrieval combinators
      , rawContents
      -- * Response-generating combinators
      , textResponse
      , mimeFileResponse
      , mimeResponse
      , exportPandoc
      , applyWikiTemplate
      -- * Content-type transformation combinators
      , pageToWikiPandoc
      , pageToPandoc
      , pandocToHtml
      , highlightSource
      -- * Content or context augmentation combinators
      , applyPageTransforms
      , wikiDivify
      , addPageTitleToPandoc
      , addMathSupport
      , addScripts
      -- * ContentTransformer context API
      , getFileName
      , getPageName
      , getLayout
      , getParams
      , getCacheable
      -- * Pandoc and wiki content conversion support
      , inlinesToURL
      , inlinesToString
      )
    where

    import qualified Control.Exception as E
    import Control.Monad.State
    import Control.Monad.Reader (ask)
    import Data.Foldable (traverse_)
    import Data.List (stripPrefix)
    import Data.Maybe (isNothing, mapMaybe)
    import Network.Gitit.Cache (lookupCache, cacheContents)
    import Network.Gitit.Export (exportFormats)
    import Network.Gitit.Framework hiding (uriPath)
    import Network.Gitit.Layout
    import Network.Gitit.Page (stringToPage)
    import Network.Gitit.Server
    import Network.Gitit.State
    import Network.Gitit.Types
    import Network.HTTP (urlDecode)
    import Network.URI (isUnescapedInURI)
    import Network.URL (encString)
    import System.FilePath
    import qualified Text.Pandoc.Builder as B
    import Text.HTML.SanitizeXSS (sanitizeBalance)
    import Text.Highlighting.Kate
    import Text.Pandoc hiding (MathML, WebTeX, MathJax)
    import Text.XHtml hiding ( (</>), dir, method, password, rev )
    import Text.XHtml.Strict (stringToHtmlString)
    #if MIN_VERSION_blaze_html(0,5,0)
    import Text.Blaze.Html.Renderer.String as Blaze ( renderHtml )
    #else
    import Text.Blaze.Renderer.String as Blaze ( renderHtml )
    #endif
    import qualified Data.Text as T
    import qualified Data.Set as Set
    import qualified Data.ByteString as S (concat)
    import qualified Data.ByteString.Char8 as SC (unpack)
    import qualified Data.ByteString.Lazy as L (toChunks, fromChunks)
    import qualified Data.FileStore as FS
    import qualified Text.Pandoc as Pandoc
    import Text.URI (parseURI, URI(..), uriQueryItems)

    #if MIN_VERSION_pandoc(1,14,0)
    import Text.Pandoc.Error (handleError)
    #else
    handleError :: Pandoc -> Pandoc
    handleError = id
    #endif
    --
    -- ContentTransformer runners
    --

    runPageTransformer :: ToMessage a
                   => ContentTransformer a
                   -> GititServerPart a
    runPageTransformer xform = withData $ \params -> do
      page <- getPage
      cfg <- getConfig
      evalStateT xform  Context{ ctxFile = pathForPage page (defaultExtension cfg)
                               , ctxLayout = defaultPageLayout{
                                                 pgPageName = page
                                               , pgTitle = page
                                               , pgPrintable = pPrintable params
                                               , pgMessages = pMessages params
                                               , pgRevision = pRevision params
                                               , pgLinkToFeed = useFeed cfg }
                               , ctxCacheable = True
                               , ctxTOC = tableOfContents cfg
                               , ctxBirdTracks = showLHSBirdTracks cfg
                               , ctxCategories = []
                               , ctxMeta = [] }

    runFileTransformer :: ToMessage a
                   => ContentTransformer a
                   -> GititServerPart a
    runFileTransformer xform = withData $ \params -> do
      page <- getPage
      cfg <- getConfig
      evalStateT xform  Context{ ctxFile = id page
                               , ctxLayout = defaultPageLayout{
                                                 pgPageName = page
                                               , pgTitle = page
                                               , pgPrintable = pPrintable params
                                               , pgMessages = pMessages params
                                               , pgRevision = pRevision params
                                               , pgLinkToFeed = useFeed cfg }
                               , ctxCacheable = True
                               , ctxTOC = tableOfContents cfg
                               , ctxBirdTracks = showLHSBirdTracks cfg
                               , ctxCategories = []
                               , ctxMeta = [] }

    -- | Converts a @ContentTransformer@ into a @GititServerPart@;
    -- specialized to wiki pages.
    -- runPageTransformer :: ToMessage a
    --                    => ContentTransformer a
    --                    -> GititServerPart a
    -- runPageTransformer = runTransformer pathForPage

    -- | Converts a @ContentTransformer@ into a @GititServerPart@;
    -- specialized to non-pages.
    -- runFileTransformer :: ToMessage a
    --                    => ContentTransformer a
    --                    -> GititServerPart a
    -- runFileTransformer = runTransformer id

    --
    -- Gitit responders
    --

    -- | Responds with raw page source.
    showRawPage :: Handler
    showRawPage = runPageTransformer rawTextResponse

    -- | Responds with raw source (for non-pages such as source
    -- code files).
    showFileAsText :: Handler
    showFileAsText = runFileTransformer rawTextResponse

    -- | Responds with rendered wiki page.
    showPage :: Handler
    showPage = runPageTransformer htmlViaPandoc

    -- | Responds with page exported into selected format.
    exportPage :: Handler
    exportPage = runPageTransformer exportViaPandoc

    -- | Responds with highlighted source code.
    showHighlightedSource :: Handler
    showHighlightedSource = runFileTransformer highlightRawSource

    -- | Responds with non-highlighted source code.
    showFile :: Handler
    showFile = runFileTransformer (rawContents >>= mimeFileResponse)

    -- | Responds with rendered page derived from form data.
    preview :: Handler
    preview = runPageTransformer $
              liftM (filter (/= '\r') . pRaw) getParams >>=
              contentsToPage >>=
              pageToWikiPandoc >>=
              pandocToHtml >>=
              return . toResponse . renderHtmlFragment

    -- | Applies pre-commit plugins to raw page source, possibly
    -- modifying it.
    applyPreCommitPlugins :: String -> GititServerPart String
    applyPreCommitPlugins = runPageTransformer . applyPreCommitTransforms

    --
    -- Top level, composed transformers
    --

    -- | Responds with raw source.
    rawTextResponse :: ContentTransformer Response
    rawTextResponse = rawContents >>= textResponse

    -- | Responds with a wiki page in the format specified
    -- by the @format@ parameter.
    exportViaPandoc :: ContentTransformer Response
    exportViaPandoc = rawContents >>=
                      maybe mzero return >>=
                      contentsToPage >>=
                      pageToWikiPandoc >>=
                      exportPandoc

    -- | Responds with a wiki page. Uses the cache when
    -- possible and caches the rendered page when appropriate.
    htmlViaPandoc :: ContentTransformer Response
    htmlViaPandoc = cachedHtml `mplus`
                      (rawContents >>=
                       maybe mzero return >>=
                       contentsToPage >>=
                       handleRedirects >>=
                       either return
                         (pageToWikiPandoc >=>
                          addMathSupport >=>
                          pandocToHtml >=>
                          wikiDivify >=>
                          applyWikiTemplate >=>
                          cacheHtml))

    -- | Responds with highlighted source code in a wiki
    -- page template.  Uses the cache when possible and
    -- caches the rendered page when appropriate.
    highlightRawSource :: ContentTransformer Response
    highlightRawSource =
      cachedHtml `mplus`
        (updateLayout (\l -> l { pgTabs = [ViewTab,HistoryTab] }) >>
         rawContents >>=
         highlightSource >>=
         applyWikiTemplate >>=
         cacheHtml)

    --
    -- Cache support for transformers
    --

    -- | Caches a response (actually just the response body) on disk,
    -- unless the context indicates that the page is not cacheable.
    cacheHtml :: Response -> ContentTransformer Response
    cacheHtml resp' = do
      params <- getParams
      file <- getFileName
      cacheable <- getCacheable
      cfg <- lift getConfig
      when (useCache cfg && cacheable && isNothing (pRevision params) && not (pPrintable params)) $
        lift $ cacheContents file $ S.concat $ L.toChunks $ rsBody resp'
      return resp'

    -- | Returns cached page if available, otherwise mzero.
    cachedHtml :: ContentTransformer Response
    cachedHtml = do
      file <- getFileName
      params <- getParams
      cfg <- lift getConfig
      if useCache cfg && not (pPrintable params) && isNothing (pRevision params)
         then do mbCached <- lift $ lookupCache file
                 let emptyResponse = setContentType "text/html; charset=utf-8" . toResponse $ ()
                 maybe mzero (\(_modtime, contents) -> lift . ok $ emptyResponse{rsBody = L.fromChunks [contents]}) mbCached
         else mzero

    --
    -- Content retrieval combinators
    --

    -- | Returns raw file contents.
    rawContents :: ContentTransformer (Maybe String)
    rawContents = do
      params <- getParams
      file <- getFileName
      fs <- lift getFileStore
      let rev = pRevision params
      liftIO $ E.catch (liftM Just $ FS.retrieve fs file rev)
                   (\e -> if e == FS.NotFound then return Nothing else E.throwIO e)

    --
    -- Response-generating combinators
    --

    -- | Converts raw contents to a text/plain response.
    textResponse :: Maybe String -> ContentTransformer Response
    textResponse Nothing  = mzero  -- fail quietly if file not found
    textResponse (Just c) = mimeResponse c "text/plain; charset=utf-8"

    -- | Converts raw contents to a response that is appropriate with
    -- a mime type derived from the page's extension.
    mimeFileResponse :: Maybe String -> ContentTransformer Response
    mimeFileResponse Nothing = error "Unable to retrieve file contents."
    mimeFileResponse (Just c) =
      mimeResponse c =<< lift . getMimeTypeForExtension . takeExtension =<< getFileName

    mimeResponse :: Monad m
                 => String        -- ^ Raw contents for response body
                 -> String        -- ^ Mime type
                 -> m Response
    mimeResponse c mimeType =
      return . setContentType mimeType . toResponse $ c

    -- | Converts Pandoc to response using format specified in parameters.
    exportPandoc :: Pandoc -> ContentTransformer Response
    exportPandoc doc = do
      params <- getParams
      page <- getPageName
      cfg <- lift getConfig
      let format = pFormat params
      case lookup format (exportFormats cfg) of
           Nothing     -> error $ "Unknown export format: " ++ format
           Just writer -> lift (writer page doc)

    -- | Adds the sidebar, page tabs, and other elements of the wiki page
    -- layout to the raw content.
    applyWikiTemplate :: Html -> ContentTransformer Response
    applyWikiTemplate c = do
      Context { ctxLayout = layout } <- get
      lift $ formattedPage layout c

    --
    -- Content-type transformation combinators
    --

    -- | Converts Page to Pandoc, applies page transforms, and adds page
    -- title.
    pageToWikiPandoc :: Page -> ContentTransformer Pandoc
    pageToWikiPandoc page' =
      pageToWikiPandoc' page' >>= addPageTitleToPandoc (pageTitle page')

    pageToWikiPandoc' :: Page -> ContentTransformer Pandoc
    pageToWikiPandoc' = applyPreParseTransforms >=>
                         pageToPandoc >=> applyPageTransforms

    -- | Converts source text to Pandoc using default page type.
    pageToPandoc :: Page -> ContentTransformer Pandoc
    pageToPandoc page' = do
      modifyContext $ \ctx -> ctx{ ctxTOC = pageTOC page'
                                 , ctxCategories = pageCategories page'
                                 , ctxMeta = pageMeta page' }
      return $ readerFor (pageFormat page') (pageLHS page') (pageText page')

    -- | Detects if the page is a redirect page and handles accordingly. The exact
    -- behaviour is as follows:
    --
    -- If the page is /not/ a redirect page (the most common case), then check the
    -- referer to see if the client came to this page as a result of a redirect
    -- from another page. If so, then add a notice to the messages to notify the
    -- user that they were redirected from another page, and provide a link back
    -- to the original page, with an extra parameter to disable redirection
    -- (e.g., to allow the original page to be edited).
    --
    -- If the page /is/ a redirect page, then check the query string for the
    -- @redirect@ parameter. This can modify the behaviour of the redirect as
    -- follows:
    --
    -- 1. If the @redirect@ parameter is unset, then check the referer to see if
    --    client came to this page as a result of a redirect from another page. If
    --    so, then do not redirect, and add a notice to the messages explaining
    --    that this page is a redirect page, that would have redirected to the
    --    destination given in the metadata (and provide a link thereto), but this
    --    was stopped because a double-redirect was detected. This is a simple way
    --    to prevent cyclical redirects and other abuses enabled by redirects.
    --    redirect to the same page. If the client did /not/ come to this page as
    --    a result of a redirect, then redirect back to the same page, except with
    --    the redirect parameter set to @\"yes\"@.
    --
    -- 2. If the @redirect@ parameter is set to \"yes\", then redirect to the
    --    destination specificed in the metadata. This uses a client-side (meta
    --    refresh + javascript backup) redirect to make sure the referer is set to
    --    this URL.
    --
    -- 3. If the @redirect@ parameter is set to \"no\", then do not redirect, but
    --    add a notice to the messages that this page /would/ have redirected to
    --    the destination given in the metadata had it not been disabled, and
    --    provide a link to the destination given in the metadata. This behaviour
    --    is the @revision@ parameter is present in the query string.
    handleRedirects :: Page -> ContentTransformer (Either Response Page)
    handleRedirects page = case lookup "redirect" (pageMeta page) of
        Nothing -> isn'tRedirect
        Just destination -> isRedirect destination
      where
        addMessage message = modifyContext $ \context -> context
            { ctxLayout = (ctxLayout context)
                { pgMessages = pgMessages (ctxLayout context) ++ [message]
                }
            }
        redirectedFrom source = do
            (url, html) <- processSource source
            return $ concat
                [ "Redirected from <a href=\""
                , url
                , "?redirect=no\" title=\"Go to original page\">"
                , html
                , "</a>"
                ]
        doubleRedirect source destination = do
            (url, html) <- processSource source
            (url', html') <- processDestination destination
            return $ concat
                [ "This page normally redirects to <a href=\""
                , url'
                , "\" title=\"Continue to destination\">"
                , html'
                , "</a>, but as you were already redirected from <a href=\""
                , url
                , "?redirect=no\" title=\"Go to original page\">"
                , html
                , "</a>"
                , ", this was stopped to prevent a double-redirect."
                ]
        cancelledRedirect destination = do
            (url', html') <- processDestination destination
            return $ concat
                [ "This page redirects to <a href=\""
                , url'
                , "\" title=\"Continue to destination\">"
                , html'
                , "</a>."
                ]
        processSource source = do
            base' <- getWikiBase
            let url = stringToHtmlString $ base' ++ urlForPage source
            let html = stringToHtmlString source
            return (url, html)
        processDestination destination = do
            base' <- getWikiBase
            let (page', fragment) = break (== '#') destination
            let url = stringToHtmlString $ concat
                 [ base'
                 , urlForPage page'
                 , fragment
                 ]
            let html = stringToHtmlString page'
            return (url, html)
        getSource = do
            cfg <- lift getConfig
            base' <- getWikiBase
            request <- askRq
            return $ do
                uri <- getHeader "referer" request >>= parseURI . SC.unpack
                let params = uriQueryItems uri
                redirect' <- lookup "redirect" params
                guard $ redirect' == "yes"
                path' <- stripPrefix (base' ++ "/") (uriPath uri)
                let path'' = if null path' then frontPage cfg else urlDecode path'
                guard $ isPage path''
                return path''
        withBody = setContentType "text/html; charset=utf-8" . toResponse
        isn'tRedirect = do
            getSource >>= traverse_ (redirectedFrom >=> addMessage)
            return (Right page)
        isRedirect destination = do
            params <- getParams
            case maybe (pRedirect params) (\_ -> Just False) (pRevision params) of
                 Nothing -> do
                    source <- getSource
                    case source of
                         Just source' -> do
                            doubleRedirect source' destination >>= addMessage
                            return (Right page)
                         Nothing -> fmap Left $ do
                            base' <- getWikiBase
                            let url' = concat
                                 [ base'
                                 , urlForPage (pageName page)
                                 , "?redirect=yes"
                                 ]
                            lift $ seeOther url' $ withBody $ concat
                                [ "<!doctype html><html><head><title>307 Redirect"
                                , "</title></head><body><p>You are being <a href=\""
                                , stringToHtmlString url'
                                , "\">redirected</a>.</body></p></html>"
                                ]
                 Just True -> fmap Left $ do
                    (url', html') <- processDestination destination
                    lift $ ok $ withBody $ concat
                        [ "<!doctype html><html><head><title>Redirecting to "
                        , html'
                        , "</title><meta http-equiv=\"refresh\" contents=\"0; url="
                        , url'
                        , "\" /><script type=\"text/javascript\">window.location=\""
                        , url'
                        , "\"</script></head><body><p>Redirecting to <a href=\""
                        , url'
                        , "\">"
                        , html'
                        , "</a>...</p></body></html>"
                        ]
                 Just False -> do
                    cancelledRedirect destination >>= addMessage
                    return (Right page)

    -- | Converts contents of page file to Page object.
    contentsToPage :: String -> ContentTransformer Page
    contentsToPage s = do
      cfg <- lift getConfig
      pn <- getPageName
      return $ stringToPage cfg pn s

    -- | Converts pandoc document to HTML.
    pandocToHtml :: Pandoc -> ContentTransformer Html
    pandocToHtml pandocContents = do
      base' <- lift getWikiBase
      toc <- liftM ctxTOC get
      bird <- liftM ctxBirdTracks get
      cfg <- lift getConfig
      let tpl = "$if(toc)$<div id=\"TOC\">\n$toc$\n</div>\n$endif$\n$body$"
      return $ primHtml $ T.unpack .
               (if xssSanitize cfg then sanitizeBalance else id) . T.pack $
               writeHtmlString def{
    #if MIN_VERSION_pandoc(1,19,0)
                            writerTemplate = Just tpl
    #else
                            writerStandalone = True
                          , writerTemplate = tpl
    #endif
                          , writerHTMLMathMethod =
                                case mathMethod cfg of
                                     MathML -> Pandoc.MathML Nothing
                                     WebTeX u -> Pandoc.WebTeX u
                                     MathJax u -> Pandoc.MathJax u
                                     _      -> JsMath (Just $ base' ++
                                                          "/js/jsMath/easy/load.js")
                          , writerTableOfContents = toc
                          , writerHighlight = True
                          , writerExtensions = if bird
                                                  then Set.insert
                                                       Ext_literate_haskell
                                                       $ writerExtensions def
                                                  else writerExtensions def
                          -- note: javascript obfuscation gives problems on preview
                          , writerEmailObfuscation = ReferenceObfuscation
                          } pandocContents

    -- | Returns highlighted source code.
    highlightSource :: Maybe String -> ContentTransformer Html
    highlightSource Nothing = mzero
    highlightSource (Just source) = do
      file <- getFileName
      let formatOpts = defaultFormatOpts { numberLines = True, lineAnchors = True }
      case languagesByExtension $ takeExtension file of
            []    -> mzero
            (l:_) -> return $ primHtml $ Blaze.renderHtml
                            $ formatHtmlBlock formatOpts
                            $! highlightAs l $ filter (/='\r') source

    --
    -- Plugin combinators
    --

    getPageTransforms :: ContentTransformer [Pandoc -> PluginM Pandoc]
    getPageTransforms = liftM (mapMaybe pageTransform) $ queryGititState plugins
      where pageTransform (PageTransform x) = Just x
            pageTransform _                 = Nothing

    getPreParseTransforms :: ContentTransformer [String -> PluginM String]
    getPreParseTransforms = liftM (mapMaybe preParseTransform) $
                              queryGititState plugins
      where preParseTransform (PreParseTransform x) = Just x
            preParseTransform _                     = Nothing

    getPreCommitTransforms :: ContentTransformer [String -> PluginM String]
    getPreCommitTransforms = liftM (mapMaybe preCommitTransform) $
                              queryGititState plugins
      where preCommitTransform (PreCommitTransform x) = Just x
            preCommitTransform _                      = Nothing

    -- | @applyTransform a t@ applies the transform @t@ to input @a@.
    applyTransform :: a -> (a -> PluginM a) -> ContentTransformer a
    applyTransform inp transform = do
      context <- get
      conf <- lift getConfig
      user <- lift getLoggedInUser
      fs <- lift getFileStore
      req <- lift askRq
      let pluginData = PluginData{ pluginConfig = conf
                                 , pluginUser = user
                                 , pluginRequest = req
                                 , pluginFileStore = fs }
      (result', context') <- liftIO $ runPluginM (transform inp) pluginData context
      put context'
      return result'

    -- | Applies all the page transform plugins to a Pandoc document.
    applyPageTransforms :: Pandoc -> ContentTransformer Pandoc
    applyPageTransforms c = do
      xforms <- getPageTransforms
      foldM applyTransform c (wikiLinksTransform : xforms)

    -- | Applies all the pre-parse transform plugins to a Page object.
    applyPreParseTransforms :: Page -> ContentTransformer Page
    applyPreParseTransforms page' = getPreParseTransforms >>= foldM applyTransform (pageText page') >>=
                                    (\t -> return page'{ pageText = t })

    -- | Applies all the pre-commit transform plugins to a raw string.
    applyPreCommitTransforms :: String -> ContentTransformer String
    applyPreCommitTransforms c = getPreCommitTransforms >>= foldM applyTransform c

    --
    -- Content or context augmentation combinators
    --

    -- | Puts rendered page content into a wikipage div, adding
    -- categories.
    wikiDivify :: Html -> ContentTransformer Html
    wikiDivify c = do
      categories <- liftM ctxCategories get
      base' <- lift getWikiBase
      let categoryLink ctg = li (anchor ! [href $ base' ++ "/_category/" ++ ctg] << ctg)
      let htmlCategories = if null categories
                              then noHtml
                              else thediv ! [identifier "categoryList"] << ulist << map categoryLink categories
      return $ thediv ! [identifier "wikipage"] << [c, htmlCategories]

    -- | Adds page title to a Pandoc document.
    addPageTitleToPandoc :: String -> Pandoc -> ContentTransformer Pandoc
    addPageTitleToPandoc title' (Pandoc _ blocks) = do
      updateLayout $ \layout -> layout{ pgTitle = title' }
      return $ if null title'
                  then Pandoc nullMeta blocks
                  else Pandoc (B.setMeta "title" (B.str title') nullMeta) blocks

    -- | Adds javascript links for math support.
    addMathSupport :: a -> ContentTransformer a
    addMathSupport c = do
      conf <- lift getConfig
      updateLayout $ \l ->
        case mathMethod conf of
             JsMathScript -> addScripts l ["jsMath/easy/load.js"]
             MathML       -> addScripts l ["MathMLinHTML.js"]
             WebTeX _     -> l
             MathJax u    -> addScripts l [u]
             RawTeX       -> l
      return c

    -- | Adds javascripts to page layout.
    addScripts :: PageLayout -> [String] -> PageLayout
    addScripts layout scriptPaths =
      layout{ pgScripts = scriptPaths ++ pgScripts layout }

    --
    -- ContentTransformer context API
    --

    getParams :: ContentTransformer Params
    getParams = lift (withData return)

    getFileName :: ContentTransformer FilePath
    getFileName = liftM ctxFile get

    getPageName :: ContentTransformer String
    getPageName = liftM (pgPageName . ctxLayout) get

    getLayout :: ContentTransformer PageLayout
    getLayout = liftM ctxLayout get

    getCacheable :: ContentTransformer Bool
    getCacheable = liftM ctxCacheable get

    -- | Updates the layout with the result of applying f to the current layout
    updateLayout :: (PageLayout -> PageLayout) -> ContentTransformer ()
    updateLayout f = do
      ctx <- get
      let l = ctxLayout ctx
      put ctx { ctxLayout = f l }

    --
    -- Pandoc and wiki content conversion support
    --

    readerFor :: PageType -> Bool -> String -> Pandoc
    readerFor pt lhs =
      let defPS = def{ readerSmart = True
                     , readerExtensions = if lhs
                                             then Set.insert Ext_literate_haskell
                                                  $ readerExtensions def
                                             else readerExtensions def
                     , readerParseRaw = True
                     }
      in handleError . case pt of
           RST        -> readRST defPS
           Markdown   -> readMarkdown defPS
    #if MIN_VERSION_pandoc(1,14,0)
           CommonMark -> readCommonMark defPS
    #else
           CommonMark -> error "CommonMark input requires pandoc 1.14"
    #endif
           LaTeX      -> readLaTeX defPS
           HTML       -> readHtml defPS
           Textile    -> readTextile defPS
           Org        -> readOrg defPS
           DocBook    -> readDocBook defPS
           MediaWiki  -> readMediaWiki defPS

    wikiLinksTransform :: Pandoc -> PluginM Pandoc
    wikiLinksTransform pandoc
      = do cfg <- liftM pluginConfig ask -- Can't use askConfig from Interface due to circular dependencies.
           return (bottomUp (convertWikiLinks cfg) pandoc)

    -- | Convert links with no URL to wikilinks.
    convertWikiLinks :: Config -> Inline -> Inline
    #if MIN_VERSION_pandoc(1,16,0)
    convertWikiLinks cfg (Link attr ref ("", "")) | useAbsoluteUrls cfg =
      Link attr ref ("/" </> baseUrl cfg </> inlinesToURL ref, "Go to wiki page")
    convertWikiLinks _cfg (Link attr ref ("", "")) =
      Link attr ref (inlinesToURL ref, "Go to wiki page")
    #else
    convertWikiLinks cfg (Link ref ("", "")) | useAbsoluteUrls cfg =
      Link ref ("/" </> baseUrl cfg </> inlinesToURL ref, "Go to wiki page")
    convertWikiLinks _cfg (Link ref ("", "")) =
      Link ref (inlinesToURL ref, "Go to wiki page")
    #endif
    convertWikiLinks _cfg x = x

    -- | Derives a URL from a list of Pandoc Inline elements.
    inlinesToURL :: [Inline] -> String
    inlinesToURL = encString False isUnescapedInURI . inlinesToString

    -- | Convert a list of inlines into a string.
    inlinesToString :: [Inline] -> String
    inlinesToString = concatMap go
      where go x = case x of
                   Str s                   -> s
                   Emph xs                 -> concatMap go xs
                   Strong xs               -> concatMap go xs
                   Strikeout xs            -> concatMap go xs
                   Superscript xs          -> concatMap go xs
                   Subscript xs            -> concatMap go xs
                   SmallCaps xs            -> concatMap go xs
                   Quoted DoubleQuote xs   -> '"' : (concatMap go xs ++ "\"")
                   Quoted SingleQuote xs   -> '\'' : (concatMap go xs ++ "'")
                   Cite _ xs               -> concatMap go xs
                   Code _ s                -> s
                   Space                   -> " "
    #if MIN_VERSION_pandoc(1,16,0)
                   SoftBreak               -> " "
    #endif
                   LineBreak               -> " "
                   Math DisplayMath s      -> "$$" ++ s ++ "$$"
                   Math InlineMath s       -> "$" ++ s ++ "$"
                   RawInline (Format "tex") s -> s
                   RawInline _ _           -> ""
    #if MIN_VERSION_pandoc(1,16,0)
                   Link _ xs _             -> concatMap go xs
                   Image _ xs _            -> concatMap go xs
    #else
                   Link xs _               -> concatMap go xs
                   Image xs _              -> concatMap go xs
    #endif
                   Note _                  -> ""
                   Span _ xs               -> concatMap go xs