    --------------------------------------------------------------------------------
    {-# LANGUAGE CPP               #-}
    {-# LANGUAGE OverloadedStrings #-}
    module Hakyll.Check
        ( Check (..)
        , check
        ) where

    --------------------------------------------------------------------------------
    import           Control.Concurrent.MVar      (MVar, newEmptyMVar, putMVar,
                                                   readMVar)
    import           Control.Exception            (SomeAsyncException (..),
                                                   SomeException (..), throw, try)
    import           Control.Monad                (foldM, forM_)
    import           Control.Monad.Reader         (ReaderT, ask, runReaderT)
    import           Control.Monad.State          (StateT, get, modify, runStateT)
    import           Control.Monad.Trans          (liftIO)
    import           Control.Monad.Trans.Resource (runResourceT)
    import           Data.ByteString.Char8        (unpack)
    import           Data.List                    (isPrefixOf)
    import qualified Data.Map.Lazy                as Map
    import           Network.URI                  (unEscapeString)
    import           System.Directory             (doesDirectoryExist,
                                                   doesFileExist)
    import           System.Exit                  (ExitCode (..))
    import           System.FilePath              (takeDirectory, takeExtension,
                                                   (</>))
    import qualified Text.HTML.TagSoup            as TS

    --------------------------------------------------------------------------------
    #ifdef CHECK_EXTERNAL
    import           Data.List                    (intercalate)
    import           Data.Typeable                (cast)
    import           Data.Version                 (versionBranch)
    import           GHC.Exts                     (fromString)
    import qualified Network.HTTP.Conduit         as Http
    import qualified Network.HTTP.Types           as Http
    import qualified Paths_hakyll                 as Paths_hakyll
    #endif

    --------------------------------------------------------------------------------
    import           Hakyll.Core.Configuration
    import           Hakyll.Core.Logger           (Logger)
    import qualified Hakyll.Core.Logger           as Logger
    import           Hakyll.Core.Util.File
    import           Hakyll.Web.Html

    --------------------------------------------------------------------------------
    data Check = All | InternalLinks
        deriving (Eq, Ord, Show)

    --------------------------------------------------------------------------------
    check :: Configuration -> Logger -> Check -> IO ExitCode
    check config logger check' = do
        ((), state) <- runChecker checkDestination config logger check'
        failed <- countFailedLinks state
        return $ if failed > 0 then ExitFailure 1 else ExitSuccess

    --------------------------------------------------------------------------------
    countFailedLinks :: CheckerState -> IO Int
    countFailedLinks state = foldM addIfFailure 0 (Map.elems state)
        where addIfFailure failures mvar = do
                  checkerWrite <- readMVar mvar
                  return $ failures + checkerFaulty checkerWrite

    --------------------------------------------------------------------------------
    data CheckerRead = CheckerRead
        { checkerConfig :: Configuration
        , checkerLogger :: Logger
        , checkerCheck  :: Check
        }

    --------------------------------------------------------------------------------
    data CheckerWrite = CheckerWrite
        { checkerFaulty :: Int
        , checkerOk     :: Int
        } deriving (Show)

    --------------------------------------------------------------------------------
    instance Monoid CheckerWrite where
        mempty                                            = CheckerWrite 0 0
        mappend (CheckerWrite f1 o1) (CheckerWrite f2 o2) =
            CheckerWrite (f1 + f2) (o1 + o2)

    --------------------------------------------------------------------------------
    type CheckerState = Map.Map URL (MVar CheckerWrite)

    --------------------------------------------------------------------------------
    type Checker a = ReaderT CheckerRead (StateT CheckerState IO) a

    --------------------------------------------------------------------------------
    type URL = String

    --------------------------------------------------------------------------------
    runChecker :: Checker a -> Configuration -> Logger -> Check
               -> IO (a, CheckerState)
    runChecker checker config logger check' = do
        let read' = CheckerRead
                        { checkerConfig = config
                        , checkerLogger = logger
                        , checkerCheck  = check'
                        }
        Logger.flush logger
        runStateT (runReaderT checker read') Map.empty

    --------------------------------------------------------------------------------
    checkDestination :: Checker ()
    checkDestination = do
        config <- checkerConfig <$> ask
        files  <- liftIO $ getRecursiveContents
            (const $ return False) (destinationDirectory config)

        let htmls =
                [ destinationDirectory config </> file
                | file <- files
                , takeExtension file == ".html"
                ]

        forM_ htmls checkFile

    --------------------------------------------------------------------------------
    checkFile :: FilePath -> Checker ()
    checkFile filePath = do
        logger   <- checkerLogger <$> ask
        contents <- liftIO $ readFile filePath
        Logger.header logger $ "Checking file " ++ filePath

        let urls = getUrls $ TS.parseTags contents
        forM_ urls $ \url -> do
            Logger.debug logger $ "Checking link " ++ url
            m <- liftIO newEmptyMVar
            checkUrlIfNeeded filePath (canonicalizeUrl url) m
        where
            -- Check scheme-relative links
            canonicalizeUrl url = if schemeRelative url then "http:" ++ url else url
            schemeRelative = isPrefixOf "//"

    --------------------------------------------------------------------------------
    checkUrlIfNeeded :: FilePath -> URL -> MVar CheckerWrite -> Checker ()
    checkUrlIfNeeded filepath url m = do
        logger     <- checkerLogger           <$> ask
        needsCheck <- (== All) . checkerCheck <$> ask
        checked    <- (url `Map.member`)      <$> get
        if not needsCheck || checked
            then Logger.debug logger "Already checked, skipping"
            else do modify $ Map.insert url m
                    checkUrl filepath url

    --------------------------------------------------------------------------------
    checkUrl :: FilePath -> URL -> Checker ()
    checkUrl filePath url
        | isExternal url  = checkExternalUrl url
        | hasProtocol url = skip url $ Just "Unknown protocol, skipping"
        | otherwise       = checkInternalUrl filePath url
      where
        validProtoChars = ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ "+-."
        hasProtocol str = case break (== ':') str of
            (proto, ':' : _) -> all (`elem` validProtoChars) proto
            _                -> False

    --------------------------------------------------------------------------------
    ok :: URL -> Checker ()
    ok url = putCheckResult url mempty {checkerOk = 1}

    --------------------------------------------------------------------------------
    skip :: URL -> Maybe String -> Checker ()
    skip url maybeReason = do
        logger <- checkerLogger <$> ask
        case maybeReason of
            Nothing     -> return ()
            Just reason -> Logger.debug logger reason
        putCheckResult url mempty {checkerOk = 1}

    --------------------------------------------------------------------------------
    faulty :: URL -> Maybe String -> Checker ()
    faulty url reason = do
        logger <- checkerLogger <$> ask
        Logger.error logger $ "Broken link to " ++ show url ++ explanation
        putCheckResult url mempty {checkerFaulty = 1}
      where
        formatExplanation = (" (" ++) . (++ ")")
        explanation = maybe "" formatExplanation reason

    --------------------------------------------------------------------------------
    putCheckResult :: URL -> CheckerWrite -> Checker ()
    putCheckResult url result = do
        state <- get
        let maybeMVar = Map.lookup url state
        case maybeMVar of
            Just m -> liftIO $ putMVar m result
            Nothing -> do
                logger <- checkerLogger <$> ask
                Logger.debug logger "Failed to find existing entry for checked URL"

    --------------------------------------------------------------------------------
    checkInternalUrl :: FilePath -> URL -> Checker ()
    checkInternalUrl base url = case url' of
        "" -> ok url
        _  -> do
            config <- checkerConfig <$> ask
            let dest = destinationDirectory config
                dir  = takeDirectory base
                filePath
                    | "/" `isPrefixOf` url' = dest ++ url'
                    | otherwise             = dir </> url'

            exists <- checkFileExists filePath
            if exists then ok url else faulty url Nothing
      where
        url' = stripFragments $ unEscapeString url

    --------------------------------------------------------------------------------
    checkExternalUrl :: URL -> Checker ()
    #ifdef CHECK_EXTERNAL
    checkExternalUrl url = do
        result <- requestExternalUrl url
        case result of
            Left (SomeException e) ->
                case (cast e :: Maybe SomeAsyncException) of
                    Just ae -> throw ae
                    _       -> faulty url (Just $ showException e)
            Right _ -> ok url
        where
            -- Convert exception to a concise form
            showException e = case cast e of
                Just (Http.HttpExceptionRequest _ e') -> show e'
                _                                     -> head $ words $ show e

    requestExternalUrl :: URL -> Checker (Either SomeException Bool)
    requestExternalUrl url = liftIO $ try $ do
        mgr <- Http.newManager Http.tlsManagerSettings
        runResourceT $ do
            request  <- Http.parseRequest url
            response <- Http.http (settings request) mgr
            let code = Http.statusCode (Http.responseStatus response)
            return $ code >= 200 && code < 300
        where
            -- Add additional request info
            settings r = r
                { Http.method         = "HEAD"
                , Http.redirectCount  = 10
                , Http.requestHeaders = ("User-Agent", ua) : Http.requestHeaders r
                }

            -- Nice user agent info
            ua = fromString $ "hakyll-check/" ++
                 (intercalate "." $ map show $ versionBranch Paths_hakyll.version)
    #else
    checkExternalUrl url = skip url Nothing
    #endif

    --------------------------------------------------------------------------------
    -- | Wraps doesFileExist, also checks for index.html
    checkFileExists :: FilePath -> Checker Bool
    checkFileExists filePath = liftIO $ do
        file <- doesFileExist filePath
        dir  <- doesDirectoryExist filePath
        case (file, dir) of
            (True, _) -> return True
            (_, True) -> doesFileExist $ filePath </> "index.html"
            _         -> return False

    --------------------------------------------------------------------------------
    stripFragments :: String -> String
    stripFragments = takeWhile (not . flip elem ['?', '#'])