{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE DeriveDataTypeable    #-}
{-# LANGUAGE OverloadedStrings     #-}
module Network.HTTP.Download
    ( DownloadRequest(..)
    , drRetryPolicyDefault
    , HashCheck(..)
    , DownloadException(..)
    , CheckHexDigest(..)
    , LengthCheck
    , VerifiedDownloadException(..)

    , download
    , redownload
    , verifiedDownload
    ) where

import qualified Data.ByteString.Lazy        as L
import           Conduit
import qualified Data.Conduit.Binary         as CB
import           Network.HTTP.Download.Verified
import           Network.HTTP.Client         (HttpException, Request, Response, checkResponse, path, requestHeaders)
import           Network.HTTP.Simple         (getResponseBody, getResponseHeaders, getResponseStatusCode, withResponse)
import           Path                        (Path, Abs, File, toFilePath)
import           Path.IO                     (doesFileExist)
import           RIO
import           RIO.PrettyPrint
import           System.Directory            (createDirectoryIfMissing,
                                              removeFile)
import           System.FilePath             (takeDirectory, (<.>))


-- | Download the given URL to the given location. If the file already exists,
-- no download is performed. Otherwise, creates the parent directory, downloads
-- to a temporary file, and on file download completion moves to the
-- appropriate destination.
--
-- Throws an exception if things go wrong
download :: HasTerm env
         => Request
         -> Path Abs File -- ^ destination
         -> RIO env Bool -- ^ Was a downloaded performed (True) or did the file already exist (False)?
download req destpath = do
    let downloadReq = DownloadRequest
            { drRequest = req
            , drHashChecks = []
            , drLengthCheck = Nothing
            , drRetryPolicy = drRetryPolicyDefault
            }
    let progressHook _ = return ()
    verifiedDownload downloadReq destpath progressHook

-- | Same as 'download', but will download a file a second time if it is already present.
--
-- Returns 'True' if the file was downloaded, 'False' otherwise
redownload :: HasTerm env
           => Request
           -> Path Abs File -- ^ destination
           -> RIO env Bool
redownload req0 dest = do
    logDebug $ "Downloading " <> display (decodeUtf8With lenientDecode (path req0))
    let destFilePath = toFilePath dest
        etagFilePath = destFilePath <.> "etag"

    metag <- do
      exists <- doesFileExist dest
      if not exists
        then return Nothing
        else liftIO $ handleIO (const $ return Nothing) $ fmap Just $
                 withSourceFile etagFilePath $ \src -> runConduit $ src .| CB.take 512

    let req1 =
            case metag of
                Nothing -> req0
                Just etag -> req0
                    { requestHeaders =
                        requestHeaders req0 ++
                        [("If-None-Match", L.toStrict etag)]
                    }
        req2 = req1 { checkResponse = \_ _ -> return () }
    recoveringHttp drRetryPolicyDefault $ catchingHttpExceptions $ liftIO $
      withResponse req2 $ \res -> case getResponseStatusCode res of
        200 -> do
          createDirectoryIfMissing True $ takeDirectory destFilePath

          -- Order here is important: first delete the etag, then write the
          -- file, then write the etag. That way, if any step fails, it will
          -- force the download to happen again.
          handleIO (const $ return ()) $ removeFile etagFilePath

          withSinkFileCautious destFilePath $ \sink ->
            runConduit $ getResponseBody res .| sink

          forM_ (lookup "ETag" (getResponseHeaders res)) $ \e ->
            withSinkFileCautious etagFilePath $ \sink ->
            runConduit $ yield e .| sink

          return True
        304 -> return False
        _ -> throwM $ RedownloadInvalidResponse req2 dest $ void res

  where
    catchingHttpExceptions :: RIO env a -> RIO env a
    catchingHttpExceptions action = catch action (throwM . RedownloadHttpError)

data DownloadException = RedownloadInvalidResponse Request (Path Abs File) (Response ())
                       | RedownloadHttpError HttpException
                       
    deriving (Show, Typeable)
instance Exception DownloadException