module Facebook.Auth
    ( getAppAccessToken
    , getUserAccessTokenStep1
    , getUserAccessTokenStep2
    , getUserLogoutUrl
    , extendUserAccessToken
    , RedirectUrl
    , Permission
    , hasExpired
    , isValid
    ) where

import Control.Applicative
import Control.Monad.IO.Class (MonadIO(liftIO))
import Data.Aeson ((.:))
import Data.Aeson.Types (parseEither)
import Data.Maybe (fromMaybe)
import Data.Text (Text)
import Data.Time (getCurrentTime, addUTCTime, UTCTime)
import Data.String (IsString(..))

import qualified Control.Exception.Lifted as E
import qualified Data.Attoparsec.Char8 as A
import qualified Data.Conduit as C
import qualified Data.Conduit.Attoparsec as C
import qualified Data.List as L
import qualified Data.Text as T
import qualified Data.Text.Encoding as TE
import qualified Data.Text.Encoding.Error as TE
import qualified Network.HTTP.Conduit as H
import qualified Network.HTTP.Types as HT


import Facebook.Types
import Facebook.Base
import Facebook.Monad


-- | Get an app access token from Facebook using your
-- credentials.
getAppAccessToken :: C.ResourceIO m =>
                     FacebookT Auth m AppAccessToken
getAppAccessToken =
  runResourceInFb $ do
    creds <- getCreds
    let req = fbreq "/oauth/access_token" Nothing $
              tsq creds [("grant_type", "client_credentials")]
    response <- fbhttp req
    lift $
      H.responseBody response C.$$
      C.sinkParser (AppAccessToken <$  A.string "access_token="
                                   <*> A.takeByteString)


-- | The first step to get an user access token.  Returns the
-- Facebook URL you should redirect you user to.  Facebook will
-- authenticate the user, authorize your app and then redirect
-- the user back into the provider 'RedirectUrl'.
getUserAccessTokenStep1 :: Credentials
                        -> RedirectUrl
                        -> [Permission]
                        -> Text
getUserAccessTokenStep1 creds redirectUrl perms =
  T.concat $ "https://www.facebook.com/dialog/oauth?client_id="
           : TE.decodeUtf8 (appId creds)
           : "&redirect_uri="
           : redirectUrl
           : (case perms of
                [] -> []
                _  -> "&scope=" : L.intersperse "," (map unPermission perms)
             )


-- | The second step to get an user access token.  If the user is
-- successfully authenticate and they authorize your application,
-- then they'll be redirected back to the 'RedirectUrl' you've
-- passed to 'getUserAccessTokenStep1'.  You should take the
-- request query parameters passed to your 'RedirectUrl' and give
-- to this function that will complete the user authentication
-- flow and give you an @'AccessToken' 'User'@.
getUserAccessTokenStep2 :: C.ResourceIO m =>
                           RedirectUrl -- ^ Should be exactly the same
                                       -- as in 'getUserAccessTokenStep1'.
                        -> [Argument]
                        -> FacebookT Auth m UserAccessToken
getUserAccessTokenStep2 redirectUrl query =
  case query of
    [code@("code", _)] -> runResourceInFb $ do
      -- Get the access token data through Facebook's OAuth.
      now   <- liftIO getCurrentTime
      creds <- getCreds
      let req = fbreq "/oauth/access_token" Nothing $
                tsq creds [code, ("redirect_uri", TE.encodeUtf8 redirectUrl)]
      response <- fbhttp req
      preToken <- lift $ H.responseBody response C.$$
                         C.sinkParser (userAccessTokenParser now)

      -- Get user's ID throught Facebook's graph.
      userInfo <- asJson =<< fbhttp (fbreq "/me" (Just preToken) [("fields", "id")])
      case (parseEither (.: "id") userInfo, preToken) of
        (Left str, _) ->
            E.throw $ FbLibraryException $ T.concat
                 [ "getUserAccessTokenStep2: failed to get the UserId ("
                 , T.pack str, ")" ]
        (Right (userId :: UserId), UserAccessToken _ d e) ->
            return (UserAccessToken userId d e)

    _ -> let [error_, errorReason, errorDescr] =
                 map (fromMaybe "" . flip lookup query)
                     ["error", "error_reason", "error_description"]
             errorType = T.concat [t error_, " (", t errorReason, ")"]
             t = TE.decodeUtf8With TE.lenientDecode
         in E.throw $ FacebookException errorType (t errorDescr)


-- | Attoparsec parser for user access tokens returned by
-- Facebook as a query string.  Returns an user access token with
-- a broken 'UserId'.
userAccessTokenParser :: UTCTime -- ^ 'getCurrentTime'
                      -> A.Parser UserAccessToken
userAccessTokenParser now =
    UserAccessToken userId
      <$  A.string "access_token="
      <*> A.takeWhile (/= '?')
      <*  A.string "&expires="
      <*> (toExpire <$> A.decimal)
      <*  A.endOfInput
    where toExpire i = addUTCTime (fromIntegral (i :: Int)) now
          userId = error "userAccessTokenParser: never here"


-- | The URL an user should be redirected to in order to log them
-- out of their Facebook session.  Facebook will then redirect
-- the user to the provided URL after logging them out.  Note
-- that, at the time of this writing, Facebook's policies require
-- you to log the user out of Facebook when they ask to log out
-- of your site.
--
-- Note also that Facebook may refuse to redirect the user to the
-- provided URL if their user access token is invalid.  In order
-- to prevent this bug, we suggest that you use 'isValid' before
-- redirecting the user to the URL provided by 'getUserLogoutUrl'
-- since this function doesn't do any validity checks.
getUserLogoutUrl :: UserAccessToken  -- ^ The user's access token.
                 -> RedirectUrl      -- ^ URL the user should be directed to in your site domain.
                 -> Text             -- ^ Logout URL in @https:\/\/www.facebook.com\/@.
getUserLogoutUrl (UserAccessToken _ data_ _) next =
    TE.decodeUtf8 $
      "https://www.facebook.com/logout.php?" <>
      HT.renderQuery False [ ("next", Just (TE.encodeUtf8 next))
                           , ("access_token", Just data_) ]


-- | URL where the user is redirected to after Facebook
-- authenticates the user authorizes your application.  This URL
-- should be inside the domain registered for your Facebook
-- application.
type RedirectUrl = Text


-- | A permission that is asked for the user when he authorizes
-- your app.  Please refer to Facebook's documentation at
-- <https://developers.facebook.com/docs/reference/api/permissions/>
-- to see which permissions are available.
--
-- This is a @newtype@ of 'Text' that supports only 'IsString'.
-- This means that to create a 'Permission' you should use the
-- @OverloadedStrings@ language extension.  For example,
--
-- > {-# LANGUAGE OverloadedStrings #-}
-- >
-- > perms :: [Permission]
-- > perms = ["user_about_me", "email", "offline_access"]
newtype Permission = Permission { unPermission :: Text }

instance Show Permission where
    show = show . unPermission

instance IsString Permission where
    fromString = Permission . fromString


-- | @True@ if the access token has expired, otherwise @False@.
hasExpired :: (Functor m, MonadIO m) => AccessToken anyKind -> m Bool
hasExpired token =
  case accessTokenExpires token of
    Nothing      -> return False
    Just expTime -> (>= expTime) <$> liftIO getCurrentTime


-- | @True@ if the access token is valid.  An expired access
-- token is not valid (see 'hasExpired').  However, a non-expired
-- access token may not be valid as well.  For example, in the
-- case of an user access token, they may have changed their
-- password, logged out from Facebook or blocked your app.
isValid :: C.ResourceIO m =>
           AccessToken anyKind
        -> FacebookT anyAuth m Bool
isValid token = do
  expired <- hasExpired token
  if expired
    then return False
    else
      let page = case token of
                   UserAccessToken _ _ _ -> "/me"
                   -- Documented way of checking if the token is valid,
                   -- see <https://developers.facebook.com/blog/post/500/>.
                   AppAccessToken _ -> "/19292868552"
                   -- This is Facebook's page on Facebook.  While
                   -- this behaviour is undocumented, it will
                   -- return a "400 Bad Request" status code
                   -- whenever the access token is invalid.  It
                   -- will actually work with user access tokens,
                   -- too, but they have another, better way of
                   -- being checked.
      in httpCheck (fbreq page (Just token) [])


-- | Extend the expiration time of an user access token (see
-- <https://developers.facebook.com/docs/offline-access-deprecation/>).
-- Returns @Left exc@ if there is an error while extending, or
-- @Right token@ with the new user access token (which could have
-- the same data and expiration time as before, but you can't
-- assume this).  Note that expired access tokens can't be
-- extended, only valid tokens.
extendUserAccessToken ::
    C.ResourceIO m =>
    UserAccessToken
 -> FacebookT Auth m (Either FacebookException UserAccessToken)
extendUserAccessToken token@(UserAccessToken _ data_ _)
    = do expired <- hasExpired token
         if expired then return (Left hasExpiredExc) else tryToExtend
    where
      tryToExtend = runResourceInFb $ do
        creds <- getCreds
        let req = fbreq "/oauth/access_token" Nothing $
                  tsq creds [ ("grant_type", "fb_exchange_token")
                            , ("fb_exchange_token", data_) ]
        eresponse <- E.try (fbhttp req)
        case eresponse of
          Right response -> do
            now <- liftIO getCurrentTime
            either (Left . couldn'tParseExc) Right <$>
              E.try (lift $ H.responseBody response C.$$
                            C.sinkParser (userAccessTokenParser now))
          Left exc -> return (Left exc)

      hasExpiredExc =
          mkExc [ "the user access token has already expired, "
                , "so I'll not try to extend it." ]
      couldn'tParseExc (exc :: C.ParseError) =
          mkExc [ "could not parse Facebook's response ("
                , T.pack (show exc), ")" ]
      mkExc = FbLibraryException . T.concat . ("extendUserAccessToken: ":)