{- git-annex key/value backends
 -
 - Copyright 2010 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module Backend (
	BackendFile,
	list,
	orderedList,
	genKey,
	lookupFile,
	chooseBackends,
	lookupBackendName,
	maybeLookupBackendName
) where

import Control.Monad.State (liftIO, when)
import Control.Applicative
import System.IO.Error (try)
import System.FilePath
import System.Posix.Files
import Data.Maybe

import Locations
import qualified Git
import qualified Annex
import Types
import Types.Key
import qualified Types.Backend as B
import Messages

-- When adding a new backend, import it here and add it to the list.
import qualified Backend.WORM
import qualified Backend.SHA
import qualified Backend.URL

list :: [Backend Annex]
list = Backend.WORM.backends ++ Backend.SHA.backends ++ Backend.URL.backends

{- List of backends in the order to try them when storing a new key. -}
orderedList :: Annex [Backend Annex]
orderedList = do
	l <- Annex.getState Annex.backends -- list is cached here
	if not $ null l
		then return l
		else do
			s <- getstandard
			d <- Annex.getState Annex.forcebackend
			handle d s
	where
		parseBackendList [] = list
		parseBackendList s = map lookupBackendName $ words s
		handle Nothing s = return s
		handle (Just "") s = return s
		handle (Just name) s = do
			let l' = lookupBackendName name : s
			Annex.changeState $ \state -> state { Annex.backends = l' }
			return l'
		getstandard = do
			g <- Annex.gitRepo
			return $ parseBackendList $
				Git.configGet g "annex.backends" ""

{- Generates a key for a file, trying each backend in turn until one
 - accepts it. -}
genKey :: FilePath -> Maybe (Backend Annex) -> Annex (Maybe (Key, Backend Annex))
genKey file trybackend = do
	bs <- orderedList
	let bs' = maybe bs (: bs) trybackend
	genKey' bs' file
genKey' :: [Backend Annex] -> FilePath -> Annex (Maybe (Key, Backend Annex))
genKey' [] _ = return Nothing
genKey' (b:bs) file = do
	r <- (B.getKey b) file
	case r of
		Nothing -> genKey' bs file
		Just k -> return $ Just (k, b)

{- Looks up the key and backend corresponding to an annexed file,
 - by examining what the file symlinks to. -}
lookupFile :: FilePath -> Annex (Maybe (Key, Backend Annex))
lookupFile file = do
	tl <- liftIO $ try getsymlink
	case tl of
		Left _ -> return Nothing
		Right l -> makekey l
	where
		getsymlink = takeFileName <$> readSymbolicLink file
		makekey l = maybe (return Nothing) (makeret l) (fileKey l)
		makeret l k =
			case maybeLookupBackendName bname of
					Just backend -> return $ Just (k, backend)
					Nothing -> do
						when (isLinkToAnnex l) $
							warning skip
						return Nothing
			where
				bname = keyBackendName k
				skip = "skipping " ++ file ++ 
					" (unknown backend " ++ bname ++ ")"

type BackendFile = (Maybe (Backend Annex), FilePath)

{- Looks up the backends that should be used for each file in a list.
 - That can be configured on a per-file basis in the gitattributes file.
 -}
chooseBackends :: [FilePath] -> Annex [BackendFile]
chooseBackends fs = do
	g <- Annex.gitRepo
	forced <- Annex.getState Annex.forcebackend
	if isJust forced
		then do
			l <- orderedList
			return $ map (\f -> (Just $ head l, f)) fs
		else do
			pairs <- liftIO $ Git.checkAttr g "annex.backend" fs
			return $ map (\(f,b) -> (maybeLookupBackendName b, f)) pairs

{- Looks up a backend by name. May fail if unknown. -}
lookupBackendName :: String -> Backend Annex
lookupBackendName s = fromMaybe unknown $ maybeLookupBackendName s
	where
		unknown = error $ "unknown backend " ++ s
maybeLookupBackendName :: String -> Maybe (Backend Annex)
maybeLookupBackendName s
	| length matches == 1 = Just $ head matches
	| otherwise = Nothing
	where matches = filter (\b -> s == B.name b) list