rework Differences data type

Eliminated complexity and future proofed. The most important change is that
all functions over Difference are now total; any Difference that can be
expressed should be handled. Avoids needs for sanity checking of inputs,
and version skew with the future.

Also, the difference.log now serializes a [Difference], not a Differences.
This saves space and keeps it simpler.

Note that [Difference] might contain conflicting differences (eg,
[Version5, Version6]. In this case, one of them needs to consistently win
over the others, probably based on Ord.

Types/Difference.hs

@@ -9,123 +9,74 @@ module Types.Difference (
 	Difference(..),
 	Differences(..),
 	getDifferences,
-	sanityCheckDifferences,
 	differenceConfigKey,
 	differenceConfigVal,
 	hasDifference,
 ) where
 
-import Utility.PartialPrelude
 import qualified Git
 import qualified Git.Config
 
 import Data.List
 import Data.Maybe
 import Data.Monoid
-import Control.Applicative
 
 -- Describes differences from the v5 repository format.
 --
--- The serilization is stored in difference.log, so avoid changes that
+-- The serialization is stored in difference.log, so avoid changes that
 -- would break compatability.
 --
--- Not breaking comparability is why a list of Differences is used, rather
+-- Not breaking compatability is why a list of Differences is used, rather
 -- than a sum type. With a sum type, adding a new field for some future
 -- difference would serialize to a value that an older version could not
 -- parse, even if that new field was not used. With the Differences list,
 -- old versions can still parse it, unless the new Difference constructor 
 -- is used.
+--
+-- The constructors intentionally do not have parameters; this is to
+-- ensure that any Difference that can be expressed is supported.
+-- So, a new repository version would be Version6, rather than Version Int.
 data Difference
-	= Version Int
-	| ObjectHashLower Bool
-	| ObjectHashDirectories Int
-	| BranchHashDirectories Int
-	deriving (Show, Read, Ord)
-
-instance Eq Difference where
-	Version a == Version b = a == b
-	ObjectHashLower a == ObjectHashLower b = a == b
-	ObjectHashDirectories a == ObjectHashDirectories b = a == b
-	BranchHashDirectories a == BranchHashDirectories b = a == b
-	_ == _ = False
+	= ObjectHashLower
+	| OneLevelObjectHash
+	| OneLevelBranchHash
+	deriving (Show, Read, Ord, Eq, Enum, Bounded)
 
 data Differences
 	= Differences [Difference]
 	| UnknownDifferences
-	deriving (Show, Read, Ord)
 
 instance Eq Differences where
-	Differences a == Differences b = simplify (defver:a) == simplify (defver:b)
-	_ == _ = False
+	Differences a == Differences b = canon a == canon b
+	_ == _ = False -- UnknownDifferences cannot be equal
 
 instance Monoid Differences where
 	mempty = Differences []
-	mappend (Differences l1) (Differences l2) = Differences (simplify (l1 ++ l2))
+	mappend (Differences l1) (Differences l2) = Differences (canon (l1 ++ l2))
 	mappend _ _ = UnknownDifferences
 
--- This is the default repository version that is assumed when no other one
--- is given. Note that [] == [Version 5]
-defver :: Difference
-defver = Version 5
-
--- Given [Version 6, Version 5], returns [Version 6]
-simplify :: [Difference] -> [Difference]
-simplify = go . sort
-  where
-	go [] = []
-	go (d:[]) = [d]
-	go (d1:d2:ds)
-		| like d1 d2 = go (d2:ds)
-		| otherwise = d1 : go (d2:ds)
-
-	like (Version _) (Version _) = True
-	like _ _ = False
+canon :: [Difference] -> [Difference]
+canon = nub . sort
 
 getDifferences :: Git.Repo -> Differences
-getDifferences r = checksane $ Differences $ catMaybes
-	[ ObjectHashLower 
-		<$> getmaybebool (differenceConfigKey (ObjectHashLower undefined))
-	, ObjectHashDirectories 
-		<$> getmayberead (differenceConfigKey (ObjectHashDirectories undefined))
-	, BranchHashDirectories
-		<$> getmayberead (differenceConfigKey (BranchHashDirectories undefined))
-	]
+getDifferences r = Differences $ catMaybes $
+	map getmaybe [minBound .. maxBound]
   where
-	getmaybe k = Git.Config.getMaybe k r
-	getmayberead k = readish =<< getmaybe k
-	getmaybebool k = Git.Config.isTrue =<< getmaybe k
-	checksane = either error id . sanityCheckDifferences
+	getmaybe d = case Git.Config.isTrue =<< Git.Config.getMaybe (differenceConfigKey d) r of
+		Just True -> Just d
+		_ -> Nothing
 
 differenceConfigKey :: Difference -> String
-differenceConfigKey (Version _) = "annex.version"
-differenceConfigKey (ObjectHashLower _) = tunable "objecthashlower"
-differenceConfigKey (ObjectHashDirectories _) = tunable "objecthashdirectories"
-differenceConfigKey (BranchHashDirectories _) = tunable "branchhashdirectories"
+differenceConfigKey ObjectHashLower = tunable "objecthashlower"
+differenceConfigKey OneLevelObjectHash = tunable "objecthash1"
+differenceConfigKey OneLevelBranchHash = tunable "branchhash1"
 
 differenceConfigVal :: Difference -> String
-differenceConfigVal (Version v) = show v
-differenceConfigVal (ObjectHashLower b) = Git.Config.boolConfig b
-differenceConfigVal (ObjectHashDirectories n) = show n
-differenceConfigVal (BranchHashDirectories n) = show n
+differenceConfigVal _ = Git.Config.boolConfig True
 
 tunable :: String -> String
 tunable k = "annex.tune." ++ k
 
-sanityCheckDifferences :: Differences -> Either String Differences
-sanityCheckDifferences d@(Differences l)
-	| null problems = Right d
-	| otherwise = Left (intercalate "; " problems)
-  where
-	problems = catMaybes (map check l)
-	check (ObjectHashDirectories n)
-		| n == 1 || n == 2 = Nothing
-		| otherwise = Just $ "Bad value for objecthashdirectories -- should be 1 or 2, not " ++ show n
-	check (BranchHashDirectories n)
-		| n == 1 || n == 2 = Nothing
-		| otherwise = Just $ "Bad value for branhhashdirectories -- should be 1 or 2, not " ++ show n
-	check _ = Nothing
-sanityCheckDifferences UnknownDifferences = Left "unknown differences detected; update git-annex"
-
 hasDifference :: (Difference -> Bool) -> Differences -> Bool
 hasDifference f (Differences l) = any f l
 hasDifference _ UnknownDifferences = False