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+{- git-annex vector clocks
+ -
+ - We don't have a way yet to keep true distributed vector clocks.
+ - The next best thing is a timestamp.
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.VectorClock where
+
+import Data.Time.Clock.POSIX
+import Control.Applicative
+import Prelude
+
+import Utility.Env
+import Logs.TimeStamp
+import Utility.QuickCheck
+
+-- | Some very old logs did not have any time stamp at all;
+-- Unknown is used for those.
+data VectorClock = Unknown | VectorClock POSIXTime
+	deriving (Eq, Ord, Show)
+
+-- Unknown is oldest.
+prop_VectorClock_sane :: Bool
+prop_VectorClock_sane = Unknown < VectorClock 1
+
+instance Arbitrary  VectorClock where
+	arbitrary = VectorClock <$> arbitrary
+
+currentVectorClock :: IO VectorClock
+currentVectorClock = go =<< getEnv "GIT_ANNEX_VECTOR_CLOCK"
+  where
+	go Nothing = VectorClock <$> getPOSIXTime
+	go (Just s) = case parsePOSIXTime s of
+		Just t -> return (VectorClock t)
+		Nothing -> VectorClock <$> getPOSIXTime