git-annex/Annex/VectorClock.hs

{- 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 Utility.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)

-- 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

formatVectorClock :: VectorClock -> String
formatVectorClock  Unknown = "0"
formatVectorClock (VectorClock t) = show t

parseVectorClock :: String -> Maybe VectorClock
parseVectorClock t = VectorClock <$> parsePOSIXTime t