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+{- git-annex presence log
+ -
+ - This is used to store presence information in the git-annex branch in
+ - a way that can be union merged.
+ -
+ - A line of the log will look like: "date N INFO"
+ - Where N=1 when the INFO is present, and 0 otherwise.
+ - 
+ - Copyright 2010-2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.Presence (
+	module X,
+	addLog,
+	readLog,
+	logNow,
+	currentLog,
+	historicalLog
+) where
+
+import Data.Time.Clock.POSIX
+
+import Logs.Presence.Pure as X
+import Common.Annex
+import qualified Annex.Branch
+import Git.Types (RefDate)
+
+addLog :: FilePath -> LogLine -> Annex ()
+addLog file line = Annex.Branch.change file $ \s -> 
+	showLog $ compactLog (line : parseLog s)
+
+{- Reads a log file.
+ - Note that the LogLines returned may be in any order. -}
+readLog :: FilePath -> Annex [LogLine]
+readLog = parseLog <$$> Annex.Branch.get
+
+{- Generates a new LogLine with the current date. -}
+logNow :: LogStatus -> String -> Annex LogLine
+logNow s i = do
+	now <- liftIO getPOSIXTime
+	return $ LogLine now s i
+
+{- Reads a log and returns only the info that is still in effect. -}
+currentLog :: FilePath -> Annex [String]
+currentLog file = map info . filterPresent <$> readLog file
+
+{- Reads a historical version of a log and returns the info that was in
+ - effect at that time. 
+ -
+ - The date is formatted as shown in gitrevisions man page.
+ -}
+historicalLog :: RefDate -> FilePath -> Annex [String]
+historicalLog refdate file = map info . filterPresent . parseLog
+	<$> Annex.Branch.getHistorical refdate file