factored out a generic MapLog from uuid-based logs

UUIDBased is just a MapLog with a UUID for the field.

Logs/MapLog.hs

@@ -0,0 +1,81 @@
+{- git-annex Map log
+ -
+ - This is used to store a Map, in a way that can be union merged.
+ -
+ - A line of the log will look like: "timestamp field value"
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.MapLog where
+
+import qualified Data.Map as M
+import Data.Time.Clock.POSIX
+import Data.Time
+import System.Locale
+  
+import Common
+
+data TimeStamp = Unknown | Date POSIXTime
+	deriving (Eq, Ord, Show)
+
+data LogEntry v = LogEntry
+	{ changed :: TimeStamp
+	, value :: v
+	} deriving (Eq, Show)
+
+type MapLog f v = M.Map f (LogEntry v)
+
+showMapLog :: (f -> String) -> (v -> String) -> MapLog f v -> String
+showMapLog fieldshower valueshower = unlines . map showpair . M.toList
+  where
+	showpair (f, LogEntry (Date p) v) =
+		unwords [show p, fieldshower f, valueshower v]
+	showpair (f, LogEntry Unknown v) =
+		unwords ["0", fieldshower f, valueshower v]
+
+parseMapLog :: Ord f => (String -> Maybe f) -> (String -> Maybe v) -> String -> MapLog f v
+parseMapLog fieldparser valueparser = M.fromListWith best . mapMaybe parse . lines
+  where
+	parse line = do
+		let (ts, rest) = splitword line
+		    (sf, sv) = splitword rest
+		date <- Date . utcTimeToPOSIXSeconds <$> parseTime defaultTimeLocale "%s%Qs" ts
+		f <- fieldparser sf
+		v <- valueparser sv
+		Just (f, LogEntry date v)
+	splitword = separate (== ' ')
+
+changeMapLog :: Ord f => POSIXTime -> f -> v -> MapLog f v -> MapLog f v
+changeMapLog t f v = M.insert f $ LogEntry (Date t) v
+
+{- Only add an LogEntry if it's newer (or at least as new as) than any
+ - existing LogEntry for a field. -}
+addMapLog :: Ord f => f -> LogEntry v -> MapLog f v -> MapLog f v
+addMapLog = M.insertWith' best
+
+{- Converts a MapLog into a simple Map without the timestamp information.
+ - This is a one-way trip, but useful for code that never needs to change
+ - the log. -}
+simpleMap :: MapLog f v -> M.Map f v
+simpleMap = M.map value
+
+best :: LogEntry v -> LogEntry v -> LogEntry v
+best new old
+	| changed old > changed new = old
+	| otherwise = new
+
+-- Unknown is oldest.
+prop_TimeStamp_sane :: Bool
+prop_TimeStamp_sane = Unknown < Date 1
+
+prop_addMapLog_sane :: Bool
+prop_addMapLog_sane = newWins && newestWins
+  where
+	newWins = addMapLog ("foo") (LogEntry (Date 1) "new") l == l2
+	newestWins = addMapLog ("foo") (LogEntry (Date 1) "newest") l2 /= l2
+
+	l = M.fromList [("foo", LogEntry (Date 0) "old")]
+	l2 = M.fromList [("foo", LogEntry (Date 1) "new")]