dropdead per-remote metadata

Had to refactor pure code into separate modules so it is accessible
inside Annex.Branch.Transitions.

This commit was sponsored by Peter on Patreon.

Logs/SingleValue/Pure.hs

@@ -0,0 +1,45 @@
+{- git-annex single-value log, pure operations
+ -
+ - Copyright 2014 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Logs.SingleValue.Pure where
+
+import Annex.Common
+import Logs.Line
+import Annex.VectorClock
+
+import qualified Data.Set as S
+
+class SingleValueSerializable v where
+	serialize :: v -> String
+	deserialize :: String -> Maybe v
+
+data LogEntry v = LogEntry
+	{ changed :: VectorClock
+	, value :: v
+	} deriving (Eq, Ord)
+
+type Log v = S.Set (LogEntry v)
+
+showLog :: (SingleValueSerializable v) => Log v -> String
+showLog = unlines . map showline . S.toList
+  where
+	showline (LogEntry c v) = unwords [formatVectorClock c, serialize v]
+
+parseLog :: (Ord v, SingleValueSerializable v) => String -> Log v
+parseLog = S.fromList . mapMaybe parse . splitLines
+  where
+	parse line = do
+		let (sc, s) = splitword line
+		c <- parseVectorClock sc
+		v <- deserialize s
+		Just (LogEntry c v)
+	splitword = separate (== ' ')
+
+newestValue :: Log v -> Maybe v
+newestValue s
+	| S.null s = Nothing
+	| otherwise = Just (value $ S.findMax s)