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/MetaData/Pure.hs

@@ -0,0 +1,111 @@
+{- git-annex metadata log, pure operations
+ -
+ - Copyright 2014-2018 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+
+module Logs.MetaData.Pure (
+	Log,
+	LogEntry(..),
+	parseLog,
+	showLog,
+	logToCurrentMetaData,
+	simplifyLog,
+	filterRemoteMetaData,
+	filterOutEmpty,
+) where
+
+import Types.MetaData
+import Logs.SingleValue.Pure
+import Types.UUID
+
+import qualified Data.Set as S
+import qualified Data.Map.Strict as M
+
+instance SingleValueSerializable MetaData where
+	serialize = Types.MetaData.serialize
+	deserialize = Types.MetaData.deserialize
+
+logToCurrentMetaData :: [LogEntry MetaData] -> MetaData
+logToCurrentMetaData = currentMetaData . combineMetaData . map value
+
+{- Simplify a log, removing historical values that are no longer
+ - needed. 
+ -
+ - This is not as simple as just making a single log line with the newest
+ - state of all metadata. Consider this case:
+ -
+ - We have:
+ -
+ - 100 foo +x bar +y
+ - 200 foo -x
+ -
+ - An unmerged remote has:
+ -
+ - 150 bar -y baz +w
+ -
+ - If what we have were simplified to "200 foo -x bar +y" then when the line
+ - from the remote became available, it would be older than the simplified
+ - line, and its change to bar would not take effect. That is wrong.
+ -
+ - Instead, simplify it to:
+ -
+ - 100 bar +y
+ - 200 foo -x
+ -
+ - (Note that this ends up with the same number of lines as the
+ - unsimplified version, so there's really no point in updating
+ - the log to this version. Doing so would only add data to git,
+ - with little benefit.)
+ -
+ - Now merging with the remote yields:
+ -
+ - 100 bar +y
+ - 150 bar -y baz +w
+ - 200 foo -x
+ -
+ - Simplifying again:
+ -
+ - 150 bar +z baz +w
+ - 200 foo -x
+ -}
+simplifyLog :: Log MetaData -> Log MetaData
+simplifyLog s = case sl of
+	(newest:rest) -> 
+		let sl' = go [newest] (value newest) rest
+		in if length sl' < length sl
+			then S.fromList sl'
+			else s
+	_ -> s
+  where
+	sl = S.toDescList s
+
+	go c _ [] = c
+	go c newer (l:ls)
+		| unique == emptyMetaData = go c newer ls
+		| otherwise = go (l { value = unique } : c)
+			(unionMetaData unique newer) ls
+	  where
+		older = value l
+		unique = older `differenceMetaData` newer
+
+{- Filters per-remote metadata on the basis of UUID.
+ -
+ - Note that the LogEntry's clock is left the same, so this should not be
+ - used except for in a transition.
+ -}
+filterRemoteMetaData :: (UUID -> Bool) -> Log MetaData -> Log MetaData
+filterRemoteMetaData p = S.map go
+  where
+	go l@(LogEntry { value = MetaData m }) = 
+		l { value = MetaData $ M.filterWithKey fil m }
+	fil f _v = case splitRemoteMetaDataField f of
+		Just (u, _) -> p u
+		Nothing -> True
+
+{- Filters out log lines that are empty. -}
+filterOutEmpty :: Log MetaData -> Log MetaData
+filterOutEmpty = S.filter $ \l -> value l /= emptyMetaData