{- git-annex general metadata storage log
 -
 - A line of the log will look like "timestamp field [+-]value [...]"
 -
 - Note that unset values are preserved. Consider this case:
 -
 - We have:
 -
 - 100 foo +x
 - 200 foo -x
 -
 - An unmerged remote has:
 -
 - 150 foo +x
 - 
 - After union merge, because the foo -x was preserved, we know that
 - after the other remote redundantly set foo +x, it was unset,
 - and so foo currently has no value.
 -
 -
 - Copyright 2014 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

{-# OPTIONS_GHC -fno-warn-orphans #-}

module Logs.MetaData (
	getCurrentMetaData,
	getMetaData,
	addMetaData,
	addMetaData',
	currentMetaData,
) where

import Common.Annex
import Types.MetaData
import qualified Annex.Branch
import Logs
import Logs.SingleValue

import qualified Data.Set as S
import Data.Time.Clock.POSIX

instance SingleValueSerializable MetaData where
	serialize = Types.MetaData.serialize
	deserialize = Types.MetaData.deserialize

getMetaData :: Key -> Annex (Log MetaData)
getMetaData = readLog . metaDataLogFile

{- Go through the log from oldest to newest, and combine it all
 - into a single MetaData representing the current state. -}
getCurrentMetaData :: Key -> Annex MetaData
getCurrentMetaData = currentMetaData . collect <$$> getMetaData
  where
	collect = foldl' unionMetaData newMetaData . map value . S.toAscList

{- Adds in some metadata, which can override existing values, or unset
 - them, but otherwise leaves any existing metadata as-is. -}
addMetaData :: Key -> MetaData -> Annex ()
addMetaData k metadata = addMetaData' k metadata =<< liftIO getPOSIXTime

{- Reusing the same timestamp when making changes to the metadata
 - of multiple keys is a nice optimisation. The same metadata lines
 - will tend to be generated across the different log files, and so
 - git will be able to pack the data more efficiently. -}
addMetaData' :: Key -> MetaData -> POSIXTime -> Annex ()
addMetaData' k metadata now = Annex.Branch.change (metaDataLogFile k) $
	showLog . simplifyLog 
		. S.insert (LogEntry now metadata) 
		. parseLog

{- 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 == newMetaData = go c newer ls
		| otherwise = go (l { value = unique } : c)
			(unionMetaData unique newer) ls
	  where
		older = value l
		unique = older `differenceMetaData` newer