{- git-annex location log
 -
 - git-annex keeps track of on which repository it last saw a value.
 - This can be useful when using it for archiving with offline storage.
 - When you indicate you --want a file, git-annex will tell you which
 - repositories have the value.
 -
 - Location tracking information is stored in `.git-annex/key.log`.
 - Repositories record their UUID and the date when they --get or --drop
 - a value.
 -
 - A line of the log will look like: "date N UUID"
 - Where N=1 when the repo has the file, and 0 otherwise.
 - 
 - Git is configured to use a union merge for this file,
 - so the lines may be in arbitrary order, but it will never conflict.
 -
 - Copyright 2010 Joey Hess <joey@kitenet.net>
 -
 - Licensed under the GNU GPL version 3 or higher.
 -}

module LocationLog (
	LogStatus(..),
	logChange,
	readLog,
	writeLog,
	keyLocations
) where

import Data.Time.Clock.POSIX
import Data.Time
import System.Locale
import qualified Data.Map as Map
import Control.Monad (when)

import qualified GitRepo as Git
import Utility
import UUID
import Types
import Locations

data LogLine = LogLine {
	date :: POSIXTime,
	status :: LogStatus,
	uuid :: UUID
} deriving (Eq)

data LogStatus = ValuePresent | ValueMissing | Undefined
	deriving (Eq)

instance Show LogStatus where
	show ValuePresent = "1"
	show ValueMissing = "0"
	show Undefined = "undefined"

instance Read LogStatus where
	readsPrec _ "1" = [(ValuePresent, "")]
	readsPrec _ "0" = [(ValueMissing, "")]
	readsPrec _ _   = [(Undefined, "")]

instance Show LogLine where
	show (LogLine d s u) = unwords [show d, show s, u]

instance Read LogLine where
	-- This parser is robust in that even unparsable log lines are
	-- read without an exception being thrown.
	-- Such lines have a status of Undefined.
	readsPrec _ string = 
		if length w == 3
			then case pdate of
				Just v -> good v
				Nothing -> bad
			else bad
		where
			w = words string
			s = read $ w !! 1
			u = w !! 2
			pdate :: Maybe UTCTime
			pdate = parseTime defaultTimeLocale "%s%Qs" $ head w

			good v = ret $ LogLine (utcTimeToPOSIXSeconds v) s u
			bad = ret $ LogLine 0 Undefined ""
			ret v = [(v, "")]

{- Log a change in the presence of a key's value in a repository,
 - and returns the filename of the logfile. -}
logChange :: Git.Repo -> Key -> UUID -> LogStatus -> IO FilePath
logChange repo key u s = do
	when (null u) $
		error $ "unknown UUID for " ++ Git.repoDescribe repo ++ 
			" (have you run git annex init there?)"
	line <- logNow s u
	let f = logFile repo key
	ls' <- readLog $ logFileOld repo key
	ls <- readLog f
	writeLog f (compactLog $ line:ls'++ls)
	return f

{- Reads a log file.
 - Note that the LogLines returned may be in any order. -}
readLog :: FilePath -> IO [LogLine]
readLog file = catch (return . parseLog =<< readFile file) (const $ return [])

parseLog :: String -> [LogLine]
parseLog s = filter parsable $ map read $ lines s
	where
		-- some lines may be unparseable, avoid them
		parsable l = status l /= Undefined

{- Writes a set of lines to a log file -}
writeLog :: FilePath -> [LogLine] -> IO ()
writeLog file ls = safeWriteFile file (unlines $ map show ls)

{- Generates a new LogLine with the current date. -}
logNow :: LogStatus -> UUID -> IO LogLine
logNow s u = do
	now <- getPOSIXTime
	return $ LogLine now s u

{- Returns a list of repository UUIDs that, according to the log, have
 - the value of a key. -}
keyLocations :: Git.Repo -> Key -> IO [UUID]
keyLocations thisrepo key = do
	ls <- readLog $ logFile thisrepo key
	ls' <- readLog $ logFileOld thisrepo key
	return $ map uuid $ filterPresent $ ls'++ls

{- Filters the list of LogLines to find ones where the value
 - is (or should still be) present. -}
filterPresent :: [LogLine] -> [LogLine]
filterPresent ls = filter (\l -> ValuePresent == status l) $ compactLog ls

type LogMap = Map.Map UUID LogLine

{- Compacts a set of logs, returning a subset that contains the current
 - status. -}
compactLog :: [LogLine] -> [LogLine]
compactLog ls = compactLog' Map.empty ls
compactLog' :: LogMap -> [LogLine] -> [LogLine]
compactLog' m [] = Map.elems m
compactLog' m (l:ls) = compactLog' (mapLog m l) ls

{- Inserts a log into a map of logs, if the log has better (ie, newer)
 - information about a repo than the other logs in the map -}
mapLog :: LogMap -> LogLine -> LogMap
mapLog m l = 
	if better
		then Map.insert u l m
		else m
	where
		better = case Map.lookup u m of
			Just l' -> (date l' <= date l)
			Nothing -> True
		u = uuid l