{- 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.
 -}

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

import Data.Time.Clock.POSIX
import Data.Time
import System.Locale
import qualified Data.Map as Map
import System.IO
import System.Directory
import Data.Char

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 date status uuid) = unwords
		[(show date), (show status), uuid]

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 -> undefined
			else undefined
		where
			w = words string
			date = w !! 0
			status = read $ w !! 1
			uuid = w !! 2
			pdate = (parseTime defaultTimeLocale "%s%Qs" date) :: Maybe UTCTime

			good v = ret $ LogLine (utcTimeToPOSIXSeconds v) status uuid
			undefined = ret $ LogLine (0) Undefined ""
			ret v = [(v, "")]

{- Log a change in the presence of a key's value in a repository. -}
logChange :: Git.Repo -> Key -> UUID -> LogStatus -> IO ()
logChange repo key uuid status = do
	log <- logNow status uuid
	ls <- readLog logfile
	writeLog logfile (compactLog $ log:ls)
	where
		logfile = logFile repo key

{- Reads a log file.
 - Note that the LogLines returned may be in any order. -}
readLog :: FilePath -> IO [LogLine]
readLog file = do
	exists <- doesFileExist file
	if exists
		then do
			s <- withFileLocked file ReadMode $ \h -> 
				hGetContentsStrict h
			-- filter out any unparsable lines
			return $ filter (\l -> (status l) /= Undefined )
				$ map read $ lines s
		else do
			return []

{- Adds a LogLine to a log file -}
appendLog :: FilePath -> LogLine -> IO ()
appendLog file line = do
	createDirectoryIfMissing True (parentDir file)
	withFileLocked file AppendMode $ \h ->
		hPutStrLn h $ show line

{- Writes a set of lines to a log file -}
writeLog :: FilePath -> [LogLine] -> IO ()
writeLog file lines = do
	createDirectoryIfMissing True (parentDir file)
	withFileLocked file WriteMode $ \h ->
		hPutStr h $ unlines $ map show lines

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

{- Returns the filename of the log file for a given key. -}
logFile :: Git.Repo -> Key -> String
logFile repo key = 
	(gitStateDir repo) ++ (Git.relative repo (keyFile key)) ++ ".log"

{- 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
	lines <- readLog $ logFile thisrepo key
	return $ map uuid (filterPresent lines)

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

{- Compacts a set of logs, returning a subset that contains the current
 - status. -}
compactLog :: [LogLine] -> [LogLine]
compactLog lines = compactLog' Map.empty lines
compactLog' map [] = Map.elems map
compactLog' map (l:ls) = compactLog' (mapLog map 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 map log = 
	if (better)
		then Map.insert (uuid log) log map
		else map
	where
		better = case (Map.lookup (uuid log) map) of
			Just l -> (date l <= date log)
			Nothing -> True