{- 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,
	logFile,
	keyLocations
) where

import Data.Time.Clock.POSIX
import Data.Time
import System.Locale
import qualified Data.Map as Map
import System.Directory
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 $ "bug detected: unknown UUID for " ++ Git.repoDescribe repo
	line <- logNow s u
	ls <- readLog logfile
	writeLog logfile (compactLog $ line:ls)
	return logfile
	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 <- readFile file
			return $ parseLog s
		else 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 the filename of the log file for a given key. -}
logFile :: Git.Repo -> Key -> String
logFile repo key = 
	gitStateDir 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
	ls <- readLog $ logFile thisrepo key
	return $ map uuid $ filterPresent 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